Department of otorhinolaryngology, ophthalmology,
paediatric surgery and neurosurgery
Methodologigal Instruction for Students
to Lesson
(practical lesson – 7 hours)
Themes: 1. Refraction and Accommodation
2. Strabismus
3. Diseases of the Eyelids, the Lacrimal System and the Orbit
:
The eye is a compound optical system comprising a cornea and a lens, as shown in Figure 1. It is an adaptive optical system because the crystalline lens changes shape to focus light from objects at a large range of distances on the retina. Unlike the components of most optical systems, typified in Figure 2, the cornea and lens are not centered on a common axis, nor are they spherically surfaced. Because a model eye is being treated here, however, it will be assumed that the surfaces are spherical and that their centers of curvature lie on the optical axis, a straight line from the vertex of the cornea to the posterior pole. Furthermore, the incident rays will be considered paraxial, that is, they lie close to the optical axis and strike the surfaces with very small angles of incidence (Fig. 3). A bundle of paraxial rays converges to a single point focus. As the diameter of the bundle of rays grows larger, the incidence angles of the marginal rays become larger so that they no longer may be considered paraxial. Spherical aberration forces them to cross the axis at different points, thus blurring the image, as illustrated in Figure
Fig. 1. Optical system of the eye: a, anterior surface of cornea; b, posterior surface of cornea; c, anterior cortex; d, anterior core, e, posterior cortex; f, posterior core; v and g, anterior and posterior poles of the eye through which the optical axis passes; line jh, visual axis.
Fig. 2. Centered system of rotationally symmetric optical elements containing an optical axis.
Fig. 4. The blur due to spherical aberration has radial symmetry.
Briefly, the corneal portion, including the tear layer, separates air from aqueous humor, and the lens portion separates aqueous from vitreous humor. Rays entering the eye are refracted first and by the greatest amount at the first surface of the cornea because of the large difference in index of refraction at the air-to-cornea interface. The second surface of the cornea has negative power; nevertheless, the cornea contributes over 70% of the approximately 64 diopters (D) of refractive power of the unaccommodated eye. The crystalline lens supplies the remaining refractive power. During accommodation, additional power is supplied by the lens, which assumes a rounder form.
TERMINOLOGY AND SIGN CONVENTION
Before the optical system of the eye is described in detail, a review of a few basic optical terms will be useful. This review presupposes a knowledge of geometric optics.
Light is assumed to travel from left to right. Positive distances are measured from left to right; negative distances are measured from right to left. Object distances are measured from the optical element to the object point. Image distances are measured from the optical element to the image point. In Figure 8, the object distance from the lens to the object point is negative, that is, it is measured from right to left, and the image distance is positive.
FOCAL POINTS AND FOCAL LENGTHS
When light from an infinitely distant source found to the left of an optical element strikes the element, the collimated paraxial rays will be converged to Fґ, the second focal point. This will be a real image point for positive elements (Fig. 5) and a virtual image point for negative elements (Fig. 6). Distance FґA is the second focal length. Light originating from the first focal point F will be collimated by the optical element, forming an image at infinity. FA is the first focal length. The idea of refractive power is derived from focal length and leads to the idea of vergence.
g. 5. Positions of the first and second focal points formed by a positive thin lens in air and positive single refracting surface. The distance from F to A is equal to the distance A to Fґ for the thin lens. The distance F to A is equal to the distance from C (the center of curvature) to Fґ for a single refracting surface.
Fig. 6. Positions of the first and second focal points formed by a negative thin lens in air and a negative single refracting surface.
VERGENCE
Light diverging from the object point in Figure 7 has negative vergence. The spherical wavefronts grow larger as their radial distances from the source increase. Because curvature is the reciprocal of the radius of curvature, the farther the wavefront is from the object, the smaller its curvature will be. Wavefront vergence in diopters equals the reciprocal of the radial distance in meters:
Fig. 7. Vergence in diopters shown in relation to a distance scale in meters. Light from the object point diverges; light converges toward the image point. The farther the wavefronts are from either of these points, the shallower their curvatures and the weaker the dioptric values of their vergences.
Vergence = 1/Distance in meters
Light that is converging toward an image has positive vergence. The wavefronts become increasingly curved as they approach the image point, and the vergence increases correspondingly. For example, at a distance of
It is assumed that light travels from left to right. Consequently, a divergent wavefront is centered about a point to the left of the wavefront. Because the wavefront is negative, the distance from the wavefront to the point (which may be a real object or a virtual image point) is negative. Convergent wavefronts are centered about real images or virtual objects to the right of the wavefront. These distances are positive.
REDUCED VERGENCE
Objects and images in media of any refractive index have reduced distances. For objects, the reduced distance is l/n, the reduced image distance is lґ/nґ. Similarly, the reduced first focal length is f/n and the reduced second focal length is fґ/nґ. Just as vergence is the reciprocal of the distance, reduced vergence is the reciprocal of the reduced distance. L = n/l is the reduced vergence of the object; the reduced vergence of the image is
Optical elements have the power to impress a vergence on incident light. The refracting power F (not to be confused with the first focal point F) is equal to the reciprocal of the reduced focal length, that is, F = n/f = nґ/fґ. The reduced vergence of the object and image and the refracting power are related in a very simple equation
The reduced vergence of the light after refraction (heading toward the image point) is the sum of the reduced vergence of the light from the object when it is incident on the lens or refracting surface plus the power of the lens or surface. For example, an object in air placed
If the lens is a positive lens with a focal length of Ѕ1/2 meter the lens power
The image vergence becomes Lґ = -0.5 + 2 = + 1.5 D.
This shows that the light leaves the lens with a convergence Lґ of 1.5 D and will be imaged in air at
The positive sign means the image is real and to the right of the lens (Fig. 8). Both the object and the image lie in air. They will lie in different media if the lens is replaced by a single refracting surface. For example, it may be supposed that a convex refracting surface of radius ј1/4 meter separating water and glass is facing an object
Fig. 8. Illustration of the vergence equation Lґ = L + F for a thin lens.
Fig. 9. Illustration of the vergence of equation Lґ = L + F for a single refracting surface separating water (n = 1.33) from glass (nґ = 1.5). (Drawing is not to scale.)
The refracting power of a single refracting surface is the difference between the second and first indices of refraction divided by the radius of curvature in meters:
The reduced vergence of the refracted light is Lґ = L + F = -0.44 + 0.68 = + 0.24 D.
