Anatomic and Topographic Peculiarities of the Visual Organ. Visual functions. Methods of investigation in ophthalmology. Refraction and accommodation. Strabismus.

I. Anatomy and Topography of the Visual Organ.

Visual organ consists from four parts:

1)    peripheral part – eyeball with ocular adnexa; 

2)    guiding pathway – optic nerve, chiasm, optic tract; 

3)    undercortex centers – lateral geniculare nucleus and optic radiation; 

4)    higher visual centers in the occipital cortex.

The eyes lie within two bony cavities, or orbits, of the skull located on each side of the root of the nose. The medial walls of the orbits border the nasal cavity anteriorly, the ethmoid air cells and the sphenoid sinus posteriorly. The lateral walls border the middle cranial, the temporal, and the pterygopalatine fossae. Superior to the orbit are the anterior cranial fossa and the frontal and supraorbital sinuses. The maxillary sinus and the palatine air cells are located inferiorly. The periorbital or nasal sinuses may offer a route for spread of infection. Mucoceles arising from the sinus are common and occasionally may confuse the clinician in the differential diagnosis of orbital tumors. Seven bones make up the bony orbit: frontal, zygoma, maxilla, ethmoid, sphenoid, lacrimal and palatine. The orbit has got: the optic foramen, the supraorbital foramen, the anterior ethmoidal foramen, the posterior ethmoidal foramen, the zygomatic foramen, the nasolacrimal duct, the infraorbital canal.

The superior orbital fissure is located between the greater and the lesser wings of the sphenoid bone. The cranial nerves III, IV, VI, the first branch of cranial nerve V, the superior ophthalmic vein, and the sympathetic nerve plexus pass through it. Therefore the symptomes of the superior orbital fissure syndrome are the following: exophthalmos, ptosis, ophthalmoplegy, mydriasis, paralysis of accommodation, and lowering of corneal and eyelid’s skin sensitivity. The inferior orbital fissure lies just below the superior orbital fissure between the lateral wall and the floor of the orbit. It transmits the maxillary and pterygoid parts of cranial nerve V, and a nerve arising from the pterygopalatine ganglion. The inferior ophthalmic vein passes through its lower portion before entering the cavernous sinus.

Tenon’s capsule divides orbit for two sections. Anteriorly, the eyeball and endings of muscles are situated. Posteriorly, optic nerve, muscles, vascular-nervous formations and adipose tissue are located.

The lacrimal gland is located in a shallow depression within the orbital portion of the frontal bone. The gland is separated from the orbit with fibroadipose tissue and is divided in two parts by a lateral expansion of the levator aponeurosis. The smaller, palpebral gland can be seen in the superolateral conjunctival fornix when the upper lid is everted. An isthmus of gladular tissue occasionally exists between the palbebral lobe and the main gland within the orbit. A variable number of thin-walled excretory ducts, blood vessels, lymphatics, and nerves pass from the orbital into the palbebral lacrimal gland. The ducts continue downward and empty into the conjunctival fornix approximately 5 mm above the superior margin of the upper tarsus.

The accessory lacrimal glands of Krause and Wolfring located in the eyelids are cytologigally identical to the main lacrimal gland, but much smaller. They appear to be under sympathetic control and furnish basal tear secretion.

The lacrimal drainage system includes the upper and lower canaliculi, the tear sac, and the nasolacrimal duct. The lacrimal papillae are located on the posterior edge of the lid margins. Each tiny opening, or lacrimal punctum is 0,3 mm in diameter. These openings lead to the lacrimal canaliculi and through a circutous route to the lacrimal sac and finally through the nasolacrimal duct to the nose.

There are six extraocular muscles: four rectus and two oblique. Five of them originate from annulus of Zinn. And only the inferior oblique muscle does not originate from the orbital apex; it arises from a shallow depression in the orbital plate of the maxilla at the anteromedial corner of the orbital floor near the lacrimal fossa.

The four rectus muscles insert on the anterior portion of the globe in a configuration called the spiral of Tillaux. The medial rectus muscle inserts nearest (5.0-5.5 mm) to the limbus, and the superior rectus muscle inserts furthest from the limbus (7.0-7.5 mm). The oblique muscles insert behind the equator.

The blood supply for the exraocular muscles is derived from the inferior and superior muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery. Exept for the lareral rectus muscle, each rectus muscle receives two anterior ciliary arteries that communicate with the major arteriole circle of the ciliary body. The lareral rectus muscle is supplied by a single vessel derived from the lacrimal artery.

The medial rectus muscle moves the eye inside and the lareral rectus muscle moves the eye outside. The superior rectus muscle moves the eye up and inside, the inferior rectus muscle moves the eye down and inside. The superior oblique muscle moves the eye down and outside, the inferior oblique muscle moves the eye up and outside.

Cranial nerve IV (trochlear) supplies the superior oblique muscle. Cranial nerve VI (abducens) supplies the lareral rectus muscle. Cranial nerve III (oculomotor) innervates the others exraocular muscles.

The interpalpebral fissure is the exposed zone between the upper and the lower eyelid. Normally, the adult fissure is 27-30 mm long and 8-11 mm wide. The upper eyelid is more mobile than the lower. Clinically important changes in the lid tissue occur in thyroid disease, Horner’s syndrome, facial palsy, and third nerve palsy.

The skin of the eyelids is the thinnest in the body. It contains the usual adnexal structures: the fine hairs, sebaceous glands, and sweat glands.

The lid margin contains several important landmarks. A small opening, or punctum, from the lacrimal canalicuulus exits at the summit of each lacrimal papilla. Along the entire length of the free margin of the lid is a delicare gray (pigmented) line, the so-called gray line or the intermargin sulcus. The eyelashes or cilia arise anterior to this line. Posterior to the line are the openings of the tarsal or meibomiam glands. The mucocutaneous border occurs at the level of the orifices of the tarsal glands. The gray line corresponds hystologically to the most superficial portion of the orbicularis muscle, known as the muscle of Riolan.

FIGURE Eyelid margin anatomy

Subcutaneous connective tissue of the lid contains no fat. Because of its arrangement, fluid from edema or hemorrhage can accumulate beneath the skin and results in rapid and dramatic swellung of the lids.

The orbicularis oculi muscle can be subdivided into orbital and palbebral parts. The orbital portion acts like a sphincter and functions solely as a voluntary muscle. The palbebral portion of the orbicularis functions both voluntarily and involuntarily for normal and reflex blinking. The orbicularis oculi muscle is innervated by cranial nerve VII (facial).

The levator palpebrae superioris muscle originates at the apex of the orbit. The anterior fibers pass through the orbicularis muscle and insert subcuneously. The medium portion attached to the tarsus and the posterior one – to the conjunctiva in the upper fornix. The levator muscle is innervated by cranial nerve III and elevates the upper lid. The orbicularis oculi muscle is its antagonist. Muller’s muscle originates from the undersurface of the levator muscle in the upper lid and from the capsulopalpebral head of the inferior rectus in the lower lid. It attaches to the upper border of the upper tarsus and to the lower body of the lower tarsus. This smooth (nonstriated). Sympathetically innervated muscle gives rise to important clinical signs in dysthyroid ophthalmopathy and Horner’s syndrome.

The tarsal plates consist of dense connective tissue, not cartilage. They are attached to the orbital margin by the medial and lateral palbebral ligaments. The tarsal (meibomian) glands are modified holocrine sweat glands that are oriented vertically in parallel rows through the tarsus. Their oily secretion passes through small orifices into the tear film at the mucocutaneous border of the upper and lower eyelids. The hair bulbs of the cilia are located anterior to the tarsus and the meibomin gland orifices.

