№ 10. Sense organs. The Eye. Diseases of the Eyes.
The Senses
The human body environment is filled with both internal and external stimuli. Humans are able to distinguish among the many different types of stimuli by means of a highly developed system of nerves, specialized cells, and organs. The humaervous system is adapted to monitor and respond to a vast range of environmental phenomena.
Humans have 5 senses: touch, taste, smell, sight, and hearing. The senses are based on receptor cells or groups of receptor cells called sense organs.
Receptors respond to stimuli and send nerve impulses along sensory neurons. (See nervous system page) The brain interprets the nerve impulse and, thus, we perceive the impulse as one of our senses.
Senses of the human body.
Receptors and Sense Organs
Receptors are cells that receive information from the environment and then send impulses via conductors to the central nervous system.
Receptors are the neural components of the sense organs, the eyes, ears, nose, mouth, and skin. When a particular sense organ receives stimulation, the receptor converts the stimulus into an electrical impulse that is sent to a specific region of the brain. The impulses generated by the different sense organs are electrically similar, but the region of the brain to which they are directed will vary.
Sensory organs.
The Sensory receptors involved in taste and smell contain receptor molecules that bind to specific chemicals. Odour receptors in olfactory receptor neurons, for example, are activated by interacting with molecular structures on the odour molecule. Similarly, taste receptors (gustatory receptors) in taste buds interact with chemicals in food to produce an action potential.
Other receptors such as mechanoreceptors and photoreceptors respond to physical stimuli. For example, photoreceptor cells contain specialized proteins such as rhodopsin to transduce the physical energy in light into electrical signals. Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.
The sensory receptor functions as the first component in a sensory system.
Sensory receptors respond to specific stimulus modalities. The stimulus modality to which a sensory receptor responds is determined by the sensory receptor’s adequate stimulus.
Sensory receptors can be classified:
by location
Cutaneous receptors
Cutaneous receptors are sensory receptors found in the dermis or epidermis.
Muscle spindles contain mechanoreceptors that detect stretch in muscles;
by morphology
Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:
Free nerve endings characterize the nociceptors and thermoreceptors and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout the dermis and epidermis.
Encapsulated receptors consist of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning; by rate of adaptation.
Somatic sensory receptors.
A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus. In this way it conveys information about the duration of the stimulus.
Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle.
A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops. It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate. An example of a phasic receptor is the Pacinian corpuscle.
Vision
Vision, stimulated by bright light.
The eyes are specialized organs that function by receiving light and transmitting impulses to the visual cortex of the brain. All the structures of the eye act together to focus light on the retina, the light-sensitive inner layer of the eye. The pupil, an opening in the iris, controls the amount of light that enters the eye. The pupil becomes larger when light is dim and smaller when it is bright. These involuntary responses are controlled by muscles in the pigmented iris.
After light passes through the pupil, it travels to a convex crystalline structure called the lens. Lying deep within the retina are rods and cones, photoreceptors that are essential in creating the vast array of visual images you perceive. There are nearly 125 million rods in a single retina. These are stimulated by weak light. The 7 million cones in the retina are
Hearing and Balance
Hearing (or audition) is one of the traditional five senses. It is the ability to perceive sound by detecting vibrations via an organ such as the ear. The inability to hear is called deafness. In humans and other vertebrates, hearing is performed primarily by the auditory system: vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain (primarily in the temporal lobe). Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation. The ear is specialized for two functions: detecting sound and maintaining balance. Sound, a form of energy consisting of air vibrations, is directed by the outer structures of the ear into the ear, where the vibrations stimulate mechanoreceptors.
Air enters the outer ear through the auditory canal. Vibrations in the air passing through the auditory canal cause the tympanic membrane or eardrum t vibrate. These vibrations are transmitted to the three small bones of the middle ear the hammer, the anvil, the stirrup. The intensity of a sound is determined by the number of cells stimulated: the more cells that are stimulated, the louder the sound.
Balance is maintained with the help of mechanoreceptors in the three semicircular canals of the inner ear. These canals are filled with fluid and lined with hair cells.
Taste and smell
Taste (or, more formally, gustation) is a form of direct chemoreception and is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons. In humans and many other vertebrate animals the sense of taste partners with the less direct sense of smell, in the brain’s perception of flavor. In the West, experts traditionally identified four taste sensations: sweet, salty, sour, and bitter. Eastern experts traditionally identified a fifth, called umami (savory). More recently, psychophysicists and neuroscientists have suggested other taste categories (umami and fatty acid taste most prominently, as well as the sensation of metallic and water tastes, although the latter is commonly disregarded due to the phenomenon of taste adaptation.[citatioeeded]) Taste is a sensory function of the central nervous system. The receptor cells for taste in humans are found on the surface of the tongue, along the soft palate, and in the epithelium of the pharynx and epiglottis. Specialized chemoreceptors allow humans to perceive variations in taste and odours. The chemoreceptors for taste are clustered in the taste buds. Most of the 10 000 taste buds are embedded between bumps called papillae on the tongue: additional taste buds are found on the roof of the mouth and in the throat.
Chemicals that dissolve in saliva stimulate different kinds of taste. Humans can taste only salty, sweet, sour and bitter flavours or combinations of these flavours. Other flavours are perceived by receptors in the nasal passages.
Taste buds.
Specialized chemoreceptors called olfactory receptors are located in the olfactory epithelium of the nasal passage. These cells, dendrites of sensory neurons, lie within the mucus lining of the epithelium. Specific olfactory receptors respond to different chemical. Chemicals as gas or particles enter the nose and dissolve in watery mucus before stimulating the olfactory receptors.
Other senses
Mechanoreceptors located throughout the skin make it possible to sense touch, pressure and tension. In humans the receptors for touch are concentrated ii the face and fingerprints. Body hair also plays an important role in the ability to sense touch. Large numbers of mechanoreceptors are found at the base of the hair follicles. The dendrites of these mechanoreceptors detect movement of the hair and relay impulses to the central nervous system.
The eye is the organ of vision. It has a complex structure consisting of a transparent lens that focuses light on the retina. The retina is covered with two basic types of light-sensitive cells-rods and cones. The cone cells are sensitive to color and are located in the part of the retina called the fovea, where the light is focused by the lens. The rod cells are not sensitive to color, but have greater sensitivity to light than the cone cells. These cells are located around the fovea and are responsible for peripheral vision and night vision. The eye is connected to the brain through the optic nerve. The point of this connection is called the “blind spot” because it is insensitive to light. Experiments have shown that the back of the brain maps the visual input from the eyes.
The brain combines the input of our two eyes into a single three-dimensional image. In addition, even though the image on the retina is upside-down because of the focusing action of the lens, the brain compensates and provides the right-side-up perception. Experiments have been done with subjects fitted with prisms that invert the images. The subjects go through an initial period of great confusion, but subsequently they perceive the images as right side up.
The range of perception of the eye is phenomenal. In the dark, a substance produced by the rod cells increases the sensitivity of the eye so that it is possible to detect very dim light. In strong light, the iris contracts reducing the size of the aperture that admits light into the eye and a protective obscure substance reduces the exposure of the light-sensitive cells. The spectrum of light to which the eye is sensitive varies from the red to the violet. Lower electromagnetic frequencies in the infrared are sensed as heat, but cannot be seen. Higher frequencies in the ultraviolet and beyond cannot be seen either, but can be sensed as tingling of the skin or eyes depending on the frequency. The human eye is not sensitive to the polarization of light, i.e., light that oscillates on a specific plane. Bees, on the other hand, are sensitive to polarized light, and have a visual range that extends into the ultraviolet. Some kinds of snakes have special infrared sensors that enable them to hunt in absolute darkness using only the heat emitted by their prey. Birds have a higher density of light-sensing cells than humans do in their retinas, and therefore, higher visual acuity.
Color blindness or “Daltonism” is a common abnormality in human vision that makes it impossible to differentiate colors accurately. One type of color blindness results in the inability to distinguish red from green. This can be a real handicap for certain types of occupations. To a colorblind person, a person with normal color vision would appear to have extrasensory perception. However, we want to reserve the term “extrasensory perception” for perception that is beyond the range of the normal.
Hearing.
The ear is the organ of hearing. The outer ear protrudes away from the head and is shaped like a cup to direct sounds toward the tympanic membrane, which transmits vibrations to the inner ear through a series of small bones in the middle ear called the malleus, incus and stapes. The inner ear, or cochlea, is a spiral-shaped chamber covered internally by nerve fibers that react to the vibrations and transmit impulses to the brain via the auditory nerve. The brain combines the input of our two ears to determine the direction and distance of sounds.
The inner ear has a vestibular system formed by three semicircular canals that are approximately at right angles to each other and which are responsible for the sense of balance and spatial orientation. The inner ear has chambers filled with a viscous fluid and small particles (otoliths) containing calcium carbonate. The movement of these particles over small hair cells in the inner ear sends signals to the brain that are interpreted as motion and acceleration.