By rearranging the equation Lґ = nґ/lґ to give lґ = nґ/Lґ = 1.5/0.24 =
MAGNIFICATION
In Figure 10, an object whose height is y is a distance l from a thin positive lens. The lens forms an inverted image at lґ with a height of yґ. Lateral magnification (m) is the ratio of the image height divided by the object height, m = yґ/y. It also can be shown that magnification is equal to the reduced vergence from the object divided by the reduced vergence to the image: m = L/Lґ.
Fig. 10. Lateral magnification of an object with height y at distance 1 from a thin lens.
In the previous lens example, we found L = -0.5 D, and Lґ = + 1.5 D; consequently, the magnification is m = L/Lґ = -0.5/150 = -1/3. The image is one third as large as the object, and the minus sign means that the image is inverted.
Similarly, in the previous example of a refracting surface separating water and glass, L = -0.44 D and Lґ = 0.24 D. The magnification is m = -0.44/0.24 = -1.83.
THE GULLSTRAND SCHEMATIC EYE
Schematic eyes are models of the optical system of the eye. They are extremely useful but limited representations of the dynamic living eye. The schematic eyes developed by Listing, Tscherning, and Helmholtz4 greatly advanced the understanding of the optics of the eye. However, it was Gullstrand who developed the most authoritative model of the eye. His model was based on the work of many researchers and some very original experiments and instrumentation of his own. The essential parameters that Gullstrand needed to find how light travels through the eye were the curvatures of the surfaces of the cornea and lens, their positions, and the indices of refraction of the ocular media. The techniques for making these determinations, described by Helmholtz and Gullstrand,4 are experimentally intricate and complicated by considerable trigonometric calculations.
Although the anterior surface of the cornea seems spherical, it is not. Centered about the vertex is an optical zone 2 to
These topographic features of the anterior surface of the cornea were determined by means of Gullstrand’s photokeratoscope, a device that took photographs of the corneally reflected image of an illuminated pattern of circles. Measurements of the photographs were used to calculate the corneal contour. The ophthalmometer also was used for this purpose. Both methods use reflection from the front surface of the cornea. This reflection forms the first of the four Purkinje images. With care, the reflex from the posterior surface of the cornea as well as faint reflections from the anterior and posterior surfaces of the lens may be seen. The anterior surface of the lens is convex and forms a virtual reflex image, but the posterior lens surface is concave and forms a real reflex image of a distant object. Each of these surfaces acts as a simple spherical mirror. Therefore, their radii may be calculated easily, provided that the optical characteristics of the optical elements preceding each surface of interest are known. Measurements of 14 ocular parameters comprising curvatures of surfaces, thicknesses of elements, and indices of the media were required for Gullstrand to define the optical system of a standard eye (see Fig. 13).
REFRACTIVE INDICES
The indices of refraction of the cornea, aqueous, lens, and vitreous had to be determined. Gullstrand found that the indices of refraction of the aqueous and vitreous humors were both equal to 1.336, which is practically identical to water. The index of refraction of the cornea was higher and is given as 1.376.
The crystalline lens structure often is compared with the layers of an onion. This laminar structure has an increasing lens density and index of refraction from the outermost layers to the center. Calculations of ray paths through a lens of continuously varying indices or gradient index are very complicated. Therefore, Gullstrand calculated an equivalent lens made up of a central core with a refractive index of 1.406 surrounded by a cortex of index 1.386. These two-index lenses closely approximated the size, shape, and power of the average real crystalline lens. Gullstrand’s data are presented.
RADII OF CURVATURE OF THE REFRACTIVE SURFACES
The radii of the various ocular surfaces were measured with an ophthalmometer. This is an instrument with an illuminated object pattern of known size and position that is reflected by the ocular surface. The reflected pattern is viewed by means of the special optical system of the ophthalmometer that may be adjusted to find the size of the image reflected by the surface of interest.
Because the ratio of the image size to the object size is equal to the ratio of the image distance to the object distance, the focal length of the surface, treated as a spherical mirror, can be calculated. The focal length of a spherical mirror is equal to one half of its radius of curvature, so the ophthalmometer can be calibrated directly to read out the radius of curvature of the surface. Clinical ophthalmometers usually are calibrated to provide the refracting power of the front surface of the cornea. Given 1.376, the index of refraction of the cornea, this merely requires solving the equation.
Similarly, the power of the posterior surface of the cornea is the result of the difference in the indices of the cornea and aqueous. This difference is much less than at the air–cornea interface (the anterior surface) so that the power at the posterior surface of the cornea is weak. In fact, it is a negative or diverging power because the light travels from a higher (cornea 1.376) to a lower (aqueous = 1.336) index medium.
POSITIONS OF THE OCULAR SURFACES
Gauges, calipers, and other mechanical devices provided the early data on the thicknesses and positions of the optical elements in the eyes of cadavers. With the invention of the slit lamp, finding these positions optically without dissecting and deforming the eye was possible. Initially, the slit lamp is focused on the anterior surface of the cornea to establish the zero position. The slit lamp then is racked forward to bring the second surface into focus, and the distance that the slit lamp is translated to accomplish this is recorded. This position represents the apparent position of the posterior corneal surface because it is being observed through the anterior surface. The anterior surface is a refracting surface that optically affects the space behind it. Therefore, the actual position of the posterior corneal surface required optical calculation. Similarly, for example, the depth of the anterior chamber would be found by focusing on the anterior surface of the lens or on the edge of the iris. This is also an apparent position. The real position depends on the power of the cornea.
GULLSTRAND’S RESULTS
Several significant factors should be noted about the Gullstrand schematic eye with relaxed or zero accommodation. The first and second principal points at about 1.348 and
TABLE 3. Complete Optical System of Gullstrand Eye
Refracting power 58.64
Position of first principal point 1.348
Position of second principal point 1.602
Position of first focal point -15.707
Position of second focal point 24.387
First focal length -17.055
Second focal length 22.785
Position of fovea centralis 24.0
Axial refraction -1.0
The power of the cornea is 43.05 D. In the unaccommodated state, the crystalline lens has a power of 19.11 D. Interestingly, although the cortex and core indices are 1.386 and 1.406, respectively, an index of 1.42 would be required if a homogeneous lens were to have the same form and power. The power of the unaccommodated eye is 58.64 D.
