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

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FIGURE 4-4A,B. (B) Pathology of amblyopia in monkey striate cortex (visual cortex). Well-defined cortex dominance columns are seen in normal specimen (B1 figures), but cortex columns are underdeveloped in specimen for amblyopic monkey (B2 figures).21

Unilateral Pattern Distortion Amblyopia

Unilateral, or asymmetrical, retinal image blur can produce amblyopia and loss of binocularity depending on the severity of the condition. The ophthalmic literature often refers to amblyopia associated with monocular image blur as “pattern deprivation amblyopia.” This term is misleading, because unilateral image blur results in pattern distortion and cortical suppression, both of which contribute to the amblyopia.

Clinically, mild image blur (e.g., blur associated with mild anisometropia) causes mild anisometropic amblyopia and allows for the development of peripheral fusion and stereopsis (i.e.,

TABLE 4-3. Classification of Amblyopia.

A.Strabismic amblyopia (suppression)

1.Congenital esotropia

2.Congenital exotropia

3.Acquired constant tropia in childhood

4.Accommodative esotropia

5.Small-angle tropia (monofixation syndrome)

6.Intermittent exotropia (rarely associated with amblyopia)

B.Monocular pattern distortion (suppression and pattern distortion)

1.Anisometropia

a.Hyperopia 1.50

b.Myopia 3.00

c.Meridional 1.50

2.Media opacities

a.Unilateral cataract

b.Unilateral corneal opacity (Peter’s anomaly)

c.Unilateral vitreous hemorrhage or vitreous opacity

C.Bilateral pattern distortion (pattern distortion)

1.Ametropia

a.Bilateral high hypermetropia 5.00

b.Bilateral meridional (astigmatic) 2.50

2.Media opacity

a.Bilateral congenital cataracts

b.Bilateral corneal opacities (Peter’s anomaly)

c.Bilateral vitreous hemorrhages

monofixation syndrome). A significant blurred image during infancy (e.g., unilateral congenital cataract or corneal opacity), however, can result in severe amblyopia. Vision can be as poor as count fingers with total loss of binocular function manifested by the development of sensory strabismus.

Anisometropic amblyopia, one of the most common types of amblyopia, is caused by a difference in refractive errors that results in a unilateral or asymmetrical image blur. Most patients with anisometropic amblyopia have straight eyes and appear “normal,” so the only way to identify these patients is through vision screening. Stereoacuity testing has had limited value in screening for anisometropic amblyopia because most patients have relatively good stereopsis (between 70 and 3000 s arc). Patients with anisometropic amblyopia usually have peripheral fusion, and most have the monofixation syndrome.35 Myopic anisometropia is generally less amblyogenic than hypermetropic anisometropia. As little as 1.00 hypermetropic anisometropia and 2.00 myopic anisometropia can be associated with amblyopia.51 Astigmatic anisometropic amblyopia does not occur unless there is a unilateral astigmatism greater than 1.50 D.51

For practical purposes, however, we do not see significant anisometropic amblyopia unless differences between the two eyes are greater than 1.50 in hyperopes and greater than 3.00 in myopes. Myopic anisometropic amblyopia is often amenable to treatment even in late childhood whereas hypermetropic amblyopia is often difficult to treat past 4 or 5 years of age, probably because high myopia is usually acquired after the critical period of visual development, and the more myopic eye is in focus for near objects (a baby’s world is up close). In contrast, patients with hypermetropic anisometropia always use the less hypermetropic eye because it requires less accommodative effort and constantly suppress the more hypermetropic eye.

Bilateral Blurred Retinal Image

Pattern distortion in its pure form without suppression occurs when there is bilateral symmetrical image blur and no strabismus. Clinically, the effects of pure image blur are seen in cases of bilateral high hypermetropia or bilateral symmetrical astigmatism, or with bilateral ocular opacities such as bilateral congenital cataracts and bilateral Peter’s anomaly. Bilateral pattern distortion causes bilateral poor vision. Depending on the extent of the distortion, some binocular fusion can develop, usually associated with gross stereopsis. If severe image blur occurs during the neonatal period so that essentially no pattern stimulation is provided, extremely poor vision and sensory nystagmus develop. Bilateral amblyopia and nystagmus will occur in cases of dense bilateral congenital opacities unless the image is cleared by 2 months of age. This type of nystagmus is called sensory nystagmus and is associated with bilateral severe amblyopia, or other causes of congenital blindness such as macular or optic nerve pathology. Sensory nystagmus does not occur with cortical blindness because extrastriate visual pathways anterior to the occipital cortex supply the fixation reflex. Acquired opacities after 6 months of age usually do not cause sensory nystagmus because the motor component of fixation has already been established. The presence of sensory nystagmus indicates severe amblyopia, usually 20/200 visual acuity or worse.