The human ear can perceive frequencies from 16 cycles per second, which is a very deep bass, to 28,000 cycles per second, which is a very high pitch. Bats and dolphins can detect frequencies higher than 100,000 cycles per second. The human ear can detect pitch changes as small as 3 hundredths of one percent of the original frequency in some frequency ranges. Some people have “perfect pitch”, which is the ability to map a tone precisely on the musical scale without reference to an external standard. It is estimated that less than one in ten thousand people have perfect pitch, but speakers of tonal languages like Vietnamese and Mandarin show remarkably precise absolute pitch in reading out lists of words because pitch is an essential feature in conveying the meaning of words in tone languages. The Eguchi Method teaches perfect pitch to children starting before they are 4 years old. After age 7, the ability to recognize notes does not improve much.
Taste.
The receptors for taste, called taste buds, are situated chiefly in the tongue, but they are also located in the roof of the mouth and near the pharynx. They are able to detect four basic tastes: salty, sweet, bitter, and sour. The tongue also can detect a sensation called “umami” from taste receptors sensitive to amino acids. Generally, the taste buds close to the tip of the tongue are sensitive to sweet tastes, whereas those in the back of the tongue are sensitive to bitter tastes. The taste buds on top and on the side of the tongue are sensitive to salty and sour tastes. At the base of each taste bud there is a nerve that sends the sensations to the brain. The sense of taste functions in coordination with the sense of smell. The number of taste buds varies substantially from individual to individual, but greater numbers increase sensitivity. Women, in general, have a greater number of taste buds than men. As in the case of color blindness, some people are insensitive to some tastes.
Smell.
The nose is the organ responsible for the sense of smell. The cavity of the nose is lined with mucous membranes that have smell receptors connected to the olfactory nerve. The smells themselves consist of vapors of various substances. The smell receptors interact with the molecules of these vapors and transmit the sensations to the brain. The nose also has a structure called the vomeronasal organ whose function has not been determined, but which is suspected of being sensitive to pheromones that influence the reproductive cycle. The smell receptors are sensitive to seven types of sensations that can be characterized as camphor, musk, flower, mint, ether, acrid, or putrid. The sense of smell is sometimes temporarily lost when a person has a cold. Dogs have a sense of smell that is many times more sensitive than man’s.
Touch.
The sense of touch is distributed throughout the body. Nerve endings in the skin and other parts of the body transmit sensations to the brain. Some parts of the body have a larger number of nerve endings and, therefore, are more sensitive. Four kinds of touch sensations can be identified: cold, heat, contact, and pain. Hairs on the skin magnify the sensitivity and act as an early warning system for the body. The fingertips and the sexual organs have the greatest concentration of nerve endings. The sexual organs have “erogenous zones” that when stimulated start a series of endocrine reactions and motor responses resulting in orgasm.
Beyond the five sense organs.
In addition to sight, smell, taste, touch, and hearing, humans also have awareness of balance (equilibrioception), pressure, temperature (thermoception), pain (nociception), and motion all of which may involve the coordinated use of multiple sensory organs. The sense of balance is maintained by a complex interaction of visual inputs, the proprioceptive sensors (which are affected by gravity and stretch sensors found in muscles, skin, and joints), the inner ear vestibular system, and the central nervous system. Disturbances occurring in any part of the balance system, or even within the brain’s integration of inputs, can cause the feeling of dizziness or unsteadiness.
Kinesthesia is the precise awareness of muscle and joint movement that allows us to coordinate our muscles when we walk, talk, and use our hands. It is the sense of kinesthesia that enables us to touch the tip of our nose with our eyes closed or to know which part of the body we should scratch when we itch.
Synesthesia.
Some people experience a phenomenon called synesthesia in which one type of stimulation evokes the sensation of another. For example, the hearing of a sound may result in the sensation of the visualization of a color, or a shape may be sensed as a smell. Synesthesia is hereditary and it is estimated that it occurs in 1 out of 1000 individuals with variations of type and intensity. The most common forms of synesthesia link numbers or letters with colors.
Human eye
1. vitreous body 2. ora serrata 3. ciliary muscle 4. ciliary zonules 5. canal of Schlemm 6. pupil 7. anterior chamber 8. cornea 9. iris 10. lens cortex 11. lens nucleus 12. ciliary process 13. conjunctiva 14. inferior oblique muscle 15. inferior rectus muscle 16. medial rectus muscle 17. retinal arteries and veins 18. optic disc 19. dura mater 20. central retinal artery 21. central retinal vein 22. optic nerve 23. vorticose vein 24. bulbar sheath 25. macula 26. fovea 27. sclera 28. choroid 29. superior rectus muscle 30. retina
General properties
The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit. The smaller frontal unit, more curved, called the cornea is linked to the larger unit called the sclera. The corneal segment is typically about
Components
The eye is made up of three coats, enclosing three transparent structures. The outermost layer is composed of the cornea and sclera. The middle layer consists of the choroid, ciliary body, and iris. The innermost is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels, which can be seen in an ophthalmoscope.
Within these coats are the aqueous humor, the vitreous body, and the flexible lens. The aqueous humor is a clear fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than the aqueous humor, present behind lens and the rest, and is bordered by the sclera, zonule, and lens. They are connected via the pupil.
Dynamic range
The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and geometrically by adjusting the iris which regulates the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) is mostly complete in thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 f-stops) is possible. The process is nonlinear and multifaceted, so an interruption by light merely starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like tobacco.
The eye includes a lens not dissimilar to lenses found in optical instruments such as cameras and the same principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about
Eye irritation
Bloodshot eyeball.
Eye irritation has been defined as “the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye”. It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are e.g. discomfort, dryness, excess tearing, itching, grating, sandy sensation, smarting, ocular fatigue, pain, scratchiness, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from severe to less severe. It has been suggested that these eye symptoms are related to different causal mechanisms.
Several suspected causal factors in our environment have been studied so far. One hypothesis is that indoor air pollution may cause eye and airway irritation. Eye irritation depends somewhat on destabilization of the outer-eye tear film, in which the formation of dry spots results in such ocular discomfort as dryness. Occupational factors are also likely to influence the perception of eye irritation. Some of these are lighting (glare and poor contrast), gaze position, a limited number of breaks, and a constant function of accommodation, musculoskeletal burden, and impairment of the visual nervous system. Another factor that may be related is work stress. In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users. Other risk factors, such as chemical toxins/irritants, e.g. amines, formaldehyde, acetaldehyde, acrolein, N-decane, VOCs; ozone, pesticides and preservatives, allergens, etc. might cause eye irritation as well.
Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation as well. Personal factors (e.g., use of contact lenses, eye make-up, and certain medications) may also affect destabilization of the tear film and possibly result in more eye symptoms. Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high. An integrated physiological risk model with blink frequency, destabilization, and break-up of the eye tear film as inseparable phenomena may explain eye irritation among office workers in terms of occupational, climate, and eye-related physiological risk factors.
There are two major measures of eye irritation. One is blink frequency which can be observed by human behavior. The other measures are break up time, tear flow, hyperemia (redness, swelling), tear fluid cytology, and epithelial damage (vital stains) etc., which are human beings’ physiological reactions. Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation. Blink frequencies are individual with mean frequencies of < 2-3 to 20-30 blinks/minute, and they depend on environmental factors including the use of contact lenses. Dehydration, mental activities, work conditions, room temperature, relative humidity, and illumination all influence blink frequency. Break-up time (BUT) is another major measure of eye irritation and tear film stability. It is defined as the time interval (in seconds) between blinking and rupture. BUT is considered to reflect the stability of the tear film as well. Iormal persons, the break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained. Studies have shown that blink frequency is correlated negatively with break-up time. This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch. Other evaluating methods, such as hyperemia, cytology etc. have increasingly been used to assess eye irritation.
There are other factors that related to eye irritation as well. Three major factors that influence the most are indoor air pollution, contact lenses and gender differences. Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparisons with random samples of the general population. These research results might indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers. Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms, dryness, redness, and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers. Studies have shown that incidence of dry eyes increases with age. especially among women. Tear film stability (e.g. break-up time) is significantly lower among women than among men. In addition, women have a higher blink frequency while reading. Several factors may contribute to gender differences. One is the use of eye make-up. Another reason could be that the women in the reported studies have done more VDU work than the men, including lower grade work. A third often-quoted explanation is related to the age-dependent decrease of tear secretion, particularly among women after 40 years of age.,
In a study conducted by UCLA, the frequency of reported symptoms in industrial buildings was investigated. The study’s results were that eye irritation was the most frequent symptom in industrial building spaces, at 81%. Modern office work with use of office equipment has raised concerns about possible adverse health effects. Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying paper. Emission of various particulate and volatile substances has been suggested as specific causes. These symptoms have been related to Sick Building Syndrome (SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.