The palpebral conjunctiva is a transparent vascularized membrane covered by a nonkeratinized epithelium that lines the inner surface of the eyelids. Continuous with the conjunctival fornices, it merges with the bulbar conjunctiva before terminating at the limbus.

The blood supply to the eyelids is derived from the facial system, which arises from the external carotid artery, and the orbital system, which originates from the internal carotid artery via branches of the ophthalmic artery. The venous drainage of the lids can also be divided into two portions, a superficial or pretarsal system that drains into the internal and external jugular veins, and a deep or posttarsal system that eventually flows into the cavernous sinus.

The conjunctiva can be divided into three geographic zone: palbebral, fornical and bulbar. The conjunctiva is a mucous membrane consisting of nonkeratizing squamous epirhelium and goblet cell on its surface and a thin, richy vascularized substantia propria. Anterior ciliary arteries provide its blood supply. Its innervation is derived from the opthalmic division of cranial nerve V. The normal conjunctiva is pale pink, smooth, transparent, moist.

The structural wall of the eyeball includes transparent cornea and opaque sclera.

The cornea occupies the center of the anterior pole of the globe. In the adult, it measures about12 mm in the horizontal meridian and about 11 mm in the vertical meridian. The cornea and the agueous humor together form a positive lens of about 43 diopters in air and therefore constitute the main refractive elements of the eye. The central third of the cornea is nearly spherical and measures about 4 mm in the diameter in the normal eye. Because the posterior surface of the cornea is more curved than the anterior surface, the central cornea is thinner (0,5 mm) than the peripheral cornea (1,0 mm). Epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium are hystologic layers of the cornea. The normal cornea is transparent, glassy, spherical, high sensitive, without vessels.

The limbus is a transition zone located between the peripheral cornea and the anterioa sclera. It is gray and tranclucent.

The sclera covers the posretior four fifth of the surface of the globe. Two potential openings exist in the sclera, one anteriorly for the cornea and the other posteriorly for the optic nerve. The sclera is thinnest (0,3 mm) at the rectus musclees and thickest (1,0 mm) at the posterior pole. The sclera, like the cornea, is essentially avascular except for the suprficial vessels of the episclera and the intrascleral vascular plexus located just posterior to the limbus. There are large numbers of channels, or emissaria, in the sclera for the passage of arteries, veins, and nerves.the opaque, porcelain whire appearance of the sclera contrasts markedly with transparent cornea and is due primarily to the sclera’s greater water content and the less uniform orientation of its collagen fibers.

The uveal tract, the vascular, middle compartment of the eye, consists of three parts: the iris and ciliary body located in the anterior uvea; and the choroid located in the posterior uvea.

The iris is made up of blood vessels and connective tissue, in addition to the melanocytes and pigment cells that are responcibile for its distinctive color. The iris must be mobile to change pupillary size. The iris diafragm subdivides the anterior segment into the anterior and posterior chambers. The ophthalmic division of cranial nerve V innervates the dilatator muscle. The postganglionic fibers travel with the short ciliary nerves to the iris sphinmcter.

The ciliary body has two principal functions: aqueous humor formation and accomodation. In addition, it may pay role in the uveoscleral outflow of aqueous humor. The ciliary body is 6-7 mm wide and consists of two parts: the pars plana and the pars plicata. Classic descriptions of the ciliary muscle suggest that it has three layers: longitudinal, radial, and circular fibers. The ciliary muscles behave like other smooth, nonstriated muscle fibers. The main innervation is derived from parasympathetic fibers of cranial nerve III. Sympathetic fibers have also been observed.

The choroid, the posterior portion of the uveal tract, the posterior portion of the uveal tract, nourisches the outer portion of the retina. Perfusion of the choroid comes from both the long and short posterior ciliary arteries and from recurent branches of the anterior ciliary arteries. The venous blood drains through the vortex system.

The retina is a thin, transparent structure that differentiates from the inner and outer layers of the optic cup. It is a peripheral receptor of visual analisator. The retina adheres to the choroid at the ora serata and optic disc. Posterior two thirds of retina form the optic part. It consists from ten layers: retinal pigment epithelium, photoreceptors layer (rods and cones), external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibers layer, internal limiting membrane. The blind part of retina from ora serrata till pupil has only two layers. In fovea centralis only cones are located. The function of cones ic central vision (visual acuity and colour vision), while the function of rods is peripheral vision (dark adaptation and field of vision).

The anterior chamber of the eye is bordered anteriorly by the cornea and posteriorly by the iris diafragm and the pupil. It is filled with aqueous humor, which is prodused by the ciliary epithelium in the posterior chamber. The fluid passes through the pupillary aperture and drains chiefly through the trabecular neshwork into the canal of Schlemm.

The posterior chamber of eye lies between iris anterirly, ciliary body externaly, lens internarly and vitreous posteriorly.

The lens is a biconvex structure located in the posterior chamber durectly behind the pupil. Cnanges in lens thickness occur during accomodation. The lens lacks innervation. After regression of the hyaloid vascular system during fetal life, it depends totally on the aqueous and vitreous for lens nourishment. Lens continues to grow throughout life and is entirely enclised from embryo stage onward by a basement membrane.

The vitreous cavity occupies four fifths of the volume of the globe. The transperarent vitreous humor plays an important role in the metabolism of the intraocular tissues because it provides a passageway for metabolism utilized by the lens, the ciliary body, and the retina. Its volume is close to 4,0 ml. The vitreous adheres to the retina peripherally at the vitreous base, which extends anterior to the ora serrata. Additional attachments exist at the disc margin, in the perifoveal region, and onto the posterior lens capsule.

Optic nerve (cranial nerve II) is formed by axons of the retinal ganglion cells. It can be divided into four portions: intraocular (nerve head), intraorbital, intracanalicular and intracranial. Its length is 45 mm. The optic nerve varies in length from 35-55 mm, but averages 40 mm.

Each optic tract begins from chiasm and contains ipsilateral temporal and contralateral nasal fibers from the optic nerves. The lateral geniculate nucleous or body is the synaptic zone for the higher visual projections. The optic radiation connects the lateral geniculate body with the cortex of the occipital lobe. The visual cortex, the thinnest area of the human cerebral cortex, has six cellular layers and occupies the superior and inferior lips of the calcarine fissure on the posterior and medial surfaces of the occipital lobes.

The fundus examination is called ophthalmoscopy. Important landmarks to be noted during ophthalmoscopy routinely are: optic disc and its cup, branches of central retinal artery and vein (superior temporal, superior nasal, inferior temporal, inferior nasal), macula (fovea) and its central depression (foveola). The degree of pigmentation observed in ocular fundus is dependent primarily on the number of pigmented melanocytes in the choroid.

The vitreous humor develops pockets of liquefied vitreous in the previously ho­mogenous gel. This vitreous syneresis predisposes to a separation of the vitreous from its attachments to the retina and optic disc, called a posterior vitreous detachment (PVD). Since the vitreous is normally adherent to the retina, a PVD (Figure 1.5) in turn can predispose the patient to retinal traction, tears, and detachment.

Arteriosclerotic changes predispose the patient to vasculopathic cranial third, fourth, and sixth nerve palsies; retinal artery and vein occlusions; and anterior ischemic optic neuropathy.

The aging eye functions differently as well. Subjective testing in patients aged 50 or older sometimes reveals a loss of visual acuity, contrast sensitivity, and visual fields. Vertical smooth-pursuit eye movements and simultaneous vertical eye-head tracking decrease. Many older patients have trouble looking up, as well as moving the head up while simultaneously looking up with the eyes. Aging delays regenera­tion of rhodopsin, slows rod-mediated dark adaptation, and may lead to relative difficulty with night vision.