The data in Table 3 show that the standard eye is axially hyperopic by 1 D; therefore, it has a length of
An optically homogeneous lens with spherical refracting surfaces would produce a significant amount of spherical aberration. As noted, spherical aberration is characterized by rays being brought to a progressively shorter focus as they strike a lens at greater heights from the optical axis. Fortunately, two countereffects exist in the eye. The cornea is not spherical but tends to flatten out at its margins, and thus, marginal rays are not refracted as much as they would have been had the spherical contour of the cornea been maintained. Similarly, the lower index in the outer zones of the lens causes less refraction of the marginal rays. These two effects compensate for spherical aberration and may, in fact, overcorrect it. Constriction of the pupil completes the mechanisms that reduce spherical aberration, at least in bright surroundings. This plays a significant role in increasing visual acuity.
During accommodation, the curvatures of the lens become steeper, the axial thickness increases, and the pupil constricts. These changes enable the eye to focus sharply near objects on the retina. The uneven capsule (Fig. 14) allows the front surface of the lens to bulge in the center while keeping the periphery less curved, which helps control spherical aberration as power is increased.
The image degrading effect of chromatic aberration of the eye is mitigated by three factors, according to Wald.8 First, the crystalline lens acts as a filter that transmits the visible spectrum but absorbs the near ultraviolet light of wavelengths shorter than 400 nm. This region of the spectrum, where chromatic aberration is increasing most rapidly, simply does not reach the rods and cones.
Second, the sensitivity of the eye shifts toward the red end of the spectrum as the illumination is increased, and the eye switches from low acuity rod vision to high acuity cone vision. The rods have a peak sensitivity at 500 nm that corresponds to blue-green, and the foveal cones have a peak sensitivity at 562 nm or in yellow-green (see Fig. 39). The significance of this shift in sensitivity is that the foveal cones respond more strongly to longer wavelengths where chromatic aberration gradually increases, whereas rods respond more strongly to the region of rapidly increasing chromatic aberration. Thus, the smaller chromatic blur experienced by the cones supports their high-acuity function.
Finally, the full chromatic blur of the eye is alleviated by the spectral transmission of the pigmentation of the macula lutea. Wald compared the spectral sensitivities of cones in the macula with cones in a colorless peripheral area of the retina. He found that the maculae pigment “takes up the absorption of light in the violet and blue regions of the spectrum just where absorption by the lens falls to very low values. Thus, the yellow patch removes for the central retina the remaining regions of the spectrum for which the color error is high.”8 Despite the “spectrum-reducing” factors described by Wald, chromatic aberration has a remarkable influence on reflexive accommodation of the eye.
The blur patterns for a myopic, emmetropic, and hyperopic eye. Both the red and blue blurs superimpose in the emmetropic eye, producing no noticeable color aberration. In absolute hyperopia, the blue rays focus on the retina and are surrounded by a red blur or halo. In myopia, the red rays are in focus on the retina and some blue halo surrounds. For example, a myope will see red neon signs most sharply, and a hyperope (with no accommodation) will see blue neon signs most sharply. The myope and hyperope will see colored haloes about points emitting a mixture of red and blue light.
MODERN SCHEMATIC EYE
Modern technological developments in the biometry of the human eye have resulted in an extensive literature describing its anatomy and optical properties. Among these are the gradient index of the lens, the aspheric curvatures of the surfaces of the cornea and the lens, and the dispersive characteristics of the ocular media. Abetted by powerful optical ray trace programs, it is easy to evaluate the optical characteristics, especially the aberrations of model eyes with these properties. One such model9 that predicts spherical and chromatic aberration is shown in Figure 17.
Fig.
All surfaces are centered on the optical axis. The aperture stop coincides with the front surface of the lens and is decentered by
Image Size of Object at Far-Point in Axial Hypermetropia
The small globe of the axially hyperopic eye causes the retina to intercept the convergent rays before they come to a focus, as shown in Figure
The position of the nodal point is
The distance from the nodal point to the retina is 21.2 – 5.7 =
Assuming once again that the virtual object is
EMMETROPIA
Although the emmetropic eye, with relaxed accommodation, forms sharp retinal images of distant objects, emmetropia is not the statistically normal refractive state. Studies of refractive state show that the peak of the distribution curve occurs at about 1 D of hyperopia, although the frequency of myopia is greater in adults than children (Fig. 58). Most infants are hyperopic, probably because the axial length of their eyeballs is too short. Consequently, hyperopia decreases with growth. Emmetropia is considered merely a point on the curve of refractive status that marks the transition from hyperopia to myopia. It occurs when the length of the eyeball, the curvature of the cornea, and the power of the unaccommodated lens all are appropriate for focusing collimated light on the retina, a remarkable condition to exist in so large a sample of the population. The young emmetrope with normal amplitude of accommodation will have distinct distant and near vision, assuming that there are no problems with binocular vision or amblyopia.
Fig. 58. Distribution of refractive errors among the population (Wulfeck JW et al: Vision in Military Aviation. WADC Technical Report 58-399. Wright Air Development Center, OH, Nov 1959.)
AMETROPIA
Ametropia exists when distant objects are not focused sharply on the retina by an eye with relaxed accommodation. The eye is too long or short for its power or too weak or strong for its length. Whether it is power or length, that is, whether it is refractive or axial ametropia, depends on establishing norms for power and length as, for example, those of Gullstrand’s schematic eye. Naturally, any given individual may suffer from both axial and refractive ametropia. In the interest of simplicity, the two types will be considered separately.
DEFINITION OF MYOPIA
Myopia is defined as that optical condition of the nonaccommodating eye in which parallel rays of light entering the eye are brought to a focus anterior to the retina. It can also be described as the condition in which the far point of focus is located at some finite distance from the cornea. For practical purposes, this is a distance of less than
Fig 1. Light rays originating at the far point (F) of a -5 D eye focus at the retina. The far point and retinal focal point (f) are said to be conjugate. (Curtin B J: The Myopias: Basic Science and Clinical Management. Philadelphia, JB Lippincott)
P = 1/f
where P is the power of the refractive error in diopters and f is the distance of the far point or focal point of the eye in meters from the eye. If the far point of an eye is at
REFRACTIVE AND AXIAL MYOPIA
Myopia can be characterized as refractive or axial, although a continuum of the two types exists. In refractive myopia the overall refractive power of the eye as determined by cornea and crystalline lens power, modified by anterior chamber depth, is excessive in relation to an eye of normal axial length (21.5 mm-
THE CORRECTION OF MYOPIA
The optical correction of myopia with spectacles, particularly the higher degrees, presents several considerations that must be addressed by the practitioner. Minus lenses cause image minification and “barrel” distortion in addition to prismatic image displacement. Although these do not present an adjustment problem to myopes accustomed to wearing spectacles, changes in prescription may bring on their unpleasant optical effects. The thick-edged lenses necessary for higher myopic corrections can present cosmetic problems for some patients. These considerations should be discussed with patients to ease the transition to, and permit a more realistic expectation of their vision with, stronger spectacles.