Many of the symptoms described in SBS and multiple chemical sensitivity (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals. A repeated measurement design was employed in the study of acute symptoms of eye and respiratory tract irritation resulting from occupational exposure to sodium borate dusts. The symptom assessment of the 79 exposed and 27 unexposed subjects comprised interviews before the shift began and then at regular hourly intervals for the next six hours of the shift, four days in a row. Exposures were monitored concurrently with a personal real time aerosol monitor. Two different exposure profiles, a daily average and short term (15 minute) average, were used in the analysis. Exposure-response relations were evaluated by linking incidence rates for each symptom with categories of exposure.
Acute incidence rates for nasal, eye, and throat irritation, and coughing and breathlessness were found to be associated with increased exposure levels of both exposure indices. Steeper exposure-response slopes were seen when short term exposure concentrations were used. Results from multivariate logistic regression analysis suggest that current smokers tended to be less sensitive to the exposure to airborne sodium borate dust.
Several actions can be taken to prevent eye irritation—
· trying to maintaiormal blinking by avoiding room temperatures that are too high; avoiding relative humidities that are too high or too low, because they reduce blink frequency or may increase water evaporation
· trying to maintain an intact tear film by the following actions. 1) blinking and short breaks may be beneficial for VDU users. Increase these two actions might help maintain the tear film. 2) downward gazing is recommended to reduce the ocular surface area and water evaporation.3) the distance between the VDU and keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by a low direction of the gaze. And 4) blink training can be beneficial.
In addition, other measures are proper lid hygiene, avoidance of eye rubbing, and proper use of personal products and medication. Eye make-up should be used with care
Eye movement
MRI scan of human eye.
The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea centralis, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.
Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.
Extraocular muscles
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.
Rapid eye movement
Rapid eye movement, or REM for short, typically refers to the sleep stage during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.
Saccades
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain. Some irregular drifts, movements, smaller than a saccade and larger than a microsaccade, subtend up to six minutes of arc.
Microsaccades
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.
Vestibulo-ocular reflex
The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa.
Smooth pursuit movement
The eyes can also follow a moving object around. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.
It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.
Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window at a moving train, the eyes can focus on a ‘moving’ train for a short moment (through smooth pursuit), until the train moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the train (through a saccade).
Near response
The adjustment to close-range vision involves three processes to focus an image on the retina.
Vergence movement
The two eyes converge to point to the same object.
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate ‘towards each other’ (convergence), while for an object farther away they rotate ‘away from each other’ (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose for example). When looking into the distance, or when ‘staring into nothingness’, the eyes neither converge nor diverge. Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.
Pupil constriction
Lenses cannot refract light rays at their edges as well as they can closer to the center. The image produced by any lens is therefore somewhat blurry around the edges (spherical aberration). It can be minimized by screening out peripheral light rays and looking only at the better-focused center. In the eye, the pupil serves this purpose by constricting while the eye is focused oearby objects. In this way the pupil has a dual purpose: to adjust the eye to variations in brightness and to reduce spherical aberration.
Accommodation of the lens
A change in the curvature of the lens, accommodation is carried out by the ciliary muscles surrounding the lens contracting. This narrows the diameter of the ciliary body, relaxes the fibers of the suspernsory ligament, and allows the lens to relax into a more convex shape. A more convex lens refracts light more strongly and focuses divergent light rays onto the retina allowing for closer objects to be brought into focus.
Effects of aging
There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.
As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of diseases of the aging eye. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of infection occurs, the STD can be transmitted to the eye.
With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities — visible as floaters — gradually increase iumber.
Various eye care professionals, including ophthalmologists, optometrists, and opticians, are involved in the treatment and management of ocular and vision disorders. A Snellen chart is one type of eye chart used to measure visual acuity. At the conclusion of a complete eye examination, the eye doctor might provide the patient with an eyeglass prescription for corrective lenses. Some disorders of the eyes for which corrective lenses are prescribed include myopia (near-sightedness) which affects about one-third[citatioeeded] of the human population, hyperopia (far-sightedness) which affects about one quarter of the population, astigmatism, and presbyopia, the loss of focusing range during aging.
Eye care professionals
The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner. These specialists, or eye care professionals, serve different functions in different countries. Each eye care professional can typically be categorized into one or a multiplicity (i.e. an ophthalmologist can perform surgery; and in some instances prescribe lenses, which is a duty often performed by optometrists) of duties of the following types of professionals:
1. Ophthalmologists
2. Orthoptists
3. Optometrists
4. Opticians
Alternative eye care practitioners
There are many practitioners who work with the human eye, influenced by a different set of theories to the orthodox optometry profession. Inspired by people such as William Horatio Bates, they believe that eye-sight can be healed using a series of exercises – some specifically employing the eye-muscles – others more generally dealing with bodily relaxation such as yogic exercises. Some believe posture plays an important role. Factors such as stress, sleep, loud music, novelty, lying, sudden exposure to light, mental discomfort (anger, depression, anxiety), physical discomfort (pain, cough, fever, extreme heat or cold) and forced concentration have all been cited as contributors to poor eye-sight. A more recent natural eye healing system than the Bates method is Natural Vision Improvement.
The eye. Diseases of the eyes.
A human eye.
Vision is a complex sense composed of many elements. The human eye, elegant in its detail and design, represents a gateway to the process we call vision. The eyeball, or globe, is spherical in shape and about
Anatomy of the mammalian eye
Anatomy of the mammalian eye
1. posterior compartment
2. ora serrata
3. ciliary muscle
4. ciliary zonules
5.
6. pupil
7. anterior chamber
8. cornea
9.iris
10. lens cortex
11. lens nucleus
12. ciliary process
13. conjuntiva
14. inferior oblique muscule
15. inferior rectus muscule
16. medial rectus muscle
17. retinal arteries and veins
18. optic disc
19. dura mater
20. central retinal artery
21. central retinal vein
The normal eye. The eyeball is so positioned in the orbit that the anterior surface of the cornea is just the line with the superior and interior orbital margins – a useful relation in the assessment of proptosis. The cornea joins the sclera at the limbus, the corneal epithelium of the conjunctiva becoming continuous with the epithelium of the conjunctiva which is adherent here to the underlying episcleral tissue. The anterior chamber is the space enclosed by the cornea anteriorly and the lens and iris posteriorly.
The pupillary margin of the iris is in constant contact with the anterior surface of the lens, although aqueous humour is able to flow from the posterior chamber through, the pupil into the anterior chamber, from which it drains through the trabeculae into Schlemm’s canal. The iris, ciliary body and choroid form a continues structure called the uveal tract.
The inner layer, the retina, is composed of visual purple, made up of rods and cones. The retina has three spots, the macula lutea or yellow spot, the fovea centralis and the optic disc, where it is pierced by optic nerve.
The eyelids consist essentially of a plate of condensed fibrous tissue lined internally by conjuctiva and covered externally by the orbicularis muscle and skin. The Meibomian glands are embedded in the tarsal plate and open on the free margin of the lid very close to its posterior border. The inner, and outer angles of the palpebral fissure are known as the inner and outer canthi; the caruncle and plica semilunaris can be seen at the inner canthus. The punctum lacrimalis, through which the tears flow, lies in the close apposition to the globe and cannot normally be seen unless the lid is everted.
The eye is moved by the eye muscles. The visual pathways include theoptic nerves, the optic chiasma, with crossed and uncrossed fibres, the medialand lateral geniculate bodies, the superior quadrigeminal bodies, opticradiations and the occipital cortex.
Visual acuity is tested by reading Snellen’s test types; the error is measured by refractions. Errors of refraction are classified as hypermetropia, myopia, presbyopia, and astigmatism.
Strabismus.
A squint or strabismus is a fairly common condition in children. It is treated by prescribing spectacles to correct the error of refraction. If one eye is amblyopic, occlusion of the good eye is often useful. If all fails, operation may be needed to correct the cosmetic defect.
The eyelids may be affected by several diseases. Orbital cellulitis is an infection that extends from the nasal sinuses giving rise to acute swelling of the orbital tissue. The condition is serious because of the possibility of meningitis or cavernous sinus thrombosis.
A common irritating inflammation of the eyelid.
Acute dacryoadenitis
Acute dacryoadenitis is rare but may occur as a complication of mumps. A painful swelling appears in the outer region of the upper lid, causing somedegree of ptosis. A dermoid cyst as well as a drooping lid on one or bothsides is a congenial defect.
Ectropion is a condition in which the lower lid falls away from the globe and becomes everted; in entropion the lid turns inwards and the lashes cause much irritation by rubbing on the cornea. Inflammatory lesions of the lids include marginal cysts. Blepharitis is a chronic inflammation of the lid margin. In severe form it affects the lash follicles, leading to destruction of the lashes and deformity of the lid margin (ulcerative blepharitis) and to molluscum contagiosum, a virus disease.