Aging does not condemn the elderly to a loss of functional vision, however. According to the Framingham Heart Study, an ongoing investigation of cardiovas­cular disease, acuity of 20/25 or better was maintained in at least 1 eye in 98% of patients ages 52 to 64, 92% ages 65 to 74, and 70% ages 75 to 85. The Framingham study found that subjective changes with age include dryness, grittiness, fatigue, burning, glare, floaters, and flashes; in addition, older patients had an increased risk of falls.

The study also noted other changes: a loss of corneal endothelial cells, yellowing and opacification of the lens, a smaller and less reactive pupil, and condensation of the vitreous gel. Retinal traction and tears were noted, as well as age-related changes in the retinal vasculature, and fewer neural cells in the retina and visual cortex.

II. Refraction and accommodation

The cornea and the lens are the refractive structures of the eye. The cornea provides approximately two-thirds of the refractive power of the eye, and the lens approximately one-third, to form an image on the retina. Reduced visual acuity will result if the axial length of the eye is either too short (hyperopia; also called hypermetropia) or too long (myopia) for the refracting power of the cornea and lens. Visual acuity also is reduced if the refracting power of the cornea and lens is different in one meridian than in another (astigmatism). These optical defects can be corrected by the use of spectacles, contact lenses, or, in selected cases, refractive surgery. A pinhole placed directly in front of the eye will narrow the effective pupillary aperture and thereby minimize the blurring induced by a refractive error. Use of a pinhole device allows an examiner to estimate what a patient’s visual potential would be with proper spectacle correction.

The ability of the ciliary muscle to contract and the lens to become more convex is called accommodation. With increasing age, the lens of every eye undergoes progressive hardening, with loss of ability to change its shape. Loss of accommodation is manifested by a decreased ability to focus oear objects (ie, presbyopia), while corrected distance visual acuity remains normal. Presbyopia develops progressively with age but becomes clinically manifest in the early to mid 40s, when the ability to accommodate at reading distance (35 to 40 cm) is lost. Presbyopia is corrected by spectacles, either as reading glasses or as the lower segment of bifocal glasses, the uppet segment of which can contain a correction for distance visual acuity if needed. Some myopic patients with presbyopia simply remove their distance glasses to read, because they do not need to accommodate in an uncorrected state.

Visual acuity

Visual acuity is a measurement of the smallest object a person can identify at a given distance from the eye. The following are common abbreviations used in recording visual acuity:

·          VA (visual acuity)

·          OD (oculus dexter): right eye

·          OS (oculus sinister): left eye

·          OU (oculus uterque): both eyes

III. Examination

All patients should have an eye examination as part of a general physical examina­tion by the primary care physician. Visual acuity, pupillary reactions, extraocular movements, and direct ophthalmoscopy through undilated pupils constitute a min­imal examination. Pupillary dilation for ophthalmoscopy is required in cases of un­explained visual loss or when fundus pathology is suspected (eg, diabetes mellitus). Distance visual acuity measurement should be performed in all children as soon as possible after age 3 because it is a more sensitive test for amblyopia than those used in the recommended younger childhood eye evaluations. The tumbling E chart or another age-appropriate testing method is used in place of the standard Snellen eye chart.

Depending on what the examination reveals and on the patient’s history, addi­tional tests may be indicated:

·        Tonometry may be performed if acute narrow-angle glaucoma is suspected. The diagnosis of open-angle glaucoma requires more complex testing than simple tonometry.

·        Anterior chamber depth assessment is indicated whearrow-angle glaucoma is suspected and prior to pupillary dilation.

·        Confrontation visual field testing is used to confirm a suspected visual field defect suggested by the patient’s history or symptoms; also used to document normal visual field.

·        Color vision testing may be part of an eye examination when requested by the patient or another agency, in patients with retinal or optic nerve disorders, and in patients taking certain medications.

·        Fluorescein staining of the cornea is necessary when a corneal epithelial defect (abrasion) or other abnormality is suspected.

·        Upper lid eversion is necessary when the presence of a foreign body is suspected.

Details on how to perform both basic and adjunctive ocular tests appear in the next section.

Equipment

Equipment for a basic eye examination consists of a few items that can be trans-ported, if necessary, with other medical instruments (Figure 1.6). The slit-lamp biomicroscope is a stationary office instrument that augments the inspection of the anterior segment of the eye by providing an illuminated, magnified view. Standard equipment in an ophthalmologist’s office, the slit lamp is also available in many emergency facilities.

Distance visual acuity testing

Distance visual acuity is usually recorded as a ratio or fraction comparing patient performance with an agreed-upon standard. In this notation, the numerator rep­resents the distance between the patient and the eye chart (usually the Snellen eye chart, Figure 1.7). The denominator represents the distance at which a person with normal acuity can read the letters. Visual acuity of 20/80 thus indicates that the patient can recognize at 20 feet a symbol that a person with normal acuity can recognize at 80 feet.

Visual acuity of 20/20 represents statistically normal visual acuity. However, many “normal” individuals actually see better than 20/20—for example, 20/15 or even 20/10. If this is the case, you should record it as such. Alternative meth­ods for recording visual acuity are decimal notation (eg, 20/20 = 1.0; 20/40 = 0.5; 20/200 = 0.1) and metric notation (eg, 20/20 = 6/6, 20/100 = 6/30).

Visual acuity is tested most often at a distance of 20 feet, or 6 meters. Greater distances are cumbersome and impractical; at shorter distances, variations in the test distance assume greater proportional significance. For practical purposes, a dis­tance of 20 feet may be equated with optical infinity.

FIGURE 1.7 Snellen eye chart

To test distance visual acuity with the conventional Snellen eye chart, follow these steps:

1.    Place the patient at the designated distance, usually 20 feet (6 meters), from a well-illuminated Snellen chart (see Figure 1.7). If glasses are normally worn for distance vision, the patient should wear them.

2.    By convention, the right eye is tested and recorded first. Completely occlude the left eye using an opaque occluder or the palm of your hand; alternatively, have the patient cover the eye.

3.    Ask the patient to read the smallest line in which he or she can distinguish more than one-half of the letters. (If the tumbling E chart is being used, have the patient designate the direction in which the strokes of the E point.)

4.    Record the acuity measurement as a notation (eg, 20/20) in which the numerator represents the distance at which the test is performed, and the denominator represents the numeric designation for the line read.

5.    Repeat the procedure for the other eye.

6.    If visual acuity is 20/40 or less in one or both eyes, repeat the test with the patient viewing the test chart through a pinhole occluder and record these results. The pinhole occluder may be used over the patient’s glasses.

If a patient cannot see the largest Snellen letters, proceed as follows:

1.    Reduce the distance between the patient and the chart. Record the new distance as the numerator of the acuity designation (eg, 5/70 if the patient is 5 feet from the chart and is only able to see the 20/70 size letters).

2.    If the patient is unable to see the largest Snellen letter at 3 feet, hold up1 hand, extend 2 or more fingers, and ask the patient to count the number of fingers. Record the distance at which counting fingers is done accurately (eg, CF 1 ft).

3.    If the patient cannot count fingers, determine whether or not he or she can detect the movement of your hand. Record a positive response as hand motion (eg, HM 2 ft).

4.    If the patient cannot detect hand motion, use a penlight to determine whether he or she can detect the direction or the perception of light. Record the patient’s response as LP with projection (light perception with direction), LP (light perception), or NLP (no light perception).

Visual Impairment Versus Visual Disability

The term visual acuity impairment (or simply visual impairment) is used to describe a condition of the eyes. Visual disability describes a condition of the individual. The disabling effect of impairment depends in part on the individual’s ability to adapt and to compensate. Two individuals with the same visual impairment measured on a Snellen eye chart may show very different levels of functional disability. Table 1.2 summarizes the differences between visual impairment and visual disability.

