STRABISMUS (SQUINT)
Strabismus, i.e. misalignment of the gaze, is one of the commonest ophthalmic problems, present in ~4-5% of under-5 children. Early detection and treatment of strabismus is essential to prevent secondary amblyopia and psychological stress.
Types: States of ocular balance may be divided in three types:
Orthophoria, i.e. ideal and exact ocular balance between two eyes with synchronized image formation in all directions and distances of gaze, which is seldom present in children.
Heterophoria (L atent squint), which manifests only in stress situations, e.g. fatigue, illnesses or after covering the normal eye. Some degree of heterophoria is common in normal children, though large heterophoria may lead to diplopia, headache and eye-strain.
Heterotropia (Manifest squint) is persistent misalignment of gaze, in one or more directions of gaze.
Heterophoria or heterotropia is further classified according of the direction of deviation in squinted eye as: (a) esophoria/esotropia or convergent squint (commonest type), (b) exophoria/exotropia or divergent squint,
(c) hyperphoria/hypertropia, i.e. upward deviation, and
(d) hypophoria/hypotropia, i.e. downward deviation.
Etiology of strabismus may be divided into: Non-paralytic (Concomitant) squint, due to ocular or visual defects, e.g. refractive errors in involved eye, is more common in children.
Paralytic (Non-concomitant) squint, due to weakness of extraocular muscles is rare, caused by: (i) congenital neuromuscular abnormality, (ii) traumatic neuromuscular damage, e.g. in birth injury or basal skull fractures, (iii) CNS infections/tumors, (iv) toxic neuropathies, e.g. diphtheria, lead poisoning, etc.
Consequences: Primary optic consequence of strabismus is diplopia due to misalignment of two visual axis, i.e. formation of two separate images on retina. However, secondary optic response to diplopia varies according to the age of child.
A visually immature child (lt;5-6 years) tends to suppress one of the two diplopic images at cortical level or by maintaining abnormal head-position, leading to amblyopia in affected eye.
An older child is usually unable to achieve this visual suppression and remains diplopic, with consequent headache and eye-strain.
Diagnosis of squint may be made by:
Corneal reflex test (Hirschberg's test), particularly useful in uncooperative children or those with poor visual fixation. When a torchlight is projected on the cornea of both eyes simultaneously with child looking directly towards the source, light reflection on both the cornea should be symmetrical and centrally placed. In strabismus, reflection is asymmetrical and off-center in affected eye. In Krimski method, prisms are placed over one or both eyes to assess the amount of prism needed to align reflections, i.e. degree of squint.
Cover test is more useful to detect latent squint, though requires child's cooperation and reasonably good vision. In Cover-uncover test, child is asked to focus on an object with both eyes. Subsequently, when one eye is covered, movement of uncovered eye to readjust the image indicates latent squint in uncovered eye. Similar procedure is repeated with other eye. In another method, i.e. Alternate-cover test, examiner rapidly covers and uncovers each eye alternately like a windshield wipers. Rapid movement of eye, as it is uncovered, indicates ocular deviation. Cover tests should be performed for both distant and near vision fixation and in different gaze positions to detect latent squint or heterophoria.
Assessment of refractive errors and other ocular or neurological conditions is necessary to identify the cause of strabismus.
Management of strabismus aims to:
• Eliminate causative factors, e.g. refractive errors
• Encourage use of affected eye to prevent secondary amblyopia, by covering the normal eye for few weeks (occlusion therapy)
• Correct visual axis misalignment by orthoptic exercises and surgery in selected cases.
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