Ametropic amblyopia (bilateral hypermetropic amblyopia) usually occurs with hypermetropia greater than 5.00 D without significant anisometropia.36 In these cases, visual acuity is decreased in each eye, the eyes are usually straight, and the patients usually have gross stereopsis. When patients are first

given their optical correction, visual acuity does not significantly improve. The lack of improvement with spectacle correction often leads the examiner to seek an organic cause for the decreased vision. The treatment of bilateral high hypermetropic amblyopia is to prescribe full hypermetropic correction. In most cases, visual acuity will slowly improve if the glasses are worn full-time, with final visual acuity usually in the range of 20/30 to 20/25 achieved over a period of 6 months to a year.

Bilateral meridional amblyopia is caused by bilateral astigmatism and, like bilateral hypermetropic amblyopia, is secondary to pattern distortion. Significant meridional amblyopia occurs with astigmatism greater than 2.50 D. To avoid meridional amblyopia, astigmatisms of 2.50 D or more should be treated in preschool children, and astigmatisms over 3.00 D to 4.00 D should be treated in infants.

Amblyopic Vision

The visual deficit associated with amblyopia has certain unique characteristics, including the crowding phenomenon, the neutral density filter effect, and eccentric fixation. The crowding phenomenon relates to the fact that patients with amblyopia have better visual acuity reading single optotype than reading multiple optotypes in a row (linear optotypes). Often, patients with amblyopia will perform 1 or 2 Snellen lines better when presented with single optotypes versus linear optotypes. This crowding phenomenon may have something to do with the relatively large receptive field associated with amblyopia. Crowding bars are often used around a single optotype to provide a more sensitive test for amblyopia.

A neutral density filter reduces overall luminance without inducing a color change. Decreased luminance of the visual target results in diminished central acuity in normal eyes. Decreased illumination of visual targets has less of an effect on amblyopic eyes because they are not using central acuity. The intraocular differences in visual acuity between the amblyopic eye and the sound eye diminish when the patient looks through a neutral density filter that lowers the luminance of the visual target. For example, a patient with a left amblyopia has 20/20 vision in the right eye and 20/60 in the left eye under photopic conditions (4 lines difference). He may have visual acuities of 20/50 right eye and 20/60 left eye under scotopic conditions (1 line difference).

A

B

FIGURE 4-5A,B. Eccentric fixation. (A) Sound eye fixes with the fovea (left) and the amblyopic eye eccentrically fixates in an area of fixation (right). (B) Right eye is covered, and eccentric fixation persists with patient viewing in an eccentric area.

All amblyopes have some degree of extrafoveal fixation. Mild amblyopes (20/40–20/100) fixate so close to the fovea that they appear to fixate centrally. Severe amblyopes, usually 20/200 to count fingers, use a large parafoveal area for viewing (Fig. 4- 5). This area of eccentric fixation is not a pinpoint location but a general area of viewing.

The presence of eccentric fixation is a clinical sign of severe amblyopia and has a poor visual prognosis. Remember that anomalous retinal correspondence is quite different from eccentric fixation. Anomalous retinal correspondence (ARC) is a binocular sensory adaptation to strabismus that allows acceptance of images on noncorresponding retinal points. ARC is only active during binocular viewing and, when one eye is covered, fixation reverts back to the true fovea. Eccentric fixation, on the other hand, is dense amblyopia without foveal fixation and is present under monocular or binocular conditions.

FIGURE 4-8. Measuring fixation preference. Patient has strong fixation preference for the left (left figure) and amblyopia in the right eye. Temporarily covering the left eye (center figure) forces fixation to the right eye, but when the cover is removed, the patient refixates to the left eye (right figure). This indicates strong fixation preference, i.e., amblyopia.

Patients with a fixation preference may have amblyopia. The strength of fixation preference indicates if amblyopia is present, with the weaker preference for one eye being the amblyopic eye. Fixation preference can be quantified by briefly covering the preferred eye to force fixation to the nonpreferred eye. Remove the cover from the preferred eye, then observe how well and how long the patient will maintain fixation with the nonpreferred eye before refixating back to the preferred eye. If fixation immediately goes back to the preferred eye after the cover is removed, then this indicates strong fixation preference for the preferred eye and amblyopia of the deviated eye (Fig. 4-8). However, if the patient maintains fixation with the nonpreferred eye through smooth pursuit, through a blink, or for at least 5 s, there is mild fixation preference and no significant amblyopia (vision within 2 Snellen lines difference) (Fig. 4-9). The ability to maintain fixation with the nonpreferred eye while following a moving target is a very reliable indicator of equal vision and detects no significant amblyopia.

The reliability of fixation preference testing for diagnosing amblyopia has been shown to be quite good in patients with large-angle strabismus, more than 10 to 15 PD.62 Patients with small-angle strabismus, however, will show strong fixation