Near visual acuity testing

Near visual acuity testing may be performed if the patient has a complaint about near vision. Otherwise, testing “at near” is usually performed only if distance testing is difficult or impossible—at the patient’s bedside, for instance. In such situations, testing with a near card may be the only feasible way to determine visual acuity.

If the patient normally wears glasses for reading, he or she should wear them dur­ing testing. This holds true for the presbyopic patient in particular. The patient holds the test card—for example, a Rosenbaum Pocket Vision Screener (Figure 1.8)—at the distance specified on the card. This distance is usually 14 inches or 35 centimeters. While the examiner occludes 1 of the patient’s eyes, the patient reads the smallest characters legible on the card. The test is then repeated for the other eye. Letter size designations and test distances vary. To avoid ambiguity, both should be recorded (eg, J 5 at 14 in, 6 point at 40 cm). Some near cards carry distance-equivalent values. These are valid only if the test is done at the recommended distance. If a standard near vision card is not available, any printed matter such as a telephone book or a newspaper may be substituted. Both the approximate type size read and the distance at which the material was held are recorded.

FIGURE 1.8 Rosenbaum pocket vision screener

Visual acuity estimation in an uncooperative patient

Occasionally, you will encounter a patient who is unwilling or unable to cooperate with standard visual acuity testing or who may be suspected of feigning visual loss. Because the typical visual acuity test will not work for such a patient, you will need to be alert to other signs. Withdrawal or a change in facial expression in response to light or sudden movement indicates the presence of vision. A brisk pupillary response to light also suggests the presence of some degree of vision. The exception to this is the patient with cortical blindness, which is due to bilateral widespread destruction of the visual cortex. In almost all cases, referral to an ophthalmologist is recommended.

Confrontation visual field testing

FIGURE 1.9 Confrontation visual field testing

The examiner takes a position about one meter in front of the patient. The patient is asked to cover the left eye with the palm of the left hand; the examiner closes the right eye. Thus, the field of the examiner’s left eye is used as a reference in assessing the field of the patient’s right eye. The patient is asked to fixate on the examiner’s left eye and then count the fingers of the examiner in each of the four quadrants of the visual field. Wiggling the fingers as a visual stimulus is not desirable. After the patient’s right eye is tested, the procedure is repeated for the left eye, with the patient covering the right eye with the palm of the right hand, and the examiner closing the left eye.

Amsler grid testing

Amsler grid testing (Figure 3.18) is a useful method of evaluating the function of the macula. Utilizing a patient’s best near correction, the test is carried out by having the patient look with one eye at a time at a central spot on a page where horizontal and vertical parallel lines make up a square grid pattern. This grid pattern may be printed with white lines against a black background or vice versa. The patient is asked to note irregularities in the lines. Irregularities may be reported as lines that arc, seem to bow or bend, appear gray or fuzzy, or are absent in certain areas of the grid, indicating a scotoma. The straight line, right angle, and square are geometric shapes that highlight areas of distortions most easily. With the chart held at a normal reading distance of 30 cm from the eye, the Amsler grid tests the central 20° of visual field. Thus, the entire macula is evaluated with this examination.

Color perception test

The normal retina contains 3 color-sensitive pigments: red-sensitive, green-sensitive, and blue-sensitive. A developmental deficiency in either the concentration or the function of one or more of these pigments causes various combinations and degrees of congenital color vision defects. Most such defects occur in males through an X-linked inheritance pattern. Color vision abnormalities also may be acquired in individuals with retinal or optic nerve disorders.

Color vision testing is performed with the use of pseudoisochromatic plates (eg, Ishihara plates), which present numbers or figures against a background of colored dots. The person with abnormal color discrimination will be confused by the pseu­doisochromatic plates, which force a choice based on hue discrimination alone while concealing other clues such as brightness, saturation, and contours.

FIGURE 3.19 Ishihara color perception test plates

The patient should wear glasses during color vision testing if they are normally worn for near vision. The color plates are presented consecutively (to each eye separately) under good illumination, preferably natural light. Results are recorded according to the detailed instructions provided with the plates. Usually, a fraction is specified, with the numerator equivalent to the number of correct responses and the denominator the total plates presented. The type of color defect can be deter­mined by recording the specific errors and using the instructions provided with the plates.

External inspection

With adequate room light, the examiner can inspect the lids, surrounding tissues, and palpebral fissure. Palpation of the orbital rim and lids may be indicated, de­pending on the history (eg, trauma or mass lesion) and symptoms. Inspection of the conjunctiva and sclera is facilitated by using a penlight and having the patient look up while the examiner retracts the lower lid or look down while the examiner raises the upper lid. The penlight also aids in the inspection of both the cornea and the iris.

Upper lid eversion

Upper lid eversion is sometimes required to search for conjunctival foreign bod­ies or other conjunctival signs. Topical anesthetic facilitates this procedure. The patient is asked to look down and the examiner grasps the eyelashes of the upper lid between the thumb and the index finger. A cotton-tipped applicator is used to press gently downward over the superior aspect of the tarsal plate as the lid margin is pulled upward by the lashes (Figure 1.9). Pressure is maintained on the everted upper lid while the patient is encouraged to keep looking down. The examiner should have a penlight within reach to inspect the exposed conjunctival surface of the upper lid for a foreign body or other abnormality. A cotton-tipped applicator soaked in topical anesthetic can be used to remove a foreign body. To return the lid to its normal position, the examiner releases the lid margin and the patient is instructed to look up.

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Ocular motility testing

The patient is asked to follow an object in 6 directions, the cardinal fields of gaze. This enables the examiner to systematically test each muscle in its primary field of action (Table 1.3). Thus, a possible isolated weakness or paralysis of muscle can best be detected.

TABLE 1.3 Muscles that are involved in eye moving in different directions

Pupillary reaction testing

Inspection of the pupils should be part of the physical examination. The patient’s direct and consensual pupillary reactions to light are evaluated in a room with reduced illumination and with the patient looking at a distant object.

To test the direct pupillary reaction to light, first direct the penlight at the patient’s right eye and see if the pupil constricts (a normal reaction). Repeat for the left pupil. To test the consensual pupillary reaction to light, direct the penlight at the right eye and watch the left pupil to see if it constricts along with the right pupil (a normal consensual response). Repeat for the left pupil, watching the tight pupil for the response. Occasionally, this examination may reveal indications of neurologic disease. (See Chapter 7 for details on pupillary examination and a description of the swinging-flashlight test for the detection of an afferent pupillary defect in the anterior visual pathway.) Pupillary inspection may reveal active or prior ocular disease with alterations in pupillary shape or size that are the result of local intraocular processes (for example, damage to the pupillary sphincter or adhesion of the iris to the lens).

Anterior chamber depth assessment

Usually, the anterior chamber is deep, and the iris has a flat contour. When the anterior chamber is shallow, the iris becomes convex as it is bowed forward over the lens. Under these conditions, the nasal iris is seen in shadow when a light is directed from the temporal side (Figure 1.10). As the shallowness of the anterior chamber increases, so do the convexity of the iris and the shaded area of the nasal iris. A shallow anterior chamber may indicate narrow-angle glaucoma (also called angle-closure glaucoma) or a narrow angle that could close with pupillary dilation, thus inducing an attack of angle-closure glaucoma. If a patient is suspected of having a narrow angle, he or she should not be dilated and should be referred to an ophthalmologist for evaluation.

To assess anterior chamber depth, follow these steps:

1.       Shine a light from the temporal side of the head across the front of the eye parallel to the plane of the iris.

2.       Look at the nasal aspect of the iris. If two-thirds or more of the nasal iris is in shadow, the chamber is probably shallow and the angle narrow.