The minus spectacle lens operates at an optical disadvantage when compared with the plus lens, and this is increasingly so with higher degrees of refractive error. As can be seen, a minus lens will require substantially greater power to correct a given refractive error than would a plus lens at the same vertex distance. Image minification also becomes more marked with greater lens powers of myopic correction and with increases in lens vertex distance.
CONVERGENCE AND ACCOMMODATION
If an object of regard is brought toward a fully corrected myope with normal binocular fixation, the increase in divergence of the light rays from the object results in their being focused behind the retina. This stimulates ocular accommodation and convergence so that the image is brought into focus at both foveas. Both accommodation and convergence are altered with the use of minus spectacle lenses.
Minus spectacle lenses have the effect of base-in prisms when the visual axes of the eyes are convergent. From Prentice’s rule, the amount of prism in diopters is equal to the displacement, in centimeters, from the optical center of the lens times the dioptric power of the lens.
P=d x p
where P = prismatic power in prism diopters, d = displacement in centimeters from the optical center of the lens, and p = lens strength in diopters. In the case of a bilateral – 15 D myope who is looking
The range of accommodation in the uncorrected myope is reduced in .nonlinear proportion to the myopic error. This range is defined as the distance from the far point to the near point (point of focus using maximal accommodation) of the eye and represents the distance over which a clear focus can be achieved. The amplitude of accommodation, on the other hand, is defined as the difference in the refraction of the eye between these two values expressed in diopters. Figure 7 demonstrates a 16 D range of accommodation in a -4 D myope with and without correction. It can be seen that with correction the range of accommodation is greatly expanded. Without correction a myope will have the advantage of a closer near point. Thus, for special vocational or avocational needs that demand extended periods of work at a close near point, the removal of glasses may often be advantageous. Medium and high myopes learn of this advantage early in life and instinctively resort to uncorrected near vision for fine, detailed near work. Another maneuver used by many presbyopic myopes is that of sliding their spectacles down on their nose to increase vertex distance. This reduces the power of the minus lens and affords better near vision.
Spectacle correction with minus lenses also requires less accommodative effort to maintain clear near vision.6 The greater the minus power and the greater the vertex distance of the lens from the eye, the less the accommodatioecessary to see a near object clearly. For the hyperope, on the other hand, the greater the plus power lens and the greater its vertex distance, the greater the accommodative effort required to see an object at the same distance. With correcting spectacles at a vertex of
Clinically, the increased accommodative effort required at near for a spectacle-corrected myope who first starts to use contact lenses frequently induces acute asthenopic symptoms. The possibility of near vision problems should always be carefully explained to prospective myopic contact lens users, especially those who are middle-aged.
ANISOMETROPIC MYOPIA
The optical correction of anisometropic myopia presents the same prismatic difficulties described above, with additional problems due to the unequal prismatic effects between the eyes. When gaze is not directed through the optical centers of unequal myopic spectacle lenses, horizontal and, more importantly, vertical phorias or tropias can be induced. Fusional vergence amplitudes can overcome the effects of these lenses so that the patient is asymptomatic. At times, however, it may be necessary for these patients to turn their heads rather than their eyes to keep the visual axis near the optical centers of the lenses. This is done instinctively if these situations are presented early in life. Whereas the anisometropic and presbyopic adult will usually have little problem adjusting to new, carefully centered, single-vision reading glasses, the transition to a bifocal can be a considerable problem. This can be handled in one of two ways. Different type bifocal segments with different locations of their optical centers can be dispensed so that at the usual reading position the induced prism between lenses is neutralized. Another, easier, method is to reduce the vertical prism induced in the lower half of a minus lens by eliminating some of the base-down prism of the stronger lens. This is done by “slabbing off” to produce a biprism lens. It has the effect of removing a portion of the base-down prism of the minus carrier lens.
Another problem encountered in high anisometropic myopia is that of unequal image minification by the correcting lenses. This can result in sizable amounts of aniseikonia, which can adversely affect binocular vision. Changes in base curve, thickness, and vertex distance of the spectacle lenses can be made to compensate for some of the difference in image size between the two eyes. Contact lenses are the best solution if they can be worn as they will result in the greatest reduction of image size disparity between the two eyes. Because most patients have this problem from birth, neurophysiologic adaptive processes will usually facilitate wearing spectacles asymptomatically if they are provided at an early age. Patients who suddenly develop a substantial change in their spectacle correction, as from retinal detachment surgery or asymmetric progression of myopia, usually become symptomatic due to aniseikonia. These cases require special consideration when dispensing new spectacles.
CONTACT LENSES
The optical benefits of contact lenses over spectacle correction in high myopia include image magnification, the elimination of prismatic object displacement with its attendant “barrel” distortion, and the elimination of image degradation caused by the spherical aberration of spectacle lenses with off-axis viewing (coma).9 As we have observed earlier, when the optical correction in myopia is brought closer to the corneal surface, image minification decreases. With a contact lens at the corneal surface, image minification is reduced to a minimum. The prismatic effects and spherical aberration of spectacle correction are eliminated because the optical axis of the contact lens moves with the visual axis of the eye. We have also noted that consideration must always be given to the effects of contact lenses on the accommodation of pre-presbyopic myopes and on the convergence amplitude in myopes with convergence insufficiency.
Contact lens technology has advanced significantly in the past several years. Rigid Polymethylmethacrylate (PMMA) contact lenses have been virtually replaced with daily and extended-wear rigid gas-permeable lenses. Both daily and extended-wear toric soft contact lenses have allowed patients who were previous hard contact lens failures to wear contacts comfortably. In addition, there is now a contact lens available that is made with a rigid gas-permeable center and bonded soft edge. Patients with astigmatism who were previously unable to wear rigid contact lenses of any type can sometimes be fitted successfully with these lenses. The availability of soft toric lenses, in addition to the dual-construction lens, brings comfortable contact lens correction of most refractive errors within the reach of the great majority of patients. Sufficient patient motivation and normal external ocular anatomy, however, are still essential for successful contact lens wear. Fitting myopes and other patients with high degrees of refractive error, including astigmatism, can be very challenging, but also quite rewarding. The reader is directed to a detailed exposition of this subject elsewhere in this text.
