Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

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FIGURE 3-7A–C. Diagonal lines are presented to each eye with the lines oriented 90° to each other (A,B). The combined binocular perception is a patchy pattern, with lines from each eye being seen; however, because of rivalry, crossing lines are not seen (C).

nism to keep the eyes aligned on visual targets as they move through space. Motor fusion also controls innate tendencies for the eyes to drift off target. These correctional eye movements that maintain binocular foveal alignment provided by motor fusion are termed fusional vergence movements.

Unlike version movements, in which both eyes move in the same direction, vergence eye movements are in the opposite direction; they are termed “disjunctive” and disobey Hering’s law. Convergence, for example, is invoked when one eye follows an object moving from distance to near and results in both eyes moving to the midline with the right eye moving left and the left eye moving right (Fig. 3-8A). You can experience convergence by fixating on a pencil at arm’s length and slowly bringing the pencil to your nose. As the pencil approaches your nose, the eyes converge to hold alignment on the pencil. Convergence movements are the strongest vergence

movements, and there are several mechanisms that contribute to convergence (see Vergence Amplitudes, following).

In addition to convergence, there are two other vergence movements: divergence and vertical vergence (Fig. 3-8B,C). Divergence is used to follow an object moving away and consists of the right eye moving right and left eye moving left. Vertical vergence is the weakest vergence movement and keeps our eyes from drifting vertically. Vertical vergence is depression of one eye with elevation of the fellow eye.

Measurement of vergence amplitudes and a discussion of the various mechanisms of convergence are presented next.

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FIGURE 3-8A–C. Vergence. (A) Convergence of the eyes as the pencil approaches from the distance. (B) Divergence as the patient changes fixation from a near target to a distance target. (C) Vertical vergence, as the patient vertically aligns the eyes to compensate for the vertical phoria or an induced deviation produced by a vertical prism.

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FIGURE 3-10A,B. Alternate cover test in patient with an esophoria. (A) Eyes are straight; the patient has a tendency to cross (esophoria), but fusional divergence maintains proper alignment. (B) Left eye is covered, dissociating fusion and allowing the left eye to manifest the esophoria. Note that the left eye turns in under the cover.

existent, as virtually all normally sighted people, with normal bifoveal fusion, have a small phoria but maintain alignment through motor fusion. Thus, most normal people are orthotropic but heterophoric.

Phorias may spontaneously become manifest under conditions such as fatigue or illness that can cause central nervous system depression and diminish motor fusion. Central nervous system depressants also diminish motor fusion, and a patient with a large phoria may manifest their deviation after imbibing alcoholic beverages or taking sedatives. (This explains why the cowboy sees double after celebrating in town with one too many whiskies.) A large phoria that is difficult to control may spontaneously become manifest, and this is called an intermittent tropia.

Strabismus most commonly occurs in infancy or childhood and is usually idiopathic or related to a refractive error. In most of these cases, the eye muscles are normal and the eye can rotate freely. Less often, mechanical restriction of eye movements (restrictive strabismus) or an extraocular muscle paresis (paralytic strabismus) causes the strabismus. A blind eye may also drift, and this is termed sensory strabismus.

FIGURE 3-12. Alternating exotropia. Top diagram: right eye is fixing with the left eye and the image falling temporal to the fovea. Bottom diagram: exotropic left eye is fixing with a right exotropia and the image falling temporal to the right fovea.

turns out and the target image is temporal to the fovea (Fig. 3- 12). Note that fixation can switch from eye to eye. According to Hering’s law, as the deviated eye moves into primary position, the fixing eye turns in the same direction to become the deviated eye (compare upper and lower drawings of Figs. 3-11 and 3-12).

Vertical strabismus can be categorized as hypertropia or hypotropia. Because of Hering’s law, a left hypertropia is the same deviation as a right hypotropia, depending on which eye is fixing (Fig. 3-13). In contrast to a horizontal deviation, when

FIGURE 3-13. Alternating left hypertropia. Top left: right eye fixing and a left hypertropia. Top right: retinal image (x) is falling on the fovea (small dot) of the right eye; however, the left fovea is rotated down (left hypertropia) so the retinal image (x) is located above the fovea (small dot). Bottom: left eye fixing with right eye turned down. Now the retinal image (x) falls below the right fovea, which is rotated up (right hypotropia).

describing a hyperdeviation we must identify which side the hypertropia is on, either right hypertropia (RHT) or left hypertropia (LHT). By convention, we usually refer to a vertical deviation as a hypertropia, rather than use the term hypotropia, unless there is an obvious restriction or paresis that keeps one eye in a hypotropic position. This convention has practical importance as it minimizes confusion over which terminology is used, thus reducing the risk of inadvertently operating for a right hypotropia when the patient actually had a right hypertropia.