3.       If you are unsure of the extent of shadow, direct the light more from the front of the eye, which will eliminate shadows, and then return the light to the temporal side of the head.

4.       Repeat the test for the other eye.

Intraocular pressure measurement

Intraocular pressure (IOP) is determined largely by the outflow of aqueous humor from the eye. The greater the resistance to outflow, the higher the intraocular pressure. Alterations in the actual production of aqueous humor also have an effect on the intraocular pressure.

Intraocular pressure varies among individuals. An IOP of 15 millimeters of mercury (mm Hg) represents the mean in a “normal” population. However, an IOP in the range from 10 to 21 mm Hg falls within 2 standard deviations of the mean.

Measurement of IOP is part of a glaucoma screening examination, along with ophthalmoscopic assessment of the optic cup. Diagnosing open-angle glaucoma requires additional testing not available to primary care physicians; therefore, IOP measurement is not indicated in this setting. However, IOP determination can be useful when the diagnosis of acute angle-closure glaucoma is being considered.

In the past, Schiotz (indentation) tonometry has been an inexpensive and simple method for primary care physicians to measure intraocular pressure. A Schi0tz tonometer (if available and the physician is skilled in its use) can be used to measure the intraocular pressure in a patient with suspected angle-closure glaucoma. With the patient in a supine position, the Schi0tz device with a given weight is placed on the patient’s anesthetized cornea and indents the cornea in an amount related to the IOP. A printed conversion table that accompanies the tonometer is used to determine the IOP in millimeters of mercury.

Currently, handheld electronic tonometers are available in some hospital emergency departments to measure intraocular pressure (Figure 1.11). These battery-operated devices can be used with the patient in any position, as opposed to other devices that require the patient to be either seated or supine. The intraocular pressure results are obtained rapidly with the electronic tonometer and correlate highly with those obtained by the Goldmann applanation tonometer (a slit-lamp-mounted instrument used by ophthalmologists) that is considered the gold standard for IOP measurement. Electronic tonometers are expensive and require daily calibration.

To perform electronic tonometry, the practitioner instills topical anesthetic in the patient’s eyes, separates the lids, and gently applies the calibrated tonometer to the patient’s cornea. The pressure reading and reliability rating displayed on the device are noted in the patient’s record.

Topical anesthetics applied for tonometry have little effect on the margins of the eyelids. If the tonometer touches the lids, the patient will feel it and squeeze the lids together, impeding IOP measurement. This can be avoided by holding the patient’s lids wide apart with the free hand while applying the tonometer tip with the other hand. Take care not to apply digital pressure to the eyeball while holding the lids apart, as it may produce a falsely high pressure reading. If the patient is wearing contact lenses, they must be removed before IOP is measured. Tonometry should never be attempted in a patient suspected of having a ruptured globe; doing so could result in further damage to the eye.

Fluorescein staining of cornea

Corneal staining with fluorescein (a yellow-green dye) is useful in diagnosing defects of the corneal epithelium. Fluorescein is applied in the form of a sterile filter-paper strip, which is moistened with a drop of sterile water, saline, or topical anesthetic and then touched to the palpebral conjunctiva. A few blinks spread the fluorescein over the cornea. Areas of bright-green staining denote absent or diseased epithelium (Figure 1.12). Viewing the eye under cobalt blue light (available on most direct ophthalmoscopes) or using a Wood lamp enhances the visibility of the fluorescence (Figure 1.13).

Two precautions to keep in mind when using fluorescein are:

1.           Use fluorescein-impregnated strips instead of stock solutions of fluorescein because such solutions are susceptible to contamination with Pseudomonas species.

2.           Have the patient remove soft contact lenses prior to application to avoid discoloration of the lenses.

 

Ophthalmoscopy

When examining the patient’s right eye, hold the direct ophthalmoscope in the right hand and use your right eye to view the patient’s eye. Use your left hand and left eye to examine the patient’s left eye. The patient’s eyeglasses are removed, and, barring large astigmatic refractive errors, most examiners prefer to remove their own glasses as well. Contact lenses worn by either patient or examiner may be left in place.

Pupillary Dilation

Pharmacologic dilation of the patient’s pupils greatly facilitates ophthalmoscopy. Recommended agents include tropicamide 1% and phenylephrine hydrochloride 2.5%. Dilation of the pupil should not be done under the following conditions:

1.                If assessment of anterior chamber depth suggests a shallow chamber and a narrow angle, do not dilate because an attack of angle-closure glaucoma might be precipitated.

2.                If a patient is undergoing neurologic observation and pupillary signs are being monitored (eg, a head-injured patient), do not dilate until the neurologist or neurosurgeon determines it is safe to do so.

3.                Dilation can cause blurred vision and light sensitivity for several hours, so patients must be informed of the potential impact on their activities such as reading and driving.

Method of Direct Ophthalmoscopy

To perform direct ophthalmoscopy, follow these steps:

1.                Have the patient comfortably seated. With the room lights dimmed, instruct the patient to look at a point on the wall straight ahead, trying not to move the eyes.

2.                Set the focusing wheel at about +8. Set the aperture wheel to select the large, round, white light.

3.                Begin to look at the right eye about 1 foot from the patient. Use your right eye with the ophthalmoscope in your right hand. When you look straight down the patient’s line of sight at the pupil, you will see the red reflex (see the next section).

4.                Place your free hand on the patient’s forehead or shoulder to aid your proprioception and to keep yourself steady.

5.                Slowly come close to the patient at an angle of about 15° temporal to the patient’s line of sight. Try to keep the pupil in view. Turn the focusing wheel in the negative direction to bring the patient’s retina into focus.

6.                When a retinal vessel comes into view, follow it as it widens to the optic disc, which lies nasal to the center of the retina.

7.                Examine the optic disc, retinal blood vessels, retinal background, and macula in that order (see the next section).

8.                Repeat for the patient’s left eye, holding the ophthalmoscope in your left hand and viewing with yout left eye.

Red Reflex

Light reflected off the fundus of the patient produces a red reflex when viewed through the ophthalmoscope at a distance of 1 foot. A normal red reflex (Figure 1.14) is evenly colored, is not interrupted by shadows, and is evidence that the cornea, anterior chamber lens, and vitreous are clear and not a significant source for decreased vision. Opacities in the media — such as a corneal scar, cataract, or vitreous hemorrhage — appear as black silhouettes and can be best appreciated when the pupil has been dilated. Most retinal pathology will not affect the red reflex.

Optic Disc

In most cases, when viewed through the ophthalmoscope, the normal optic disc (Figure 1.15) is slightly oval in the vertical meridian and has a pink color that is due to extremely small capillaries on the surface. Detail of these small vessels cannot be discerned, which differentiates them from pathologic vessels on the optic disc. The disc edge or margin should be identifiable (sharp). A central whitish depression in the surface of the disc is called the physiologic cup. The optic disc can be thought of as the yardstick of the ocular fundus. Lesions seen with the ophthalmoscope are measured in disc diameters (one disc diameter equals approximately 1.5 mm).

A great deal of normal variation exists in the appearance of the optic disc. The size of the physiologic cup varies among individuals. The pigmented coats of the eye — the retinal pigment epithelium and the choroid — frequently fail to reach the margin of the optic disc, producing a hypopigmented crescent (Figure 1.16, left). Such crescents are especially common in myopic eyes on the temporal side of the optic disc. Conversely, an excess of pigment may be seen in some eyes, producing a heavily pigmented margin along the optic disc (see Figure 1.16, right). The retinal nerve fibers (ganglion cell axons) ordinarily are nonmyelinated at the optic disc and in the retina, but occasionally myelination may extend on the surface of the optic disc and retina, producing a dense, white superficial opacification with feathery edges (Figure 1.17).