Blepharitis.
Among skin conditions of the lids contact dermatitis is very common, which may follow the topical application of drugs, cosmetics or any foreign material. The thin skin and loose subcutaneous tissue of the lids predispose to oedema in this region. Local inflammations, renal disease, myxoedema are some of the many causes.
Xanthelasma, deposition of lipid material, does not cause any symptoms but may require excision for cosmetic reasons.
Herpes opthalmicus may occur in the area of the ophthalmic division of the trigeminal nerve.
Hemangioma, neurofibroma, papilloma are neoplasms of the lids of benign type. Malignant neoplasms must be excised and careful follow-up examination is necessary for many years.
The most common diseases of the conjuctiva are inflammations: acute catarrhal conjunctivitis, vernal, chronic, angular, and viral conjunctivitis.
The cornea, being an avascular structure, is very rarely the site of originof a tumour. Many new growths, however, arise from the limbus, which isthe site of transition from conjuctival to corneal epithelium, and these aregrouped together under he title of bulbar tumours (epibulbar dermoid,hemangioma, cystic nevus, epithelioma, sarcoma). Inflammations of thecornea include the dendritic ulcer, mycotic ulcer, superficial, interstitial, ordisciform keratitis. Keratoconus is of the degenerative character. Deficiencyof tear production leads to keratitis sicca, a characteristic picture in the corneaand conjunctiva that loses its normal lustre and on microscopicexamination with the slit-lamp the precorneal tear film can be seen to bereduced.
The sclera may be affected by episcleritis probably allergic iature in some type and by less common scleritis. Scleromalacia perforans is characterized by the appearance of degenerate areas in the sclera.
Acute iridocyclitis is accompanied with pain, photophobia lacrimation and blurring of vision. Chronic anterior uveitis occurs in two main types: one caused by granulomatous disease such as tuberculosis and sarcoidosis is characterized by severe destructive lesions, often with nodules in the iris; the other type is a quiet process involving more particularly the ciliary body.
The lens, being an avascular structure, is never the site of a new growth, but the iris and ciliary body may be the site of tumorous growths similar to those occurring in the choroid, the commonest being neuroectodermal in origin, such as nevus, leiomyoma of the iris, malignant melanoma of the ciliary body or iris, ring sarcoma of the iris. The anterior segment may suffer some trauma. Perforation wounds of the cornea frequently involve the iris and lens. The thinnest part of the iris is at its origin from the ciliary body, and a contusion of the eye may tear the iris at this point, causing iridodialysis. Contusion injuries may rupture the zonule of the lens and cause its dislocation, laterally or downwards.
A wide variety of foreign bodies may enter the eye in injuris. Theimmediate symptoms may be slight, and a small corneal wound is easilyoverlooked. Radiography of the eye should be taken after every injury inwhich it is at all revealed by radiography, an examination with a slit-lampmicroscope may enable it to be seen in the segment.
H00-H06 Disorders of eyelid, lacrimal system and orbit
· (H00.0) Hordeolum (“stye” or “sty”) — a bacterial infection of sebaceous glands of eyelashes.
An external stye or sty (/ˈstaɪ/), also hordeolum (/hɔrˈdiːələm/), is an infection of the sebaceous glands of Zeis at the base of the eyelashes, or an infection of the apocrine sweat glands of Moll. External styes form on the outside of the lids and can be seen as small red bumps. Internal styes are infections of the meibomian sebaceous glands lining the inside of the eyelids. They also cause a red bump underneath the lid with only generalized redness and swelling visible on the outside. Styes are similar to chalazia, but tend to be of smaller size and are more painful and usually produce no lasting damage. They contain water and pus and the bacteria will spread if the stye is forcefully ruptured. Styes are characterized by an acute onset and usually short in duration (7–10 days without treatment) compared to chalazia that are chronic and usually do not resolve without intervention. Styes are usually caused by staphylococcus aureus bacterium.
Signs and symptoms
The first sign of a stye is a small, yellowish spot at the center of the bump that develops as pus expands in the area.
Other stye symptoms may include:
o A lump on the top or bottom eyelid
o Localized swelling of the eyelid
o Localized pain
o Redness
o Tenderness
o Crusting of the eyelid margins
o Burning in the eye
o Droopiness of the eyelid
o Scratchy sensation on the eyeball (itching)
o Blurred vision
o Mucous discharge in the eye
o Irritation of the eye
o Light sensitivity
o Tearing
o Discomfort during blinking
o Sensation of a foreign body in the eye
Cause
Styes are commonly caused by the blocking of an oil gland at the base of the eyelash. Although they are particularly common in infants, styes are experienced by people of all ages. Styes can be triggered by poor nutrition, sleep deprivation, lack of hygiene, lack of water, and rubbing of the eyes. Sharing of washcloths or face towels should be curtailed to avoid spreading the infection between individuals. Styes can last from one to two weeks without treatment, or as little as four days if treated properly.
Medical professionals will sometimes lance a particularly persistent or irritating stye with a needle to accelerate its draining. A stye’s expansion can also be fought with erythromycin ophthalmic ointment. Medical professionals may also treat styes with other antibiotics, such as chloramphenicol or amoxicillin. Chloramphenicol is used successfully in many parts of the world, but contains a black box warning in the
If a stye bursts, care must be taken to cleanse the wound to prevent reinfection.
Prevention
Stye prevention is closely related to proper hygiene. Proper hand washing can reduce the risks of developing not only styes, but also all other types of infections.
Upon awakening, application of a warm washcloth to the eyelids for five to ten minutes, three to six times a day, may be beneficial in decreasing the occurrence of styes by liquefying the contents of the oil glands of the eyelid and thereby preventing blockage. Some studies suggest oral flaxseed supplementation to prevent the occurrence of styes.
To prevent developing styes, it is recommended to never share cosmetics or cosmetic eye tools with other people. People should also keep their eye tools clean and generally practice proper eye hygiene. It is recommended to remove makeup every night before going to sleep and discard old or contaminated eye makeup and try to avoid feeling stressed.
The primary mode of treatment for a stye is application of warm compresses. Incision and drainage is performed if resolution does not begin in the next 48 hours after warm compresses are started.
As a part of self-care at home, patients may cleanse the affected eyelid with tap water or with a mild, nonirritating soap or shampoo (such as baby shampoo) to help clean crusted discharge. Cleansing must be done gently and while the eyes are closed to prevent eye injuries.
Treatment with moxifloxacin, an antibiotic also used to treat pink eye, is often prescribed. Over-the-counter options include ointments like polymyxin B sulfate (bacitracin zinc).
Patients are highly advised not to lance the stye, as serious infection can occur as a result. The infection could spread to the surrounding tissues and areas.
Eye stye sufferers should avoid eye makeup (e.g., eyeliner), lotions, and wearing contact lenses, since these can aggravate and spread the infection (sometimes to the cornea).
Medical treatment can also be provided by a doctor and it is aimed at relieving the symptoms. Pain relievers such as acetaminophen may be prescribed and in some cases, antibiotics may be needed. Antibiotics are normally given to patients with multiple styes or with styes that do not seem to heal, and to patients who also suffer from blepharitis or rosacea. Commonly, the ophthalmologist prescribes oral or intravenous antibiotics, such as doxycycline, only when the infection has spread. Topical antibiotic ointments or antibiotic/steroid combination ointments can also be administered in stye treatment.
Surgery is the last resort in stye treatment. Styes that do not respond to any type of therapies are usually surgically removed. Stye surgery is performed by an ophthalmologist, and generally under local anesthesia. The procedure consists of making a small incision on the inner or outer surface of the eyelid, depending if the stye is pointing externally or not. After the incision is made, the pus is drained out of the gland, and very small and unnoticeable sutures are used to close the lesion. It is common for the removed stye to be sent for histopathological examination to rule out the possibility of skin cancer.
Complications
Stye complications occur in very rare cases. However, the most frequent complication of styes is progression to a chalazion that causes cosmetic deformity, corneal irritation, and often requires surgical removal. Complications may also arise from the improper surgical lancing, and mainly consist of disruption of lash growth, lid deformity or lid fistula. Styes that are too large may interfere with one’s vision.
Eyelid cellulitis is another potential complication of eye styes, which is a generalized infection of the eyelid. Progression of a stye to a systemic infection (spreading throughout the body) is extremely rare, and only a few instances of such spread have been recorded.
Prognosis
Although styes are harmless in most cases and complications are very rare, styes often recur. They do not cause intraocular damage, meaning they do not affect the eye. Styes normally heal on their own by rupturing within few days to a week, causing the relief of symptoms. Few people require surgery as part of stye treatment. With adequate treatment, styes tend to heal quickly and without complications.