BINOCULAR VISION
Cortical integration of similar images on each retina into a unified perception is binocular vision. The subject of binocular vision is complicated by the fact that it is comprised of three independent components and contained in two separate reflexes. The components of binocular vision are simultaneous perception, fusion, and stereopsis. The duality of binocular vision results from macular and extramacular binocular vision being distinct reflexes. The many differences between the macular and extramacular binocular vision reflexes are discussed elsewhere in these volumes.
ABSENCE OF BINOCULAR VISION
Binocular vision is an acquired reflex that normally develops during the first three to four months of life. Its development demands certain requisites. The infant must be capable of seeing with each eye and both eyes must be aligned with one another, permitting similar retinal images to project onto corresponding retinal areas during the critical period for binocular vision development, which extends to approximately two years of age. The patients with congenital or very early onset strabismus do not receive the essential stimulation from similar images projecting onto corresponding retinal areas; consequently, their binocular vision reflexes are not developed. Absence of binocular vision is elicited by sensory tests yielding responses that prove the absence of simultaneous perception of images on each retina. Despite the absence of binocular vision, if the attention directed to the object of regard results in fixating first with one eye for a moment and then fixating with the other eye for the next moment, such alternate fixation ensures normal monocular macular function. But if the attention to the object of regard is fixated exclusively with one eye, amblyopia will be apparent in the unused eye.
Patients without binocular vision experience no untoward visual symptoms; they have neither diplopia nor visual confusion. Their only visual handicap is absence of stereopsis, which they are as unable to comprehend as the colorblind person is unable to comprehend a deficiency in color perception.
Pertinent to this chapter on the adaptations of binocular vision in strabismus and the most important of the many differences between macular and extramacular binocular vision reflexes is the absence of any adaptation by the macular binocular vision reflex in the strabismic patient. The reflex is impossible to adapt to the newly acquired strabismus because it ceases to function the moment the eyes deviate from being straight. For an adaptation of the binocular vision reflex to occur, the reflex must continue functioning during strabismus, which causes annoying symptoms that are eliminated by the adaptation. Only the extramacular binocular vision reflex continues functioning after the onset of the strabismus. Hence, the adaptation of the binocular vision to strabismus is restricted to the extramacular binocular vision reflex .
VISUAL SYMPTOMS IN STRABISMUS
Once the extramacular binocular vision reflex has developed, annoying visual phenomena are produced when the eyes deviate from being straight. The same phenomena occur when placing a 15D prism before one eye. Viewing with the strabismic eye, or through the prism with straight eyes, precludes the opportunity for similar retinal images to project onto corresponding retinal areas. The visual environment is now seen doubled on itself. The doubling is caused by the two separate visual phenomena known as diplopia and visual confusion.
DIPLOPIA
Diplopia is the simultaneous perception of two images of one object resulting from these similar images projecting onto noncorresponding retinal areas. The simultaneous perception of images under these circumstances yields the impression that the object of regard is simultaneously located at two points in space (Fig. 1). If the deviation is sufficiently small to displace only objects in the area of conscious regard onto noncorresponding retinal areas, peripheral objects still project onto corresponding retinal areas (Fig. 2). Diplopia extends to all objects in visual space when their images are projected onto noncorresponding retinal areas; therefore, simultaneously there is diplopia for both the area of conscious regard (central diplopia) and the peripheral vision (peripheral diplopia).
Fig. 1. Diplopia results from simultaneous perception of similar images projecting onto noncorresponding retinal areas. Esotropia causes homonymous diplopia; clear image on macula of right eye is illustrated by solid lines, blurred image oasal retina of left eye is illustrated by broken lines. The illustrated diplopic images are according to the patient”s drawing of them. Inset. Heteronymous diplopia in exotropia. (Symposium on Strabismus, Transactions of the
Fig. 2. The heterotropic deviation may be sufficiently small to cause the object of regard to be diplopic, but the peripheral objects displaced equidistant to the object of regard from the horopter may still be within Panum”s area. Thus, it is possible for a patient with a small heterotropic deviation to have extramacular fusion in the absence of macular fusion. (Symposium on Strabismus, Transactions of the
ADAPTATIONS TO STRABISMUS
PHYSICAL ADAPTATIONS
The onset of strabismus in a person who has developed extramacular binocular vision produces three distinct, annoying symptoms: central diplopia, peripheral diplopia, and peripheral visual confusion. Two factors offer partial relief from central diplopia. One is the perceptual difference in the sharpness and clearness of the two similar images since one is projected onto the fovea and the other onto an extramacular area. This difference, however, pertains only to visual stimuli with contour value that are viewed during the photopic state. To compensate for this visual circumstance, important clues are provided for the strabismic patient to locate the object of regard correctly in space. However, when using scotopic vision or when viewing a contourless object such as a light, the patient does not have these differential clues of sharpness and clearness for the object of regard; consequently, the diplopia persists unabated.
Secondly, the ability to be attentive to only one of the images of the object of regard is subconsciously enhanced by a rapid blinking of the non-dominant eye. The image that correctly localizes the object of regard is immediately identified, and as long as the patient’s attention remains attached to this image, correct localization of the object continues. This functions satisfactorily as long as the object of regard remains unchanging, but with scotopia, if the patient is viewing a series of changing contourless objects such as multiple oncoming headlights during nighttime driving, only sustained voluntary closure of the nondominant eye may avert catastrophe.
CORTICAL ADAPTATIONS
Eventual relief from the troublesome symptoms of central and peripheral diplopia and peripheral visual confusion is obtained for some patients through cortical adaptations that occur within the neurophysiology of extramacular binocular vision. Development of these complex adaptations is easy for the very young strabismic patient; children over 10 years of age are capable of slowly acquiring them. As already mentioned, once extramacular binocular vision is acquired, it is never surrendered. Also, once single binocular vision has been enjoyed, the patient would like to maintain it; however, the young patient successfully develops suppression and anomalous retinal correspondence (ARC), the adaptations necessary to permit continuation of single extramacular binocular vision. Older patients are more inclined to be permanent victims of the annoying symptoms caused by their continuing binocular vision.
AMBLYOPIA
A wide variety of ocular disorders in infancy or childhood may be responsible for abnormal visual experience that causes amblyopia. Traditionally clinicians have classified persons with amblyopia into several categories on the basis of apparent etiology (strabismus, anisometropia, isoametropia, visual deprivation due to adnexal normality or media opacity, and so on).