Retinal Circulation

The retinal circulation is composed of arteries and veins, visible with the ophthalmoscope (compare Figure 1.15 with Figure 1.18). The central retinal artery branches at or on the optic disc into divisions that supply the four quadrants of the inner retina; these divisions lie superficially in the nerve fiber layer. A similarly arranged system of retinal veins collects at the optic disc, where spontaneous pulsation (with collapse during systole) may be observed in 80% of normal eyes. The ratio of normal vein-to-artery diameter is 3:2. Arteries are usually lighter in color and typically have a more prominent light reflex than veins. The examiner should follow arteries from the disc and veins back to the disc in each quadrant, noting in particular the arteriovenous (A/V) crossing patterns.

Fundus Background

The normal fundus background is a uniform red-orange color primarily due to the pigmentation of the retinal pigment epithelium. The blood and pigment of the choroid also contribute to the appearance of the fundus background. For example, in heavily pigmented eyes, the fundus may have a darker color due to increased choroidal pigment content.

Fovea

The normal fovea, at the center of the macula (see Figures 1.15 and 1.18), is located directly temporal and slightly inferior to the optic disc and usually appears darker than the surrounding retina because the specialized retinal pigment epithelial cells of the fovea are taller and more heavily pigmented. In some eyes, the fovea may appear slightly yellow due to the xanthophyll pigment in the retina. The central depression of the fovea may act as a concave mirror during ophthalmoscopy and produce a light reflection known as the foveal reflex.

Summary steps in eye examination

An accurate history must be obtained before beginning the physical examination.

1.                Measure the visual acuity for each eye.

2.                Perform a confrontation field test for each eye.

3.                Inspect the lids and the surrounding tissues.

4.                Inspect the conjunctiva and sclera.

5.                Test the extraocular movements.

6.                Test the pupils for direct and consensual responses.

7.                Inspect the cornea and iris.

8.                Assess the anterior chamber for depth and clarity.

9.                Assess the lens for clarity through direct ophthalmoscopy.

10.           Use the ophthalmoscope to study the fundus, including the disc, vessels, and macula.

11.           Perform tonometry when acute angle-closure glaucoma is suspected, if a reliable tonometer is available.

Management or referral

The American Academy of Ophthalmology recommends that patients ages 40 to 65 be examined by an ophthalmologist every 2 to 4 years (after receiving a baseline exam at age 40 if not previously done), and every 1 to 2 years for patients over age 65. Children should undergo an evaluation in the first few months of life, then again at 6 months, 3 years, and 5 years of age by their primary care physician. Any abnormalities should be evaluated by an ophthalmologist.

Reduced visual acuity

The following guidelines apply for patients in whom reduced visual acuity is found, unless the patient has been seen by an ophthalmologist and the condition has been confirmed as stable.

VA Less Than 20/20

Any patient with visual acuity less than 20/20 in 1 or both eyes should be referred to an ophthalmologist if visual symptoms are present. Reduced visual acuity is the best single criterion by which to differentiate potentially blinding conditions from less serious ocular disorders, although 20/20 visual acuity does not preclude a pa¬tient from having serious eye disease.

VA Less Than 20/40

Any patient with visual acuity less than 20/40 in both eyes is an equally important candidate for referral, even in the absence of complaints. Although many such patients suffer only from uncorrected refractive errors, undetected painless but progressive loss of vision does occur in many disorders of the eyes and visual system.

Asymmetry

Any patient with a difference in visual acuity between the eyes of 2 lines or more on the Snellen chart should be referred promptly, even if visual acuity in 1 or both eyes is better than 20/40. Generally, visual function is nearly identical between the eyes; thus, in the absence of known causes of reduced vision, asymmetry of visual acuity may be a sign of occult disease. Patients may be unaware of even severe vi-sion loss in 1 eye if the other eye sees normally.

Presbyopia

Presbyopia is manifested by reduced near vision with no change in distance visual acuity. Middle-aged or elderly patients complaining of this combination will benefit from a referral for the prescription of corrective lenses.

Abnormal fundus appearance

Only after performing numerous fundus examinations will the practitioner be able to recognize the great range of normal ophthalmoscopic appearances. When an abnormality is suspected, further studies or consultation may be required because fundus abnormalities can indicate significant ocular or systemic diseases. Ophthalmologic consultation should be sought for fundus changes accompanied by acute or chronic visual complaints or in patients with systemic disease known to manifest in the eye.

Photographs of the fundus are taken with a special camera that provides a greater field of view than is possible with the direct ophthalmoscope. Many fundus abnormalities have 3-dimensional qualities, such as elevation or depression, but the examiner is limited to a monocular, 2-dimensional view with the direct ophthalmoscope or photographs. It is necessary to learn to think in 3 dimensions in order to grasp the pathophysiology.

Shallow anterior chamber depth/elevated intraocular pressure

A patient suspected of having shallow anterior chamber depth (at risk for angle-closure glaucoma) should be referred to an ophthalmologist for further evaluation.

Points to remember

·        An inadequate history can lead the physician to misinterpret the examination findings.

·        To prevent patients from reading the visual acuity chart with both eyes, either intentionally or unintentionally, the examiner must ensure that one eye is completely occluded.

·        A well-lighted hallway often provides an acceptable location for distance visual acuity testing with a standard Snellen chart.

·        To avoid measurement error when performing tonometry, the examiner must keep the lids apart by holding them firmly against the bony margins of the orbit, rather than by pressing them against the globe.

IV. Amblyopia, strabismus

Amblyopia

Amblyopia is a reduction in visual acuity in the absence of detectable organic dis­ease (such as cataract, retinoblastoma, or other inflammatory or congenital ocular disorders) that results from a disruption of the normal development of vision. It is usually unilateral, but it can (rarely) affect both eyes. Amblyopia does not cause learning disorders.

Amblyopia may develop in young children who receive visual information from one eye that is blurred or conflicts with information from the other eye. To under­stand how amblyopia may develop in this way, consider that the brain is receiv­ing 2 stimuli for each visual event: 1 from a visually aligned (fixating) eye and 1 from an “abnormal” eye (vision blurred or eye misaligned). If this abnormal visual experience is prolonged, the brain continually “favors” the eye with better vision, to the eventual detriment of visual development in the other eye. For this reason, amblyopia is often referred to colloquially as “lazy eye.”

A number of predisposing factors can lead to the development of amblyopia. These are summarized below.

Strabismic Amblyopia

A child can develop amblyopia in the context of strabismus. The eye used habitually for fixation retains normal acuity and the nonpreferred eye often develops decreased vision. Adult-onset strabismus gener­ally will cause diplopia (double vision) because the 2 eyes are not aligned on the same object. The brain of a child, on the other hand, is more adaptive. In a similar strabismic situation, the child’s brain ignores (suppresses) the image from one of the eyes—usually the one that provides the blurrier image.

Sometimes the degree of misalignment between the 2 eyes is very slight, making detection of strabismus and suspicion of strabismic amblyopia difficult. Even with a small angle of strabismus, amblyopia may be quite severe.

Refractive Amblyopia

Amblyopia can result from a difference in refractive error between the 2 eyes. The eye with the lesser refractive error provides the clearer image and usually is favored ovei the other eve; consequently, amblyopia develops. Children with asymmetric hvperopia are susceptible, because unequal accommodation is impossible; the child can bring only 1 eye at a time into focus. Refractive amblyopia may be as severe as that found in strabismic amblyopia. However, the pediatrician or family physician may overlook the possibility of amblyopia because there is no obvious strabismus. Detection of amblyopia must be based on an abnormality found in visual acuity testing.

