Ординатура / Офтальмология / Английские материалы / Oxford American Handbook of Ophthalmology_Tsai, Denniston, Murray_2011

576 CHAPTER 17 Strabismus

Amblyopia

Amblyopia is a developmental defect of central visual processing leading to reduced visual form sense. Effectively, this means that during the first 8 to 10 years of life, our capacity for high-level vision is vulnerable. Anything less than perfect, balanced foveal images from both eyes can lead to loss of vision in one or both eyes.

With increasing age, this is harder to reverse and by about 10–12 years of age is usually permanent. However, some recent studies have shown success of amblyopia treatment even in older children.

Causes of amblyopia

No or reduced image

Deprivation amblyopia

Constant monocular occlusion for >1 week/year of life is very likely to lead to amblyopia in those <6 years. Significant unilateral congenital cataracts require urgent removal with optical correction in the first few weeks of life; significant bilateral congenital cataracts should be removed in the first 6–8 weeks of life.

Image blurring from refractive error

•Anisometropic amblyopia: there is significant risk with difference of refraction of >2.5D, and increased risk if present >2 years. This is a highly amblyogenic stimulus.

•Ametropic amblyopia: significant risk if refractive error is greater than +5.00D or –10.00D; bilateral amblyopia may occur if uncorrected.

•Astigmatic/meridional amblyopia: significant risk if >0.75D cylinder; risk is increased if there is a different axis and/or magnitude between the two eyes.

Abnormal binocular interaction

•Strabismic amblyopia: significant risk if one eye is preferred for fixation; if it is freely alternating, then there is low risk. This is more common in esotropia than in exotropia.

Clinical features

•Reduced visual acuity in the absence of an organic cause and despite correction of refraction.

•Exaggeration of the crowding phenomenon (better visual acuity with single optotypes).

•Tolerance of a neutral density filter (for a specific filter, VA is reduced significantly less in amblyopia than in organic lesions).

Treatment

The critical period during which visual development may be influenced is up to 10 years. Newer research shows promise at up to 12 or more years of age, but with less effect.

At younger ages, there is more rapid reversal of amblyopia but increased risk of inducing occlusion amblyopia in the covered eye.

578 CHAPTER 17 Strabismus

Binocular single vision

Binocular single vision (BSV) is the ability to view the world with two eyes, form two separate images (one for each eye), and yet fuse these centrally to create a single perception.

The development of BSV depends on correct alignment and similar image clarity of both eyes from the neonatal period. This permits normal retinal correspondence in which an image will stimulate anatomically corresponding points of each retina and subsequent stimulation of functionally corresponding points in the occipital cortex to produce a single perception.

The points in space that project onto these corresponding retinal points lie on an imaginary plane. The horopter. Panum’s fusional area is the narrow plane in front and behind the horopter in which, despite disparity, points will be seen as single.

Levels of binocular single vision

Binocular vision may be graded as follows:

1.Simultaneous perception: simultaneously perceives an image on each retina;

2.Fusion: stimulation of corresponding points allows central fusion of image;

3.Stereopsis: images are fused but slight horizontal disparity gives a perception of depth.

Fusion has sensory and motor components. Whereas sensory fusion generates a single image from corresponding points, motor fusion adjusts eye position to maintain sensory fusion. Fusional reserves (also called fusional amplitudes) indicate the level at which these mechanisms break down (usually seen as diplopia) (Table 17.3).

Abnormalities of BSV

Confusion and diplopia

These are abnormalities of simultaneous perception.

•Confusion is the stimulation of corresponding points by dissimilar images (i.e., two images appear superimposed in the same location).

•Diplopia is the stimulation of noncorresponding points by the same image (i.e., double vision).

Adaptive mechanisms

Adaptive mechanisms include suppression, abnormal retinal correspondence, and abnormal head posture.

•Suppression is a cortical mechanism to ignore one of the images causing confusion (central suppression at the fovea) or diplopia (peripheral suppression). Monocular suppression leads to amblyopia if not treated; alternating suppression (between the two eyes) does not, but depth perception and stereopsis will be decreased. The size and density of the suppression scotoma is also variable.