Only recently, however, has evidence from laboratory investigations begun to justify the assumption that different pathophysiologic disturbances underlie the occurrence of amblyopia in different clinical settings. Some of these findings will be reviewed in later sections of this chapter. Distinctions tend to blur in practice because of the frequent coincidence of multiple causal disorders in a single patient. There also seems to be considerable heterogeneity even in cases with a “pure” etiologic basis, possibly attributable to variation in the proportionate contributions of the different amblyopiogenic mechanisms described above to visual loss, or to varying age of onset or duration of the condition.
With an overall prevalence of 2% to 4%, amblyopia is the most frequent cause of visual impairment in children and young adults in our population.6 A majority of cases are due to strabismus with constant unilateral fixation, which leads to amblyopia in the deviating eye. Amblyopia generally does not develop if fixation alternates, providing each eye with similar access to higher visual centers, or if strabismic deviation is intermittent (as a result of fusional vergence or incomitance), so that there are periods of normal binocular interaction that preserve the integrity of the visual system. Chronic rejection of disparate input from the deviating eye by the binocular visual centers seems to be the principal factor responsible for strabismic amblyopia, but blurring of its foveal image due to inappropriate accommodation (determined by the distance from the viewer of the fixating eye’s object of regard) also may contribute.
Amblyopia is considerably more common in persons who are esotropic than in those who are exotropic, and it can often be determined that an older amblyopic, exotropic patient actually developed the condition while esotropic at an earlier age. This observation is probably a reflection of the fact that most young persons with exodeviation are only intermittently tropic. The occasionally encountered child with constant exotropia is as likely to develop amblyopia as an esotropic one of the same age.
Uncorrected refractive error is the second most common cause of abnormal visual experience leading to amblyopia, although it develops in only a minority of cases. Amblyopic visual loss becomes evident when optical correction is provided. Unilateral blur resulting from anisometropia may be the sole source of amblyopia (acting through either or both possible amblyopiogenic mechanisms), or it may interact with strabismus by determining a constant preference for fixation
with the less ametropic eye. Modest degrees (+ 1 to +3) of unilateral excess hyperopia may cause mild to moderate amblyopia, especially when there is significant hyperopia in the less ametropic eye as well.7 When the more ametropic eye is mildly myopic, with a far point that approximates normal near viewing distance, amblyopia generally does not develop. Bilateral myopic shift during late childhood or adolescence may account for the occasional finding of amblyopia in the emmetropic eye of an adult with unilateral myopia, which during early childhood was more hyperopic than its fellow. AMBLYOPIC VISION
The visual defect in amblyopia is complex and distinctive. Patients with unilateral amblyopia find it easy to recognize a qualitative difference between the vision in their normal and amblyopic eyes even when acuity is rendered equal by optical blur or diffusion. However, precise characterization of amblyopic vision remains difficult for clinicians and even laboratory researchers. It is still not generally possible to determine definitely whether amblyopia is present on the basis of vision testing alone.
Amblyopia primarily affects spatial or form vision, although abnormalities of the light sense may also be found in at least some cases. The following discussion considers a number of specific aspects of amblyopic vision, with emphasis on points that either are important in clinical practice or seem to shed light on the pathophysiology or anatomic localization of the underlying visual system disturbance.
ACCOMMODATION
The function of accommodation is to bring retinal images into sharp focus, a state with which the amblyopic eye often lacks experience and which it may have difficulty recognizing. It is therefore not surprising that amblyopia impairs the ability to control accommodation, typically resulting in a subnormal response.50,51 Moderate hyperopia that would not be expected to affect the vision of a normal eye, which will accommodate to sharpen its retinal image, may cause significant optical blur in association with amblyopia.
Baseline acuity should be determined for an amblyopic eye with full hyperopic correlation under cycloplegia. Even if there is initially no difference between corrected and uncorrected acuity, glasses should be considered for substantial degrees of hyperopia to eliminate a possible obstacle to successful treatment.
STRABISMIC AMBLYOPIA
Efforts to improve amblyopic vision are best made before surgical correction of strabismus. At this stage, parental motivation, a key factor in successful therapy, generally is greatest. In the preverbal child, the effectiveness of treatment usually can be assessed only by observing each eye’s frequency of deviation and ability to hold fixation, and this information becomes unavailable once the eyes are straight. Preoperative sensory optimization may aid in achieving the surgical goal of good, stable ocular alignment with some degree of binocular cooperation, whereas postoperative occlusion therapy for amblyopia may interfere with the establishment or maintenance of useful binocularity. Only rarely is the need for surgery obviated by eliminating amblyopia.
OCCLUSION
Full-time occlusion of the preferred eye is the most effective method for treating strabismic amblyopia. The occluded eye is deprived of all form vision throughout the waking day except for a brief period, during which the child has an opportunity to maintain the occluded eye’s ability to fixate and the parents are able to judge the degree of preference for one eye or the other.
OPTICAL BLUR
In some cases, amblyopia can be effectively treated by optically degrading the quality of the preferred eye’s retinal image (often referred to as penalization). The most common means of accomplishing this is producing cycloplegia in the nonamblyopic eye with atropine (Fig. 2). Blur also may be generated with spectacle or contact lenses.
Fig. 2. Treatment of strabismic amblyopia with atropinization of the good eye. Notice the blur-induced shift of fixation to the amblyopic eye.
Optical blur has several advantages over occlusion. The devices used are not irritating to the skin and are cosmetically more acceptable than occlusive patches. With atropinization, it is difficult for the child to thwart efforts at treatment. Because blur is a less-potent modality than occlusion, follow-up evaluations need not be as frequent. Finally, unlike occlusion, blur does not preclude the possibility of binocular vision, so that when the eyes are aligned, peripheral fusion can continue to be exercised during treatment (see later).
The principal limitation of optical blur in the treatment of amblyopia is that in severe cases, it may not be possible to degrade vision in the preferred eye sufficiently to cause the patient to fixate with the amblyopic eye. If fixation is maintained with the nonamblyopic eye in a strabismic patient, therapy will fail. In the amblyope with straight eyes, measured acuity in the preferred eye must be reduced below the level of the amblyopic eye, but this in itself does not ensure that treatment will succeed. Some amblyopes prefer to use the blurred eye, even when its Snellen acuity is poorer than that of the amblyopic eye.
ORBITAL DISEASE
Orbital disorders typically present in one of five ways and the type of presentation is helpful in steering the examiner to the correct anatomic location and hence the correct differential diagnosis.