The prognosis is better if one does not attempt to squeeze or puncture the stye, as infection may spread to adjacent tissues. A stye usually will heal within a few days to a week, but if it does not improve or it worsens within two weeks, a doctor’s opinion should be sought. Also, patients are recommended to call a doctor if they encounter problems with vision, the eyelid bumps becomes very painful, the stye bleeds or reoccurs or the eyelid or eye becomes red.
· (H00.1) Chalazion — a cyst in the eyelid (usually upper eyelid)
A chalazion (pron.: /kəˈleɪziən/; plural chalazia /kəˈleɪziə/), also known as a meibomian gland lipogranuloma, is a cyst in the eyelid that is caused by inflammation of a blocked meibomian gland, usually on the upper eyelid. Chalazia differ from styes (hordeola) in that they are subacute and usually painless nodules. They may become acutely inflamed but, unlike a stye, chalazia usually point inside the lid rather than on the lid margin.
Signs and symptoms
o Swelling on the eyelid
o Eyelid tenderness
o Sensitivity to light
o Increased tearing
o Heaviness of the eyelid
A chalazion or meibomian cyst can sometimes be mistaken for a stye.
Treatment
Topical antibiotic eye drops or ointment (e.g. chloramphenicol or fusidic acid) are sometimes used for the initial acute infection, but are otherwise of little value in treating a chalazion. Chalazia will often disappear without further treatment within a few months and virtually all will re-absorb within two years.
If they continue to enlarge or fail to settle within a few months, then smaller lesions may be injected with a corticosteroid or larger ones may be surgically removed using local anesthesia. This is usually done from underneath the eyelid to avoid a scar on the skin. If the chalazion is located directly under the eyelid’s outer tissue, however, an excision from above may be more advisable so as not to inflict any unnecessary damage on the lid itself. Eyelid epidermis usually mends well, without leaving any visible scar. Depending on the chalazion’s texture, the excision procedure varies: while fluid matter can easily be removed under minimal invasivion, by merely puncturing the chalazion and exerting pressure upon the surrounding tissue, hardened matter usually necessitates a larger incision, through which it can be scraped out. Any residual matter should be metabolized in the course of the subsequent healing process, generally aided by regular appliance of dry heat. The excision of larger chalazia may result in visible hematoma around the lid, which will wear off within three or four days, whereas the swelling may persist for longer. Chalazion excision is an ambulant treatment and normally does not take longer than fifteen minutes. Nevertheless, owing to the risks of infection and severe damage to the eyelid, such procedures should only be performed by a medical professional.
It is rare that a chalazion will recur and they will usually be biopsied to rule out the possibility of a tumour.
Complications
A large chalazion can cause astigmatism due to pressure on the cornea.
As laser eye surgery involves shaping the cornea by burning parts of it away, weakening its structure, post operation patients can be left predisposed to deformation of the cornea from small chalazia.
Complications including, but not limited to hypopigmentation may occur with corticosteroid injection.
Recurring chalazia in the same area may sometimes be a symptom of sebaceous cell carcinoma, albeit rarely. This is a type of cancer.
Sometimes, as a last resort, surgery is performed. The eyelid is injected with a local anesthetic, a clamp is put on the eyelid, then the eyelid is turned over, an incision is made on the inside of the eyelid, and the chalazion is drained and scraped out. A scar on the upper lid can cause discomfort as some patients feel the scar as they blink. Of course as surgeries are intrusive and damage healthy tissue (e.g. leaving behind scar tissue or possibly even causing blepharitis), given other options, less intrusive treatment is always preferable. Similarly, chalazia may recur once the eye is predisposed and surgical intervention each time is not possible. So surgery should be considered only as a last resort, performed on as few as 5% of all chalazia patients.
Blocked meibomian glands may also be treatable with hot towels or microwaveable ‘eye bags’, which liquify the oils on the gland.
Chalazion surgery
Chalazion surgery is normally performed by an ophthalmologist at an eye hospital. This type of surgery is a simple procedure which is generally performed as a day operation and the patient does not need to remain in the hospital for further medical care.
Chalazion removal surgery is performed under local or general anesthesia. Commonly, general anesthesia is administered in children to make sure they stay still and no injury to the eye occurs. Local anesthesia is used in adults and it is applied with a small injection into the eyelid. The discomfort of the injection is minimized with the help of an anesthetic cream which is applied locally.
The chalazion may be removed in two ways, depending on the size of cyst. Relatively small chalazia are removed through a small cut at the back of the eyelid. The surgeon lifts the eyelid so they can have access to the back of its surface and makes an incision of approximately 3mm just on top of the chalazion. The lump is then removed and pressure is applied for a few minutes to stop any oozing of blood that may occur because of the operation. Surgery of small chalazia does not require stitches as the cut is at the back of the eyelid and therefore the cut cannot be seen and the cosmetic result is excellent.
Larger chalazia are removed through an incision in front of the eyelid. Larger chalazia usually push on the skin of the eyelid and this is the main reason why doctors prefer removing them this way. The incision is not usually larger than
When surgery for a chalazion is considered, patients who take aspirin or any other blood-thinning medications are advised to stop taking them one week prior to the procedure as they may lead to uncontrollable bleeding. There are several tests taken prior the surgery to make sure the patient is in good condition for the operation.
In rare cases, patients are kept overnight in the hospital after chalazion surgery. This includes cases in which complications occurred and the patient needs to be closely monitored. In most cases however, patients are able to go home after the operation has ended.
The recovery process is easy and quite fast. Most patients experience some very minor discomfort in the eye which can be easily controlled by taking painkilling medication. Patients are, however, recommended to avoid getting water in the eye for up to 10 days after surgery. They may wash, bathe or shower but they must be careful to keep the area dry and clean. Makeup may be worn at least one month post-operatively. Patients are recommended to not wear contact lenses in the affected eye for at least eight weeks to prevent infections and potential complications.
Commonly, patients receive eye drops to prevent infection and swellings in the eye and pain medication that will help them cope with the pain and discomfort in the eyelid and eye. One can use paracetamol/Tylenol rather than aspirin to control the pain. Also, after surgery, a pad and protective plastic shield are used to apply pressure on the eye in order to prevent leakage of blood after the operation; this may be removed 6 to 8 hours after the procedure.
People who undergo chalazion surgery are normally asked to visit their eye surgeon for post-op follow-up three to four weeks after surgery has been performed. They may start driving the day after surgery and they may return to work in one or two days.
Chalazion surgery is a safe procedure and complications occur very seldom. Serious complications that require another operation are also very rare. Among potential complications, there is infection, bleeding or the recurrence of the chalazion.
· (H01.0) Blepharitis — inflammation of eyelids and eyelashes; characterized by white flaky skiear the eyelashes
Blepharitis (pron.: /blɛfərˈaɪtɨs/ BLEF-ər-EYE-tis) is an eye condition characterized by chronic inflammation of the eyelid, the severity and time course of which can vary. Onset can be acute, resolving without treatment within 2–4 weeks (this can be greatly reduced with lid hygiene), but more generally is a long standing inflammation varying in severity. It may be classified as seborrhoeic, staphylococcal, mixed, posterior or meiobomitis, or parasitic.
Signs and symptoms
Signs and symptoms that are associated with the chronic inflammation can be:
o Redness of the eyelids.
o Flaking of skin on the lids.
o Crusting at the lid margins, this is generally worse on waking.
o Cysts at the lid margin (hordeolum).
o Red eye.
o Debris in the tear film, seen under magnification (improved contrast with use of fluorescein drops).
o Gritty sensation of the eye or foreign-body sensation.
o Reduced vision.
o Eye itching
People who wear contact lenses usually have more trouble in coping with their symptoms because although they need contact lenses, they cannot wear them. Many such patients complain of being unable to wear their lenses for long periods of time or that the lenses are causing them even more irritation of the eye.
The lids may become red and may have ulcerative, non-healing areas which may actually bleed. Blepharitis does not tend to cause problems with the patient’s vision whatsoever, but due to a poor tear film, one may experience blurred vision.
Eye redness and swelling tend to appear in more advanced cases, and they are rarely primary symptoms. The symptoms can slightly vary based on the exact cause of the condition. Blepharitis due to an allergy can cause dark lids, symptom which is known as “allergic shiner” and which tends to be more frequent in children rather than adults. Infectious blepharitis is accompanied by a yellow- or green-colored discharge which is more abundant in the morning and which leads to stuck lids. Blepharitis may also cause eyelid matting or “gluing” of the lashes.
Other blepharitis symptoms include sensitivity to light, eyelashes that grow abnormally or even loss of eyelashes. Also, the tears might seem frothy or bubbly iature and mild scarring might occur to the eyelids. The symptoms and signs of blepharitis are often erroneously ascribed by the patient as being due to “recurrent conjunctivitis”.
