Amblyopia
Definition: unilateral, or rarely bilateral, subnormal best corrected VA caused by form vision deprivation and/or abnormal binocular interaction, for which there is no identifiable pathology of the eye or visual pathway.
Classification
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Strabismic: resulting from abnormal binocular interaction in which there is continued monocular suppression of the deviating eye.
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Anisometropic: caused by a difference in refractive error between the eyes.
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Stimulus deprivation: caused by marked vision deprivation—may be unilateral or bilateral and is caused by opacities in the media (e.g. cataract) or ptosis.
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Ametropic: resulting from high bilateral but symmetrical refractive error (usually hypermetropia).
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Diagnosis
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In the absence of an organic lesion, a difference in best corrected VA of two Snellen lines or more (or >1 log unit) is indicative of amblyopia.
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VA in an amblyopic eye is usually better when reading single letters than letters in a row (‘crowding’ phenomenon).
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Treatment
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It is essential to examine the fundi to diagnose any organic disease prior to commencing treatment.
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If VA does not respond to treatment, investigations (e.g. electrodiagnostics, imaging) should be considered.
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The sensitive period during which VA can be improved is usually up to about 8 years in strabismic amblyopia but may be longer for anisometropic.
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Occlusion of the normal eye is the most effective treatment, with the regimen depending on age and amblyopia density.
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The younger the patient, the more rapid the likely improvement, but the greater the risk of inducing amblyopia in the normal eye; it is essential to monitor VA in both eyes.
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In general, the better the VA at the start of treatment, the shorter the duration required.
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If there has been no improvement after 6 months of effective occlusion, further treatment is unlikely to be fruitful.
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Penalization of the normal eye with atropine is an alternative method sometimes used when compliance with occlusion is poor.
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Heterophoria
Definition: heterophoria (latent squint) implies a tendency of the eyes to deviate when fusion is blocked; the ‘phoria’ can be either a small inward imbalance (esophoria) or an outward imbalance (exophoria).
Classification: both esophoria and exophoria can be classified by the distance at which the angle is greater: (a) convergence excess or weakness, (b) divergence excess or weakness, and (c) mixed. Slight phoria is present in most normal individuals and is overcome by the fusion reflex. When fusional amplitudes are insufficient to maintain alignment, sometimes at times of stress or poor health, the phoria is described as decompensating.
Diagnosis
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Presentation: symptoms of binocular discomfort (asthenopia) or double vision (diplopia).
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Tests: (a) cover–uncover test detects heterophoria and should be performed both for near using an accommodative target, and for distance, and (b) Maddox wing ( Fig. 18.1 ) which dissociates the eyes for near fixation and measures heterophoria.
Fig 18.1
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Treatment
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Refraction: correct significant refractive error.
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Orthoptic treatment: of most value in convergence weakness.
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Symptom relief: with temporary stick-on Fresnel prisms that may subsequently be incorporated into spectacles (maximum usually 10–12 Δ, split between the eyes).
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Surgery: occasionally required for large deviations.
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Vergence abnormalities
Convergence insufficiency
Pathogenesis: convergence insufficiency (CI) typically affects individuals with excessive near visual demand. Accommodative insufficiency (AI) is often also present. CI is usually idiopathic and sometimes post-viral, and typically affects school-age children.
Diagnosis
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Presentation: with asthenopia or diplopia.
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Signs: (a) reduced near point of convergence independent of any heterophoria, and (b) reduced near point of accommodation in AI.
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Treatment
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Orthoptic exercises aimed at normalizing the near point and maximizing fusional amplitudes.
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With good compliance, symptoms should be eliminated within a few weeks but if persistent can be treated with base-in prisms, and very occasionally surgery.
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With AI, the minimum reading correction to give clear vision is prescribed but is often difficult to discard.
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Divergence insufficiency
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Pathogenesis: divergence paresis or paralysis is a rare condition typically associated with underlying neurological disease such as intracranial space-occupying lesions, cerebrovascular accidents, and head trauma.
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Presentation: at any age and may be difficult to differentiate from 6th nerve palsy, but is primarily a concomitant esodeviation with reduced or absent divergence fusional amplitudes.
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Treatment: difficult; prisms are the best option.
Near reflex insufficiency
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Pathogenesis: complete paralysis, in which no convergence or accommodation can be initiated, may be of functional origin, due to midbrain disease or may follow head trauma.
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Signs: (a) dual convergence and accommodation insufficiency; (b) mydriasis may be seen on attempted near fixation.
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Treatment: reading glasses, base-in prisms, and possibly botulinum toxin (orthoptic exercises have no effect); it is difficult to eradicate.
Spasm of the near reflex
Pathogenesis: functional condition affecting patients of all ages who are mainly female.
Diagnosis
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Presentation: diplopia, blurred vision, and headaches.
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Signs: (a) esotropia, (b) pseudomyopia, and (c) miosis. Spasm may be triggered when testing ocular movements; observation of miosis is the key to the diagnosis; refraction with and without cycloplegia confirms pseudomyopia, which must not be corrected optically.
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Treatment
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Reassurance: with advice to discontinue any activity that triggers the response.
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If persistent: atropine and a full reading correction are prescribed, but it is difficult later to abandon treatment without recurrence.
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Esotropia
Definition
Esotropia (manifest convergent squint) may be concomitant or incomitant; in a concomitant esotropia the variability of the angle of deviation is within 5 Δ in different horizontal gaze positions, and in an incomitant deviation the angle differs in various positions of gaze as a result of abnormal innervation or restriction. This section deals only with concomitant esotropia (classification is shown in Table 18.1 ).
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Early onset (essential) esotropia
Definition: idiopathic esotropia developing within the first 6 months of life in an otherwise normal infant with no significant refractive error and no limitation of ocular movements.
Diagnosis
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Angle: fairly large (>30 Δ) and stable.
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Refraction: normal for age (+1 to +2 D).
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Fixation: (a) alternates between right and left in the primary position, and (b) cross-fixating in side gaze, so that the child uses the left eye in right gaze ( Fig. 18.2a ) and the right eye on left gaze ( Fig. 18.2b )—may give a false impression of bilateral 6th nerve palsy.
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