PROPTOSIS. Proptosis implies an axial protrusion of the globe from the bony socket. It is helpful in many cases to determine if the globe is being displaced in another direction, since a lesion presenting in one quadrant of the orbit will frequently displace the globe into the opposite quadrant as well as producing a degree of proptosis. For example, lacrimal gland tumors frequently displace the eye infranasally in addition to creating proptosis.
Pseudoproptosis can occur if the pathologic side is enophthalmic. Occasionally this may cause confusion. The most frequent cause of enophthalmos is blowout fracture, but metastatic breast carcinoma may cause enophthalmos.18
PAIN. Patients with orbital disorders frequently complain of pain or discomfort on the affected side. The pain may range in severity from very mild to very severe. The quality of the pain is an important characteristic. Burning, scratching, and irritation are frequently described in patients presenting with Graves’ disease and are often related to the desiccation of the cornea and conjunctiva. Patients with inflammatory lesions, whether benign or malignant, frequently complain of an aching, throbbing, or boring pain that resides behind the eye and may radiate into the forehead, cheek, or temporal areas. Hyperesthesias, in the distribution of the supraorbital nerve or the infraorbital nerve, are encountered following orbital trauma, especially during the recovery period as the initial posttraumatic hypoesthesia is resolving.
DIPLOPIA. Diplopia is a common symptom in orbital disorders, related to a paralysis of the extraocular muscles or a restriction of ocular movement. Typically, lesions residing in the cavernous sinus or posterior orbit are responsible for paralytic abnormalities of ocular movement. More anterior orbital lesions typically produce diplopia by means of a mechanical effect. This mechanical restriction of function in the extraocular muscles may be caused by lesions that are immediately adjacent to the extraocular muscles or diseases that involve the muscle tissue, such as myositis and Graves’ disease.
DECREASE IN VISUAL ACUITY. Loss of visual acuity usually implies involvement of the optic nerve, either by compression, infiltration, vascular compromise, or inflammation. Signs of optic nerve dysfunction, such as relative afferent pupillary defect, loss of color vision, visual field cuts, and increased latency in the visually evoked responses, may be identified.
LID MALPOSITION. Lid malpositions are less common but may be associated with orbital masses in the anterior extraconal space. For example, lateral ptosis of the upper eyelid is often associated with lacrimal gland masses. Superior orbital tumors may produce upper eyelid ptosis because of interference with the normal function of the levator muscle. Lid retraction of the upper and lower eyelids is one hallmark of
ORBITAL EXAMINATION
The orbital examination consists of the routine ophthalmic examination plus some specialized evaluations that are pertinent to orbital disease.
ROUTINE
VISION. The vision is an integral part of any orbital examination. Visual acuity provides an indicator of the extent of orbital disease and decreased vision suggests involvement of the optic nerve or globe. Vision gives a baseline from which to measure progression of disease and is a medical-legal necessity. This is all the more so in patients presenting with orbital trauma. It is vital to perform a visual acuity examination before taking trauma patients to surgery even if this requires using a Desmarres retractor to open the swollen eyelids. Non-ophthalmic surgeons who are called on to operate for trauma in the orbital area frequently neglect this vital part of the orbital examination.
PUPILS. Examination of the pupils is very important and may provide information about the underlying orbital condition. The small pupil of a Horner’s syndrome may be the clue that one is dealing with a pseudoenophthalmos. The large pupil of a third cranial nerve palsy indicates that the associated diplopia is due to a paralytic abnormality as opposed to a restrictive phenomenon. The presence of a relative afferent pupillary defect is confirmation that the optic nerve is compromised by orbital disease, even though many patients have an optic nerve head that appears normal on funduscopic examination.
EXTRAOCULAR MOVEMENTS. The cross cover test is invaluable for assessing ocular position in primary position, upgaze, downgaze, right gaze, and left gaze. In most orbital disorders the deviation will vary in different positions of gaze. The non-concomitance of strabismus associated with orbital disease helps to distinguish it from other common forms of strabismus. It is important to note abnormal ductions since a slight limitation in one eye may be the only evidence of an abnormality and may be very useful in localizing the orbital lesion,
SLIT LAMP EXAMINATION. Slit lamp examination is most useful for assessing the status of the cornea which may demonstrate exposure keratopathy. caused by proptosis or lid retraction. Many orbital disorders cause dilatation of conjunctival vessels and conjunctival edema. These findings are relatively nonspecific, but large tortuous conjunctival veins may be a sign of carotid cavernous fistula.
INTRAOCULAR PRESSURE. Intraocular pressure may be elevated in patients with orbital disease, through a variety of mechanisms. Carotid cavernous fistulas cause an increase in episcleral venous pressure, and therefore impede aqueous outflow. Mass lesions may press directly against the globe, especially when a retrobulbar hemorrhage causes a sudden expansion of the mass. Restriction of the extraocular muscles, such as may occur with blowout fracture or thyroid eye disease, produces a factitious elevation of intraocular pressure when the eye attempts to rotate against the restricted muscle. In cases of mild thyroid eye disease, the progressive increase in intraocular pressure with elevation of the eye may help to confirm the diagnosis.
FUNDUS EXAMINATION. Fundus examination is important in order to assess the optic nerve. Tumors may compress the optic nerve, causing disc edema or optic atrophy. Optic nerve meningiomas may produce shunt vessels on the disc. Choroidal striae may be noted if a mass is indenting the globe. On occasion intraocular tumors may extend through the sclera into the orbit, and the correct diagnosis is made when the intraocular tumor is identified.
COLOR VISION. Assessment of color vision is an important test for optic nerve dysfunction although one must be careful not to attribute optic nerve dysfunction to patients who have a congenital abnormality of color vision. Sophisticated tests for color vision measurement, such as the Farnsworth-Munsell test, are not generally required for clinical assessment of optic nerve dysfunction. The Ishihara color plates are satisfactory for clinical use.
SPECIALIZED ORBITAL EXAMINATION
Inspection
Inspection of the face and more specifically the periocular adnexa is the simplest of all examinations and yet this inspection is frequently overlooked by neophyte examiners. It is important to look at the entire face in order to get a sense of the facial proportion and symmetry.