Blepharitis that localizes in the skin of the eyelids may cause styes or chalazia, which appear like red bumps, sometimes with a yellow spot if infection is present. Although pain is not common among blepharitis symptoms, if the condition persists or becomes painful, the individual is recommended to seek medical attention.
Chronic blepharitis may result in damage of varying severity which may have a negative effect upon vision and therefore upon the eyeglass prescription.
Infectious blepharitis can cause hard crusts around the eyelashes which leave small ulcers that may bleed or ooze after cleaning.
As a general rule, blepharitis symptoms which do not improve, despite good hygiene consisting of proper cleaning and care of the eye area, should be referred to a doctor.
Staphylococcal blepharitis
Staphylococcal blepharitis is caused by infection of the anterior portion of the eyelid by Staphylococcal bacteria. Patients notice a foreign body sensation, matting of the lashes, and burning. Collarette around eyelashes, a ring-like formation around the lash shaft, can be observed, which is an important sign of this condition. Other symptoms include loss of eyelashes or broken eyelashes. The condition can sometimes lead to a chalazion or a stye.
Staphylococcal blepharitis is a condition which may start in childhood and continue through adulthood. It is commonly recurrent and it requires special medical care. The prevalence of Staphylococcus aureus in the conjunctival sac and on the lid margin varies among countries, probably due to climate.
Staphylococcal blepharitis is normally treated with antibiotics such as Chloramphenicol ointment. Fusidic acid is usually the choice of antibiotics in cases when Chloramphenicol is contraindicated. Antibiotics are given for at least four to six weeks, which may be enough to completely cure the infection. A short course of topical steroids are administered to control the inflammation. The infection is only treated effectively if given at the same time with extra cautious eyelid hygiene. This consists of proper cleaning of the eyelid, removing crusts and debris.
Posterior blepharitis or rosacea-associated blepharitis
“Internal hordeolum“
Posterior blepharitis is inflammation of the eyelids secondary to dysfunction of the meibomian glands. Like anterior blepharitis it is a bilateral chronic condition and manifested by a broad spectrum of symptoms involving the lids including inflammation and plugging of the meibomian orifices and production of abnormal secretion upon pressure over the glands. It may be associated with skin rosacea.
Treatment and management
The single most important treatment principle is a daily routine of lid margin hygiene, as described below. Such a routine needs to be convenient enough to be continued for life to avoid relapses as blepharitis is often a chronic condition. But it can be acute, and one episode does not mean it is a lifelong condition.
A typical lid margin hygiene routine consists of four steps. The steps are more challenging to perform by visually disabled or frail patients as it requires good motor skills:
Softening of lid margin debris and oils: Apply a warm wet compress to the lids – such as a washcloth with hot water – for about two minutes. Warm wet cloths do not stay warm for long enough but dry compress masks can be conveniently warmed in a micro-wave oven and maintain a comfortable
Mechanical removal of lid margin debris: After warm compresses, wash your face with a wash cloth. Use facial soap or non-burning baby shampoo (make sure to dilute the soap solution 1/10 with water first). Gently and repeatedly rub along the lid margins while eyes are closed. Too much soap or shampoo may remove the essential oily layer of the eyes’ own tear film and create further problems with dry eye discomfort. A moist cotton bud soaked in a cup of water with a drop of baby shampoo may be used to rub along the lid margins while tilting the lid outward with the other hand.
Antibiotic reduction of lid margin bacteria (at the discretion of a physician): After lid margin cleaning, spread small amount of prescription antibiotic ophthalmic ointment with finger tip along lid fissure while eyes closed. Use prior to bed time as opposed to in the morning to avoid blurry vision.
Avoid the use of eye make-up until symptoms subside.
Often the above is advised together with mild massage to mechanically empty glands located at the lid margin (Meibomian glands, Zeis glands, Moll glands).
Microbial blepharitis is treated with antibiotics such as Sulfacetamide eye ointment applied on a cotton applicator once daily to the lid margins. Ophthalmologists may prescribe low-dose oral antibiotics such as Doxycycline and occasionally weak topical steroids.
Physicians may consider allergy testing and ocular antihistamines. Allergic responses to dust mite feces and other allergens can cause lid inflammation, ocular irritation, and dry eyes. Prescription ocular antihistamines and over-the-counter ocular antihistamines can bring relief to patients whose lid inflammation is caused by allergies.
Researchers have found Omega-3 supplementation in the form of Fish Oil or Flaxseed to be beneficial in reducing the primary symptoms of Blepharitis.
· (H02.0) Entropion and trichiasis
Entropion is a medical condition in which the eyelid (usually the lower lid) folds inward. It is very uncomfortable, as the eyelashes constantly rub against the cornea and irritate it. Entropion is usually caused by genetic factors and very rarely it may be congenital when an extra fold of skin grows with the lower eyelid (epiblepharon)[citatioeeded]. Entropion can also create secondary pain of the eye (leading to self trauma, scarring of the eyelid, or nerve damage). The upper or lower eyelid can be involved, and one or both eyes may be affected. When entropion occurs in both eyes, this is known as “bilateral entropion.” Trachoma infection may cause scarring of the inner eyelid, which may cause entropion. In human cases, this condition is most common to people over 60 years of age.
Symptoms
Symptoms of entropion include:
o Redness and pain around the eye
o Sensitivity to light and wind
o Sagging skin around the eye
o Epiphora
o Decreased vision, especially if the cornea is damaged
Treatment
Treatment is a relatively simple surgery in which excess skin of the outer lids is removed or tendons and muscles are shortened with one or two stitches. General anesthesia is sometimes used before local anesthetics are injected into the muscles around the eye. Prognosis is excellent if surgery is performed before the cornea is damaged.
· (H02.1) Ectropion
Ectropion is a medical condition in which the lower eyelid turns outwards. It is one of the notable aspects of newborns exhibiting congenital Harlequin-type ichthyosis, but ectropion can occur due to any weakening of tissue of the lower eyelid. The condition can be repaired surgically. Ectropion is also found in dogs as a genetic disorder in certain breeds.
Causes
o Congenital
o Aging
o Scarring
o Mechanical
o Allergic
o Facial nerve palsy
Anti-cancer treatments such as erlotinib, cetuximab, and panitumumab, which block the function of EGFR (the epidermal growth factor receptor).
· (H02.2) Lagophthalmos
Lagophthalmos is defined as the inability to close the eyelids completely.
Blinking covers the eye with a thin layer of tear fluid, thereby promoting a moist environment necessary for the cells of the exterior part of the eye. The tears also flush out foreign bodies and wash them away. This is crucial to maintain lubrication and proper eye health. If this process is impaired, as in lagophthalmos, the eye can suffer abrasions and infections. Lagopthalmos leads to corneal drying and ulceration.
Pathophysiology
Lagophthalmos can arise from a malfunction of the Facial nerve. Lagopthalmos can also occur in comatose patients having a decrease in orbicularis tone, in patients having palsy of the facial nerve (7th cranial nerve), and in people with severe skin disorders such as ichthyosis.
Today, lagophthalmos may arise after an overenthusiastic upper blepharoplasty. Blepharoplasty is an operation performed to remove excessive skin overlying the upper eyelid (suprapalpebral hooding) that often occurs with aging. This can appreciably improve the patient’s appearance, and make the patient look younger. If, however, excessive skin is removed, the appearance is unnatural and “lagophthalmos” is one of the signs of such excessive skin removal.
Treatment
Treatment of lagopthalmos can include both supportive care methods as well as surgical. If unable to receive surgery, artificial tears should be administered at least four times a day to the cornea to preserve the tear film. Leading up to a surgery, a patient can undergo a tarsorrhaphy which partially sews the eye shut temporarily to further protect the cornea as the patient waits for care. Multiple surgical treatments exist for Lagopthalmos but the most prevalent method includes weighing the upper eyelid down by surgically inserting a gold plate. Due to possible complications in conjunction with both the upper and lower eyelid, it might also be required to undergo a second surgery to tighten and elevate the lower eyelid to ensure both the upper and lower eyelids can fully close and protect the cornea.
· (H02.3) Blepharochalasis
Blepharochalasis is an inflammation of the eyelid that is characterized by exacerbations and remissions of eyelid edema, which results in a stretching and subsequent atrophy of the eyelid tissue resulting in redundant folds over the lid margins. It typically affects only the upper eyelids, and may be unilateral as well as bilateral.
Pathophysiology
Blepharochalasis results from recurrent bouts of painless eyelid swelling, each lasting for several days. This is thought to be a form of localized angioedema, or rapid accumulation of fluid in the tissues. Recurrent episodes lead to thin and atrophic skin. Damage to the levator palpebrae superioris muscle causes ptosis, or drooping of the eyelid, when the muscle cao longer hold the eyelid up.
Causes
Blepharochalasis is idiopathic in most cases, i.e., the cause is unknown. Systemic conditions linked to blepharochalasis are renal agenesis, vertebral abnormalities, and congenital heart disease.