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Strabismus: assessment

Although the patient’s (or parents’) primary concern is likely to be the ocular misalignment (strabismus), it is imperative to step back and consider the whole child, their visual development, and their ophthalmic status. Proper assessment requires taking a history (visual, birth, developmental; see Table 17.4), appropriate measurement of vision, refraction and ophthalmic examination (Table 17.5), and consideration of any amblyopic risk.

Strabismus may be the first presentation of a serious ocular pathology (e.g., retinoblastoma, cataract), thus careful ophthalmic examination (including dilated fundoscopy) is essential.

The general ophthalmic approach to examining the child (p. 606) must be adapted to include orthoptic examination and refraction. Turn the examination into a game whenever possible. Efficient examination helps reduce patient (and examiner) fatigue. When there is concern over possible systemic abnormalities, refer the child to a pediatrician.

The individual tests are discussed as part of clinical methods (pp. 29–32).

History

Table 17.4 An approach to assessing strabismus— history

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Strabismus: outline

Esodeviations: the eye that turns in

Is there a deviation?

Abnormalities of the face, globe, or retina may simulate an esodeviation.

Table 17.6 Causes of pseudo-esotropia

Esophoria vs. esotropia

Phorias are latent deviations that are controlled by fusion. In certain circumstances (specific visual tasks, fatigue, illness, etc.), fusion can no longer be maintained and the eyes deviate.

Tropias are manifest deviations (Table 17.7). Some individuals may be phoric in one situation (e.g., for distance) and tropic in another (e.g., for near).

Table 17.7 Esotropia

STRABISMUS: OUTLINE 583

Exodeviations: the eye that turns out

Is there a deviation?

As with esodeviations, structural abnormalities may simulate an exodeviation. Angle kappa (the difference between the pupillary axis and the optical axis) is usually slightly positive. An abnormally large positive angle kappa simulates an exodeviation.

A negative angle occurs from abnormal nasal positioning of the fovea (high myopia, traction, etc.). This simulates esodeviation.

Table 17.8 Causes of pseudo-exotropia

Exophoria vs. exotropia

Exophorias are latent deviations that are generally asymptomatic. However, when fusion can no longer be maintained, they decompensate with symptoms of asthenopia (eye strain), blurred vision, or diplopia. Exotropias are manifest deviations that may be variable or constant (Table 17.9).

Table 17.9 Exotropia

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Comitant strabismus: esotropia

Esotropia is a manifest inward deviation of the visual axes relative to each other. It is the most common form of strabismus. The condition may be primary, secondary (most commonly due to poor vision), or consecutive (after surgery for an exodeviation). Primary esotropias are classified as accommodative or nonaccommodative.

As with all strabismus, the assessment should include refraction, full ophthalmic examination, and addressing of amblyopic risk. It is essential to detect or rule out underlying pathology (e.g., intraocular tumor, cataract) at the outset.

Accommodative esotropia

Accommodative esotropia usually presents between 1 and 5 years of age. It may be refractive or nonrefractive. In the refractive group, increased accommodation tries to compensate for uncorrected hypermetropia and is accompanied by excessive convergence.

In the nonrefractive group, there is an abnormal accommodative con- vergence–accommodation (AC:A) ratio. There may be overlap between these groups.

Refractive: fully accommodative esotropia

•Esotropia fully corrected for distance and near by hypermetropic (usually +2 to +7D) correction; normal AC:A ratio; normal BSV if corrected; often intermittent initially (e.g., with fatigue, illness).

Treatment

Full hypermetropic correction is needed; treat any associated amblyopia. Orthoptic exercises may overcome suppression or improve fusion range.

Refractive: partially accommodative esotropia

•Esotropia only partially corrected by hypermetropic correction; BSV absent, or limited with ARC; ± bilateral IO overaction.

Treatment

Full hypermetropic correction is needed; treat amblyopia. Consider surgery if there is potential for BSV (aim to convert to a fully accommodative esotropia) or cosmesis (if cosmetically unacceptable despite glasses).

Nonrefractive: convergence excess esotropia

•Esotropia for near due to high AC:A ratio; orthoor esophoric for distance; dBSV for near, normal BSV for distance; usually hypermetropic.

Treatment

Treat any associated hypermetropia or amblyopia. Consider surgery (bilateral MR recession ± posterior fixation sutures), orthoptic exercises, executive bifocal glasses, or miotics.