Cyclotropia, or torsion, refers to a twisting misalignment around the Y axis of Fick. Excyclotropia (extorsion) is a temporal rotation of the 12 o’clock position, whereas incyclotropia (intorsion) means a nasal rotation of the 12 o’clock position. Normally the fovea should be aligned between the middle and the lower pole of the optic disc (Fig. 3-14, top). If the fovea is below the lower pole of the optic disc by direct view (above the disc in the indirect ophthalmoscopic view), this indicates objective extorsion (Fig. 3-14, bottom left). A fovea oriented above the middle of the optic disc by direct view (below the middle in the indirect ophthalmoscopic view) indicates intorsion (Fig. 3-14, bottom right). Torsion can also be measured by the Maddox rod test, and this is termed subjective torsion. Torsional motor

Prisms and Strabismus

Prisms are important tools for the diagnosis and treatment of strabismus, as they are used to measure and neutralize ocular deviations. A prism bends light toward the base of the prism (Fig. 3-15) because light has both particle and wave characteristics. As light passes through the prism, the part of the light wave closest to the prism base has more prism to traverse than the part of the wave closest to the apex. This is analogous to a row of soldiers marching through a triangle of sand; the soldiers walk slowly through sand so those at the base of the triangle exit the sand after the soldiers at the apex. The direction of the marching soldiers turns toward the base of the triangle as they exit.

The ability of a prism to bend light is measured in prism diopters (PD). Light travels slower through the plastic prism than it does through air, so light toward the base of the prism takes longer to exit than light traversing the apex. The exit time differential causes the light to bend toward the base of the prism. One prism diopter will shift light 1 centimeter (cm) at 1 meter

(m) or a displacement of approximately 0.5°. A 20 PD esotropia

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FIGURE 3-15A–C. Diagram of the effect of a prism over one eye. (A) Patient fixates on the X. (B) A prism is introduced, and the image is displaced toward the base of the prism and off the fovea. Note that the patient will perceive the image to jump in the opposite direction. Thus, a patient will perceive the image to jump in the direction of the apex of the prism. (C) Patient refixates to place the image on the fovea by rotating the eye toward the apex of the prism. Note that when a prism is introduced, the patient will always refixate by rotating the eye in the direction of the apex of the prism.

would mean the eye turns in approximately 10°. When a prism is placed in front of one eye, it moves the image off the fovea, causing a perceived image “jump.” The retinal image will shift toward the base of the prism, but the perceived image jump is in the opposite direction, toward the apex of the prism; this is because the retinal images are reversed, right/left and up/down (Fig. 3-15A,B). To refixate on the shifted image, the eye will move in the direction of the prism’s apex, thus aligning the fovea with the new image location (Fig. 3-15C).

Prism Neutralization of a Deviation

Prisms can be used to optically neutralize or correct strabismus. A prism acts to change the direction of the incoming image so the retinal image in each eye falls directly on the fovea. Neutralization occurs when enough prism is placed in front of the eye so the two foveas are aligned on the same object of regard. For example, when a base-out prism (prism held horizontally with the apex directed toward the nose) is placed in front of the deviated eye of a patient with esotropia, the retinal image shifts temporally toward the fovea (Fig. 3-16). If the correct amount of prism is used, the retinal image will fall directly on the fovea of the deviated eye. Thus, as seen in Figure 3-16B, the deviation has been optically neutralized by the prism even though the eye is still anatomically deviated.

The rule for neutralizing a deviation is to orient the prism so the apex is in the direction of the deviation. For esotropia, the apex is directed nasally and, for exotropia, the apex is directed temporally. The apex is directed superiorly over a hypertropic eye and inferiorly over a hypotropic eye.

The prism can also be placed in front of the fixing eye (straight eye) to neutralize the deviation. If the prism is placed base-out in front of the fixing eye (Fig. 3-17), the retinal image will move temporal to the fovea (Fig. 3-17A,B). The fixing eye will see the image shift and will immediately rotate nasally to reestablish foveal fixation (Fig. 3-17). As the fixing eye rotates nasally, the deviated eye rotates temporally causing a version movement to the right (Fig. 3-17B). Therefore, when a base-out prism is placed in front of the fixing eye, both eyes move in the same direction as the apex of the prism, and both foveas shift into alignment (Fig. 3-17B,C). In Figure 3-17C, both eyes have shifted to the right, with the left eye now turned in nasally and the right eye now straight in primary position. The previously