Epidemiology
It is encountered more commonly in younger rather than older individuals.
Complications
Complications of blepharochalasis may include conjunctival hyperemia (excessive blood flow through the moist tissues of the orbit), chemosis, entropion, ectropion, and ptosis.
Differential diagnosis
Dermatochalasis is sometimes confused with blepharochalasis, but these are two different conditions.
Treatment/Surgery
A surgeon trained to do eyelid surgery, such as a plastic surgeon or ophthalmologist, is required to decide and perform the appropriate surgical procedure. Following procedures have been described for blepharochalasis:
o External levator aponeurosis tuck
o Blepharoplasty
o Lateral canthoplasty
o Dermis fat grafts
These are used to correct atrophic blepharochalasis after the syndrome has run its course.
· (H02.4) Ptosis
Ptosis (pron.: /ˈtoʊsɪs/) (from Greek Ptosis or πτῶσις, to “fall”) is a drooping or falling of the upper or lower eyelid. The drooping may be worse after being awake longer, when the individual’s muscles are tired. This condition is sometimes called “lazy eye”, but that term normally refers to strabismus. If severe enough and left untreated, the drooping eyelid can cause other conditions, such as amblyopia or astigmatism. This is why it is especially important for this disorder to be treated in children at a young age, before it can interfere with vision development.
Treatment
Aponeurotic and congenital ptosis may require surgical correction if severe enough to interfere with vision or if cosmetics is a concern. Treatment depends on the type of ptosis and is usually performed by an ophthalmic plastic and reconstructive surgeon, specializing in diseases and problems of the eyelid.
Surgical procedures include:
o Levator resection
o Mьller muscle resection
o Frontalis sling operation
Non-surgical modalities like the use of “crutch” glasses or special Scleral contact lenses to support the eyelid may also be used.
Ptosis that is caused by a disease will improve if the disease is treated successfully.
· (H02.6) Xanthelasma of eyelid
Xanthelasma (or xanthelasma palpebrarum) is a sharply demarcated yellowish deposit of fat underneath the skin, usually on or around the eyelids. While they are neither harmful nor painful, these minor growths may be disfiguring and can be removed. They are common in people of Asian origin and those from the Mediterranean region.
Because of the hereditary component, they may or may not indicate high blood levels of cholesterol. Where there is no family history of xanthelasmata, they usually indicate high cholesterol and may correlate with a risk of atheromatous disease.
A xanthelasma may instead be referred to as a xanthoma when becoming larger and nodular, assuming tumorous proportions. Still, xanthelasma is often classified simply as a subtype of xanthoma.
Treatment
Xanthelasmata can be removed with a trichloroacetic acid peel, surgery, lasers or cryotherapy. Removal can cause scarring and pigment changes, but it is unusual after treatment with trichloroacetic acid.
· (H03.0*) Parasitic infestation of eyelid in diseases classified elsewhere
– Dermatitis of eyelid due to Demodex species ( B88.0+ )
– Parasitic infestation of eyelid in:
· leishmaniasis ( B55.-+ )
· loiasis ( B74.3+ )
· onchocerciasis ( B73+ )
· phthiriasis ( B85.3+ )
· (H03.1*) Involvement of eyelid in other infectious diseases classified elsewhere
Involvement of eyelid in:
1. herpesviral (herpes simplex) infection ( B00.5+ )
2. leprosy ( A30.-+ )
3. molluscum contagiosum ( B08.1+ )
4. tuberculosis ( A18.4+ )
5. yaws ( A66.-+ )
6. zoster ( B02.3+ )
o (H03.8*) Involvement of eyelid in other diseases classified elsewhere
o Involvement of eyelid in impetigo ( L01.0+ )
o (H04.0) Dacryoadenitis
o (H04.2) Epiphora
o (H06.2*) Dysthyroid exophthalmos it is shown that if your eye comes out that it will shrink because the optic fluids drain out
VIDEO
Sense Organs Disorders Glossary
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aural discharge |
discharge from the ear |
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otitis |
inflammation of the ear |
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subcutaneous |
relating to or located below the epidermis |
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epidermis |
outer layer of skin (can be divided into four strata) |
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dermatology |
the study of the skin |
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myringotomy |
surgical incision into the eardrum (to relieve pressure or release pus from the middle ear) |
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tympanoplasty surgical repair of the middle ear ocular |
of or relating to the eye (or sense of sight) |
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ophthalmoscope |
medical instrument for examining the retina of the eye |
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aphakia |
absence of the natural lens of the eye (usually resulting from the removal of cataracts) |
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pilosebaceous
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pertaining to hair follicles and sebaceous glands. |
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retinopathy |
disease of the retina |
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trichophagia |
compulsive eating of hair |
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subungual |
pertaining to under the nail |
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onycholysis |
Nail disease (Nail conditions that show signs of infection or inflammation) |
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Acoustic Neurom |
Benign tumor arising from the acoustic vestibulocochlear nerve in the brain. Initially, the tumor causes tinnitus, vertigo, and decreased hearing. |
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Acoustic |
Pertaining to sound or to the sense of hearing. |
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Acute Otitis Media |
Infection of the middle ear, often following an upper respiratory infection. |
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Amblyopia |
Strabismus in children that results in the partial loss of vision or lazy eye. It is reversible until the retina is fully developed at about 7 years of age. |
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Anisocoria |
Inequality of the size of the pupils. May be congenital or associated with aneurysms, head trauma, nervous system disease, brain lesions, or paresis. |
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Astigmatism
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Absence of the crystalline lens of the eye. SYN: Aphakia. |
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Aphakia |
Defective curvature of the cornea or lens of the eye. This causes light rays to be uneven and not sharply focused on the retina, so that the image is distorted.
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Audiogram
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Record of audiometer results. |
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Audiometer
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Electrical device that delivers acoustic stimuli of specific frequencies to determine a patient’s hearing loss for each frequency. |
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Audiometry |
Testing the sense of hearting. |
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Blepharitis
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Inflammation of the eyelid, causing redness, crusting, and swelling oalong lid margins. |
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Blepharoptosis
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Drooping of the upper eyelid. Also called ptosis. |
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Cararact |
Degenerative eye disease which causes clouding of the lens that decreases vision. |
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Chalazion |
Small, hard, cystic mass (granuloma) on the eyelid which is formed as a result of chronic inflammation of a sebaceous gland (meibomian gland) along the margin of the eyelid. |
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Cholesteatoma |
Collection of skin cells and cholesterol in a sac within the middle ear. Usually the result of chronic otitis media and are associated with perforations of the tympanic membrane. |
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Chorioretinitis |
Inflammation of the choroids and retina, often caused by infection. SYN: Choroidoretinitis. |
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Choroidoretinitis
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Inflammation of the choroids and retina, often caused by infection. SYN: Chorioretinitis. |
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Cochlear Implant |
Surgically implanted device allowing sensorinerual hearing-impared person to understand speech. Electrical signals are sent directly into the auditory nerve. |
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Conductive Deafness |
Results from impairment of the middle ear ossicles and membranes transmitting sound waves into the cochlea. |
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Conjunctivitis |
Inflammation of the conjuctiva. |
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Cycloplegia |
Paralysis of the ciliary muscle. |
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Dacryoadenitis
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Inflammation of the lacrimal gland. |
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Dacryocystitis |
Blockage, inflammation, and infection of a nasolacrimal duct and lacrimal sac, causing redness and swelling of the lower eyelid. |
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Diabetic Retinopathy
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Retinal effects of diabetes mellitus. Includes microaneurysms, hemorrhages, dilation of retinal veins, and neovascularization (new blood vessels form in the retina). Macular edema occurs as fluid lease from blood vessels into the retina and vision is blurred. Exudates appear in the retina as yellowish-white spots. |
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Diplopia |
Double vision. |
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Dry Macular Degeneration |
Form of macular degeneration characterized by atrophy and degeneration of retinal cells, and deposits of clumps of extracellular debris (drusen). There is no treatment for this form. |
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Ear Thermometry |
Measurement of the temperature of the tympanic membrane by detection of infrared radiation from the eardrum. |
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Ectropion |
Outward sagging and eversion of the eyelid, leading to improper lacrimatio and corneal drying and ulceration. |
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Entropion |
Inversion of the eyelid, causing the lashes to rub against the eye, that may lead to corneal abrasion. |
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Enucleation |
Removal of the entire eyeball. |
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Esotropia |
Marked turning inward of the eye. Also known as cross-eyed. |
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Exotropia |
Abnormal turning outward of one or both eyes. |
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Floaters |
Seen as block spots, are usually composed of vitreous clumps that detach from the retina. |
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Fluorescein Angiography |
Intravenous injection of fluorescein, followed by a series of photographs of the retina through the dilated pupil. |
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Glaucoma |
Increased intraocular pressure results in damage to the retina and optic nerve, with loss of vision. The pressure is increased because of the inability of aqueous humor to drain from the eye. |
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Hemianopsia
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Absence of vision in half of the visual field.