Form Deprivation and Occlusion Amblyopia

Form-deprivation amblyopia (amblyopia ex anopsia) can result when opacities of the ocular media — such as cataracts, corneal scarring, or even dropping of the upper lid (ptosis) — prevent adequate sensory input and thus disrupt vi­sual development. The amblyopia can persist even when the cause of the media opacity or ptosis is corrected. Rarely, occlusion amblyopia can result from patching of the normal eye.

FIGURE Cerulean cataract. The left eye of the boy with cerulean cataract is shown. Multiple bluish and white opacities predominantly in the lens cortex with occasional radial central lesions are apparent.

FIGURE Corneal scar in keratoconus

Amblyopia testing

Amblyopia can be detected by testing the visual acuity in each eye separately. Al­though there is no specific Mendelian pattern of inheritance, strabismus and am­blyopia sometimes cluster in families. Restoration of normal visual acuity can be successful only if treatment is instituted during the first decade of life, when the visual system is still in the formative stage. Techniques for measuring or estimating visual acuity (or visual function) and detecting amblyopia vary with the child’s age, as described below.

Newborns

True visual acuity is difficult to measure iewborns. However, infants’ general ocular status should be assessed through corneal light reflex testing, evaluation of the red reflex, pupillary testing, and, if possible, fundus examination.

Infants to 2-Year-Olds

With infants, it is possible only to assess visual function, not visual acuity. To test for amblyopia in infants (from a few months to about age 2), cover each eye in turn with the hand or, preferably, an adhesive patch and note how the child reacts. The infant should be able to maintain central fixation with each eye. If amblyopia is present, the child will likely protest—vocally or by evasive movements—the cov­ering of the “good” eye. Visual function, including ocular motility, may be further assessed by passing an interesting object, such as a ring of keys, before the baby and noting how the infant watches and follows the moving object. Moving the child’s head can be used to demonstrate full ocular motility if not otherwise documented by following movements.

Age 2 to 4 or 5

A picture card (Figure 6.6) may be used to test visual acuity in children between 2 and 3 years of age. At age 3 (or before, if the child can follow directions and communicate adequately), visual acuity can be tested with the tumbling E chart (Figure 6.7). In the tumbling E test, the child is asked to point with his or her fin­gers to indicate the direction of the “arms” of the E. The HOTV test is another vi­sion assessment for preliterate children (Figure 6.8). A card with the 4 very distinct letters H, O, T, and V, is given to the child. One of the letters is highlighted on the chart, and the child points to the appropriate letter. Use of an adhesive patch is the best way to ensure full monocular occlusion and accurate acuity measurement in children at these ages (Figure 6.9).

FIGURE 6.6 Paediatric test card

FIGURE 6.7 Tumbling E chart. Visual acuity testing in children should be done at 6 meters (20 feet) with charts such as the tumbling E shown here. The child indicates the direction of the “arms” of the E by pointing with their fingers.

FIGURE 6.8 HOTV chart. The child is given a handheld chart with the letters HOTV on it and asked to point to the letter that is highlighted on an eyechart with the same letters, HOTV, on it.

Vision should be rechecked annually once visual acuity has been determined to be normal in each eye. Young children may not quite reach 20/20 acuity; this is no cause for concern as long as vision is at least 20/40 and both eyes are equal. A recent advance in early detection of amblyogenic factots is photoscreening. A computerized camera takes photographs of the child’s undilated eyes. Refractive er­rors, strabismus, anisometropia, and media opacities are visible in the photos. This technique permits screening of preverbal children and those unable to cooperate with other types of testing. Photoscreening is not a substitute for accurate visual acuity measurement, but it can provide significant information about factors that may lead to amblyopia.

Age 4 or 5 and up

The Snellen chart may be used to test visual acuity in children age 4 or 5 and up who know the letters of the alphabet.

Strabismus

Strabismus is a misalignment of the 2 eyes, so that both eyes cannot be directed toward the object of regard. Strabismus may cause or be caused by the absence of binocular vision; as with amblyopia, strabismus does not cause learning disabilities. It is clinically useful to distinguish between concomitant (nonparalytic) and incomitant (paralytic or restrictive) strabismus. Additionally, a number of terms are used to describe and classify strabismus. These distinctions and terms are sum­marized below.

Strabismus is called concomitant or nonparalytic when the angle (or degree) of mis­alignment is approximately equal in all directions of gaze. The in­dividual extraocular muscles are functioning normally, but the 2 eyes are simply not directed toward the same target. Most concomitant strabismus has its onset in childhood. In children, it often causes the secondary development of suppression to overcome double vision and thus leads to strabismic amblyopia. Concomitant strabismus in patients under age 6 is rarely caused by serious neurologic disease. Strabismus arising later in life may have a specific and serious neurologic basis. Concomitant strabismus may occur in an adult who loses most or all of the vision in one eye from intraocular or optic nerve disease. A blind eye in an adult will fre­quently drift outward, while in a child the eye will turn inward.

Strabismus is called incomitant, paralytic, or restrictive when the degree of misalign­ment varies with the direction of gaze. One or more of the extraocular muscles or nerves may not be functioning properly, or normal movement may be mechanically restricted. This type of strabismus may well indicate either a serious neurologic disorder, such as third cranial nerve paresis, or orbital disease or trauma, such as the restrictive ophthalmopathy of thyroid disease or a blowout fracture.

Heterophoria is a latent tendency for misalignment of the 2 eyes that becomes manifest only if binocular vision is interrupted, such as by covering 1 eye. During binocular viewing, the 2 eyes of a patient with heterophoria are aligned perfectly; both eyes are directed at the same object of regard. However, when 1 eye is covered, that eye will drift to its position of rest. Once the cover is removed, the eye will realign itself with the other eye. A minor degree of heterophoria is normal for most individuals.

Heterotropia is really another term for strabismus. In general, tropia refers to a manifest deviation that is present when both eyes are open (no covers). Usually, binocular vision is reduced. Some patients, however, can demonstrate an intermit­tent heterotropia and thus achieve binocular vision part of the time.

Heterotropia and heterophoria can be subdivided further according to the di­rection of the deviation involved. In esotropia and esophoria, the deviating eye is directed inward toward the nose. Esotropia is a manifest deviation and is the most common type of deviation in childhood. Exotropia is much more likely to be inter­mittent than esotropia, with an outward deviation of an eye alternating with align­ment of the eyes. Children with this condition suppress double vision when the deviating eye is turned out and achieve some degree of binocular vision when the 2 eyes are straight. Vertical heterotropias and heterophorias have many different causes, including paralysis or dysfunction of vertically acting extraocular muscles. When vertical deviations are described, the deviating eye (right or left) should be specified. Table 6.1 summarizes the directions of deviation in heterophoria and heterotropia. Figure 6.5 depicts the different kinds of heterotropia.

TABLE 6.1 Summary of Heterophoria and Heterotopia

Strabismus testing

Strabismus testing for children (and adults) consists of general inspection, the corneal light reflex test, and the cover test. These techniques are described below.

Children up to age 3 or 4 months may exhibit temporary uncoordinated eye movements and intermittent strabismus. However, it occasional deviation persists beyond this age, a referral to an ophthalmologist should be made. Constant deviations should be referred at any age.

Epicanthus (Figure 6.10), in which epicanthal skin folds extend toward the upper eyelid and brow and the nose bridge is flat, may give an infant the appearance of esotropia, especially if the head or eyes are turned slightly to the right or left. As the child’s head grows and the nose bridge develops, the epicanthus becomes less obvious. This may be mistakenly interpreted as the child outgrowing presumed strabismus; however, a child does not outgrow a true strabismus. The cover test and evaluation of the corneal light reflex will distinguish between pseudostrabismus (epicanthus) and true strabismus. However, it is important to keep in mind that strabismus can also occur in the presence of epicanthus.