GLOBE DISPLACEMENT. Globe displacement (horizontal or vertical) does not always result in diplopia. Even so, patients may present with the complaint that the eye does not “look right.” It is important to determine whether the apparent displacement of the globe is real or illusory. Posttraumatic telecanthus or large congenital epicanthal folds may create the false impression that eyes are displaced medially. Tripod malar fractures may produce a displacement of the lateral canthal complex and the illusion that the eye is abnormally positioned, when the true anatomic abnormality involves the zygoma and eyelids. Vertical displacement of the globe is frequently seen after blowout fractures or orbital decompression. Orbital roof fractures or tumor masses present in the superior orbit may cause the globe to be displaced inferiorly. This displacement may or may not be associated with ipsilateral ptosis and/or decreased supraduction. The horizontal position of the globes is measured from the center of the bridge of the nose using a ruler to determine the distance from the midline to the center of the pupil (Fig. 1). Previous trauma or surgery to the nose may make this measurement unreliable.
Fig. 1. The ruler is held horizontally across the bridge of the nose. The distance from the midline to the pupil is recorded on each side.
Vertical displacement of the globe is evaluated by placing a ruler in a horizontal position across the bridge of the nose. The relative positions of the globes may be assessed and the vertical displacement can be either measured or estimated in millimeters. Once again significant trauma to the midface renders this measurement inaccurate.
COLOR. The color of the eyelids may provide a clue to the underlying orbital pathology. In many cases there will be no discoloration of the eyelids. However, inflammatory lesions often cause an erythema of the eyelids. This redness may be associated with edema and tenderness. Bruising of the eyelids is most frequently seen following trauma but may be associated with hemorrhage into an underlying orbital lesion. Spontaneous hemorrhage most frequently occurs with lymphangioma and hemangioma but may be seen with other tumors, such as childhood neuroblastoma. Lymphangiomas and deep seated hemangiomas have a bluish tint. More superficial vascular lesions are red or maroon.
PULSATIONS. Pulsation of the eyelids and orbital contents is an infrequent finding, and may be easier to appreciate with palpation rather than visual inspection. Pulsation is most commonly seen ieurofibromatosis, where the absence of sphenoid bone allows brain pulsations to be transmitted to the orbital contents. Extremely vascular tumors may occasionally pulsate due to the high blood flow in these lesions.
Palpation
Palpation of the orbit is a simple maneuver that often provides important information. The upper and lower eyelids should be palpated gently. It is possible to insert the tip of a small finger between the globe and the orbital rims on all four sides so that the anterior orbital contents may be palpated. In this way abnormalities of the lacrimal gland or the lacrimal sac can be easily identified, and abnormalities of the orbital rim are easily appreciated. This technique is especially useful for trauma patients who may have a step deformity of the orbital rim.
Retropulsion of the globes is a test that has received considerable attention. This test is performed by placing the fingers over both eyes and gently pushing them in a posterior direction. A firm lesion present behind the globe will resist retropulsion and this sensation is palpable. Unfortunately, this test is nonspecific and has little diagnostic value.
Digital examination of the superior fornix is a particularly useful technique for evaluating superior orbital lesions. In order to perform this examination the conjunctival sac is anesthetized with a topical agent. A gloved finger is gently inserted behind the upper eyelid into the superior fornix (Fig. 2). The finger is moved medially and laterally, anteriorly and posteriorly in order to palpate
Fig. 2. The conjunctival sac is anesthetized. A gloved finger is inserted into the superior fornix. The patient is more comfortable in downgaze. Palpation of the superior orbital mass is gently performed.
any lesions present in the superior part of the orbit. Bimanual palpation is sometimes helpful (Fig. 3). It is useful to have the patient look down during the examination since this position is more comfortable. In addition, downward rotation of the globe will sometimes bring an orbital mass more anteriorly so that it may be more readily palpated. Small orbital rim fractures may also be identified in this way. Lacrimal gland abnormalities may be easily palpated. This technique is particularly useful if the upper eyelid is swollen, making palpation of the orbital mass through the eyelid difficult. The same technique may be used in the inferior fornix.
Fig. 3. Bimanual palpation is sometimes helpful in more anterior lesions.
Sensation
The first and second divisions of the trigeminal nerve provide sensation to the face in the orbital area. The ophthalmic branch provides sensation in the forehead, upper lid, and globe, whereas the maxillary division via the infraorbital nerve gives sensation to the cheek and upper lip. Sensation is usually tested by touching these areas with a wisp of cotton and asking if the patient is able to feel the touch. It is useful to compare one side to the other since the sensory deficit is incomplete in many cases of orbital disease. Hypoesthesia of the cheek and lip is a typical finding in patients with blowout fractures because of injury to the infraorbital nerve as it travels through its bony canal in the orbital floor.
Exophthalmometry
Exophthalmometry is an accurate technique for the measurement of proptosis. This measurement may be accomplished in several ways, but in each case the reference points are the lateral orbital rim and the apex of the cornea. The generally accepted normal value is less than
Fig. 4. The Hertel exophthalmometer rests against the lateral orbital rims. By means of mirrors, the distance from the lateral orbital rim to the apex of the cornea is measured for each side.
Although the Hertel exophthalmometer is the most widely used instrument, exophthalmometry readings may be obtained by simply placing a transparent ruler on the lateral orbital rim and measuring the position of the cornea. Inspection from behind the patient looking over the forehead down onto the proptosed globes is also a way to estimate proptosis (Fig. 5).
Fig. 5. The examiner stands behind the patient and looks down over the forehead. The amount of proptosis is estimated.
Forced Duction Test
The forced duction test is a useful test for distinguishing paralytic strabismus from restrictive strabismus. The forced duction test is used to detect an inelastic extraocular muscle that prevents the globe from rotating. For example, a tight inferior rectus muscle will not allow the globe to rotate superiorly, The test is carried out by placing topical anesthetic into the conjunctival sac. The globe is grasped with a pair of toothed forceps at either the limbal area (where Tenon’s capsule is tightly adherent to the globe) or the insertion of the extraocular muscle and an attempt is made to “force” the globe to rotate in the desired direction (Fig. 6). Restriction may be felt when the globe reaches the end of its “tether.” (For some patients, especially in the pediatric age group, application of the forceps to the globe is too frightening or painful. In these situations the forceps may be replaced with a cotton swab and the globe gently pushed.) The forced duction test is particularly useful for differentiating between a medial wall blowout fracture with medial rectus entrapment and a sixth cranial nerve palsy.
Fig. 6. The conjunctiva is anesthetized and grasped with a toothed forceps at the limbus. The globe is rotated (in this case upward) until it reaches the end of its “tether.” This restriction of movement can be felt and constitutes a positive forced duction test.
Prepared by Ph. D.,
assistant of professor Ostrovsks L.O.
Adopted at the chair sitting « » ________ 2006