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Hordeolum
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Localized, purulent, inflammatory staphylococcal infection of a sebaceous gland in the eyeball. Also known as a stye. |
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Hyperacusis |
Abnormal acute sensitivity to sounds. |
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Hypermetropia |
Farsightedness (sees clearly objects that are far away). Parallel rays of light tend to “focus” behind the retina, which results in a blurred image. SYN: Hyperopia. |
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Hyperopia |
Farsightedness (sees clearly objects that are far away). Parallel rays of light tend to “focus” behind the retina, which results in a blurred image. SYN: Hypermetropia. |
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Hypertensive Retinopathy
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Retinopathy associated with hypertension, toxemia of pregnancy, or glomerulonephritis. |
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Hypertropia |
Upward deviation of one eye. |
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Iridectomy |
Portion of the iris is removed to improve drainage of aqueous numor or to extract a foreign body. |
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Iridic
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Relating to the iris. |
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Iritis
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Inflammation of the iris. Characterized by pain, sensitivity to light, and lacrimation. |
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Keratitis |
Inflammation and ulceration of the cornea. |
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Keratoplasty |
Surgical repair of the cornea. Also known as corneal transplant. |
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Lacrimation |
Secretion and discharge of tears. |
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Laser In Situ Keratomileus (LASIK)
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Use of an excimer laser to correct errors of refraction (myopia, hyperopia, and astigmatism). |
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Laser Photocoagulation |
Intense, precisely focused light beam creates an inflammatory reaction that seals retinal tears and leaky retinal blood vessels. |
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Macrotia |
Abnormally large ears. |
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Macular Degeneration |
Progressive damage to the macula of the retina. |
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Mastoiditis |
Inflammation of the mastoid sinuses, usually as a result of infection spread from acute otitis media. |
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Meniere Disease |
Disorder of the labyrinth of the inner ear with elevated endolymph pressure within the cochlea and semicircular canals. |
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Microtia |
Unusually small external ear. |
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Miosis |
Abnormal contraction of the pupils, possibly due to irritation of the occulomotor system or paralysis of dilators. |
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Miotic |
Drug which contracts the pupil of the eye. |
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Mydriasis |
Enlargement of pupils. |
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Myopia |
Nearsightedness (sees clearly at short distances). Light rays are “focused” in front of the retina. |
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Myringitis |
Inflammation of the tympanic membrane. |
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Myringotomy |
Incision of the tympanic membrane. Most often performed on children with acute otitis media. |
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Nerve Deafness |
Results from impairment of the cochlea or auditory nerve. SYN: Sensorinerual Hearing Loss. |
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Nyctalopia |
Inability to see well in low light or at night. Occurs in retinitis pigmentosa and choroidoretinitis, or with vitamin A deficiency. |
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Nystagmus |
Repetitive rhythmic movements of one or both eyes. |
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Open-Angle Glaucoma |
The most common type of glaucoma. There are open change angles, but resistance to the flow of aqueous humor. |
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Ophthalmic |
Pertaining to the eye. |
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Ophthalmoplegia |
Paralysis of ocular muscles. |
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Ophthalmoscopy |
Visual examination of the interior of the eye. |
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Otitis Media |
Inflammation of the middle ear. |
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Otolaryngology |
Division of medical science that includes otology, rhinology, and laryngology. |
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Back Otomycosis
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Infection of the external auditory meatus of the ear caused by a fungus. SYN: Myringomycosis and Otitis Mycotica. |
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Otorrhea |
Inflammation of the ear with purulent discharge. |
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Otosclerosis |
Hardening of the bony tissue of the middle ear. The result of this hereditary condition in that bone forms around the oval window and causes fixation or ankylosis of the stapes bone (ossicle). |
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Otoscopy
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Visual examination of the ear with an otoscope. |
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Palpebral |
Concerning the eyelid. |
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Papilledema |
Edema and inflammation of the optic nerve at its point of entrance into the retina. Caused by increased intracranial pressure, often due to a tumor of the brain pressing on the optic nerve. SYN: Papillitis. |
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Papillitis
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Edema and inflammation of the optic nerve at its point of entrance into the retina. Caused by increased intracranial pressure, often due to a tumor of the brain pressing on the optic nerve. SYN: Papilledema. |
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Phacoemulsification |
Ultrasonic vibrations break up the lens, which then is aspirated through the ultrasonic probe. Used for cataract removal. |
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Photocoagulation |
Pinpoint burns which form scar tissue and seal holes in small retinal tears. |
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Photophobia |
Sensitivity to light. |
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Photopsia |
Bright flashes of light. |
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Pneumatic Retinopexy |
Used for selected retinal detachments, a gas bubble is injected into the vitreous cavity to put pressure on the area of retinal tear until the retina is reattached. |
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Prebyacusia |
Progressive hearing loss with aging. SYN: Presbyopia. |
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Prebycusis |
Progressive hearing loss with aging. SYN: Prebyacusia. |
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Presbyopia |
Impairment of vision as a result of old age. As a person ages, there is a loss of elasticity of the ciliary body, which impairs its abilty to adjust the lens for accommodation to near vision. The light rays “focus” behind the retina, as in hyperopia. |
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Ptosis |
Drooping of upper lid margin as a result of neuromuscular problems. |
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Pupillary |
Concerning the pupil. Retinal Detachment Two layers of the retina separate from each other. |
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Retinitis Pigmentosa |
Group of hereditary degenerative retinal diseases marked by defective night vision followed by a progressive loss of the field of vision. |
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Retinopathy |
Any disorder of the retina. |
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Rinne Test
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Tuning fork test where the examiner places the base of the vibrating fork against the patient’s mastoid bone and in front of the auditory meatus. |
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Salpingopharyngeal |
Concerning the eustachian tube and the pharynx. |
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Scleral Buckle |
Suture of a silicone band to the sclera over a detached portion of the retina. The band pushed the two parts of the retina against each other to bring together the two layers of the detached retina. |
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Scleritis |
Inflammation of the sclera. SYN: Sclerotitis. |
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Scotoma
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An island-like blind spot in the visual field. Can be caused by damage to the retina or optic nerve. |
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Serous Otitis Media |
Noninfectious inflammation with accumulation of serous fluid. Ofter results from a dysfunctional or obstructed auditory tube. |
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Slit Lamp Microscopy |
Microscopic examination of anterior ocular structures. |
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Stapedectomy |
Excision of the stapes to improve hearing, especially in cases of otosclerosis. |
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Strabismus |
Abnormal deviation of the eye. |
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Tinnitus |
Sensation of noises (ringing, buzzing, etc.) in the ears. Caused by irritation of the delicate hair cells in the inner ear. |
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Tonometry
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Measures the intraocular pressure to detect glaucoma. |
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Tuning Fork Test |
Test of ear conduction using a vibration source (tuning fork). |
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Tympanoplasty
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Any one of several surgical procedures designed either to cure a chronic inflammatory process in the middle ear or to restore function to the sound-transmitting mechanism of the middle ear. |
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Uveitis |
Any intraocular inflammatory disorder. |
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Vertigo
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Sensation of irregular or whirling motion either of oneself or of external objects. Can result from disease in the labyrinth of the inner ear or in the nerver that carries messages fro the semicircular canals to the brain. |
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Visual Acuity Test |
Clarity of vision is assessed. The patient reads from a Snellen chart |
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Visual Field Test |
Measures the area within which objects are seen with the eyes are fixed, looking straight ahead, without movement of the head. |
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Vitrectomy |
Removal of the vitreous humor. |
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Webber Test |
Tuning fork test where the tuning fork is placed on the center of the forehead. If the loudness of sound is equal in both ears, the hearing is normal. |
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Wet Macular Degeneration |
Form of macular degeneration which results from the formation of new (neovascular) and leaky (exudative) blood vessels close to the macula.
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REFERENCES:
- Atema, Jelle (1980) “Chemical senses, chemical signals, and feeding behavior in fishes” p. 57–101. In: Bardach, JE Fish behavior and its use in the capture and culture of fishes’, The WorldFish Center, ISBN 978-971-02-0003-0.
- Brain Areas Critical To Human Time Sense Identified”. UniSci –
Daily University - Halder, G.; Callaerts, P.; Gehring, W.J. (1995). “New perspectives on eye evolution”. Curr. Opin. Genet. Dev. 5 (5): 602–609. doi:10.1016/0959-437X(95)80029-8. PMID 8664548.
- Functional MR Imaging of Regional Brain Activation Associated with the Affective Experience of Pain – Robert K. Fulbright et al., American Journal of Roentgenology, 2001; vol. 177, pages 1205–121.