General Inspection

For infants and older children, a general inspection may reveal an identifiable deviation of one eye. Having the patient look in the 6 cardinal positions of gaze (Figure 6.11) may reveal whether the deviation is approximately the same in all fields — indicating concomitant strabismus — or is significantly different in 1 field of gaze—indicating a possible incomitant strabismus. Involuntary eye jerks known as nystagmus may be detected in primary or other fields of gaze. The patient may assume an abnormal head posture (a tilt or turn to one side) to reduce the nystagmus and improve the visual acuity or to obtain binocular vision in cases of congenital cranial-nerve palsy. All infants or children with nystagmus should be examined and followed up by an ophthalmologist.

Corneal Light Reflex

Observation of the corneal light reflex constitutes an objective assessment of ocular alignment. Certainly iewborns and often in young children, it may be the only feasible method of testing for strabismus.

The patient is directed to look at a penlight held directly in front of the eyes by the examiner at a distance of 2 feet. The examiner aligns his or her eye with the light source and compares the position of the light as reflected by the cornea of each eye (Figure 6.12). Normally, the light is reflected on each cornea symmetrically and in the same position relative to the pupil and visual axis of each eye. In a deviating eye, the light reflection will be eccentrically positioned and in a direction opposite to that of the deviation. The size of the deviation can be estimated by the amount of displacement of the light reflex, but this is a relatively gross estimate.

Cover Test

The cover test (Figure 6.13) is easy to perform, requires no special equipment, and detects almost every case of tropia. It can be used on any patient over the age of 6 or 7 months. To perform the test, have the patient look at a fixation point, such as a detailed or interesting target (eg, a toy) or the Snellen chart. Note which eye seems to be the fixating eve. Cover the fixating eye and observe the other eye. If the uncovered eye moves to pick up the fixation, then it can be reasoned that this eve was not directed toward the object of regard originally (when both eyes were uncovered). If the eye moves inward to fixate, then originally it must have been deviated outward and hence is exotropic. If the eye moves outward to fixate, then it was deviated inward and is esotropic. If the eye moves up or down, then it is hypotropic or hypertropic, respectively; the deviating eye must be specified in a hypertropia or hypotropia. Of course, each eye must be tested separately because there is no way of knowing which eye may be expressing the deviation.

No shift on cover testing means there is no tropia, but a phoria could still be present. A phoria is detected by alternate cover testing. Each eye is alternately occluded, and the examiner observes the uncovered eye for a refixation shift. The patient has an esophoria if the uncovered eye moves outward to fixate and an exophoria if the eye moves inward to fixate.

A very small angle deviation may be difficult to detect by evaluating the corneal light reflex or performing the cover test. For this reason, visual acuity testing is important in all cases of suspected strabismus for detection of amblyopia.

Pupillary Testing

Abnormal pupillary responses may indicate neurologic disease or other ocular defects.

Red Reflex

Light is reflected off the fundus as red when it is examined through the ophthalmoscope from a distance of about 1 foot (Figure 6.14). Media opacities appear in the red reflex as black silhouettes. Leukocoria (“white pupil”) is a white reflex that may signify the presence of cataract or retinoblastoma (Figure 6.15).

 All infants and children should be evaluated for the red reflex; pupillary dilation may be necessary to achieve a red reflex (phenylephrine 1.0% and cyclopen-tolate 0.2% in infants, readily available in combination as Cyclomydril). Other alternatives are 2.5% phenylephrine and 0.5% cyclopentolate in the more deeply pigmented infant. (Caution: cyclopentolate can cause paralytic ileus in premature and neonatal infants) If the examiner cannot elicit a red reflex, the infant or child should be referred to an ophthalmologist urgently.

Ophthalmoscopy

A careful ophthalmoscopic examination of both eyes through dilated pupils is mandatory for any patient with reduced vision or with strabismus. In this way, the examiner can detect potentially serious intraocular lesions, such as cataract, malignancies such as retinoblastoma, or other abnormalities.

Management or referral

The early detection of amblyopia and strabismus is an important responsibility for those involved in infant and child health care. Delayed diagnosis may have serious consequences for visual acuity, eye disease, or systemic disease. If an abnormality is suspected, the patient should be referred promptly to an ophthalmologist.

Amblyopia

In children younger than 5, strabismic amblyopia can usually be treated effectively by the ophthalmologist through occlusion of the unaffected eye (Figure 6.16). The child wears an adhesive patch over the good eye, forcing the brain to utilize the previously nonpreferred eye. In general, the success of occlusion treatment for am­blyopia patients between the ages of 5 and 9 will depend on the age of the pa­tient, the degree of the amblyopia, and the persistence of patient compliance with treatment. Treatment is better tolerated by younger children but can be successful in children as old as 18. A treatment program started early in life often must be continued throughout the patient’s first decade. Amblyopia treatment by patch occlusion of the unaffected eye must be monitored carefully, especially during the younger years, to avoid causing amblyopia through sensory deprivation of the oc­cluded eye. New studies show that over a quarter of older children with moderate amblyopia will also respond to amblyopic therapy.

Treatment of refractive amblyopia consists first of wearing glasses, followed by patching of the better eye if the visual acuity difference persists after 4 to 8 weeks of wear. Equal vision in both eyes is readily achievable with parental cooperation in almost all cases. In general, the earlier the individual with amblyopia is diagnosed and treated, the better the chance of achieving equal vision. An alternative to oc­clusion therapy with an adhesive patch is the use of dilating drops (atropine 1%) daily to the better-seeing eye. This blurs the vision in the better-seeing eye and forces the child to use the amblyopic eye.

After cessation of treatment, the child must be monitored for recurrence of am­blyopia. This most commonly occurs in the first 3 months after discontinuation of amblyopia therapy.

Strabismus

The most effective way to support fusion (binocular vision) is to treat the am­blyopia and equalize the vision. Glasses can treat some or all of the esotropia in a farsighted, or hyperopic, individual (Figure 6.17) and may decrease the frequency of deviation in a myopic individual with exotropia. However, surgical correction of the misalignment may still be necessary, particularly in those children who develop esotropia before the age of 6 months (congenital esotropia). Even when binocular vision may not be achievable, the impact of a disfiguring strabismus on a patient’s self-image is a valid indication for surgery. It must be stressed that surgery is not an alternative to glasses and patching when amblyopia is present. “Vision training” has no proven value for the treatment of amblyopia or strabismus.

Serious intraocular lesions

Ocular abnormalities such as leukocoria or glaucoma require immediate referral to an ophthalmologist. Leukocoria may be the presenting sign for intraocular tumors such as retinoblastoma or for a visually significant cataract. Both conditions require prompt ophthalmic treatment. Glaucoma presents in the infant with photophobia and tearing, corneal enlargement, and clouding (Figure 6.18). If glaucoma is sus­pected, immediate referral to an ophthalmologist is indicated.

Points to remember

·        Amblyopia must be detected early and referred to an ophthalmologist to be treated successfully.

·        The importance of visual acuity testing in detecting amblyopia cannot be overemphasized. Amblyopia may be present in eyes without strabismus, so the vision in each eye may not be normal even if the eyes appear normally aligned.

·        Several serious organic conditions cause strabismus as one manifestation of the disease; therefore, all patients with strabismus should be referred to an ophthalmologist at the time of diagnosis for further testing.

·        Children may have cataracts, glaucoma, and retinal diseases, so children with unusually large eye(s), decreased or no red reflex, or poor vision should be referred to an ophthalmologist.

·        Vision training has no proven value in the treatment of amblyopia or strabismus.