Strabismus
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Direct answer
Strabismus (squint) is a misalignment of the visual axes, classified as apparent (pseudostrabismus from epicanthic folds or wide inter-pupillary distance), latent (phoria, controlled by fusion) or manifest (tropia), and as concomitant (deviation equal in all directions of gaze, the common childhood type) versus incominant (deviation varies with gaze, from paralytic or restrictive causes). Esotropia (convergent) includes infantile esotropia with a large, constant angle presenting before 6 months, and accommodative esotropia of hyperopic children corrected by cycloplegic spectacles; exotropia (divergent) is often intermittent and worse for distance. Evaluation uses the Hirschberg corneal reflex, cover-uncover and alternate cover tests with prism measurement; management treats amblyopia first, then refractive correction, and surgery on extraocular muscles for significant deviations.
What you must remember
- Definitions: phoria is a latent deviation held in check by fusion, broken by cover test; tropia is a manifest deviation; esotropia converges, exotropia diverges, hypertropia deviates upward.
- Concomitant squint: angle the same in all gazes, full ocular movements, no diplopia typically (suppression and abnormal retinal correspondence develop in children); incomitant squint: angle varies with gaze direction, diplopia maximal in the field of action of the affected muscle, secondary deviation exceeds primary.
- Clinical tests: Hirschberg test (corneal reflex position, 1 mm displacement equals about 7 degrees or roughly 15 prism dioptres), cover-uncover test reveals a tropia, alternate cover test breaks fusion and reveals the total deviation, prism cover test quantifies it in prism dioptres.
- Infantile (congenital) esotropia: large angle (often 30 to 40 degrees or more) before 6 months of age, no significant hyperopia, cross-fixation, inferior oblique overaction and dissociated vertical deviation later; treated surgically, with alignment ideally in the first 2 years to permit binocular development.
- Accommodative esotropia: typically at 2 to 3 years of age from hyperopia; the child converges to accommodate; fully corrected with cycloplegic refraction (atropine or cyclopentolate) and hyperopic glasses; high accommodative convergence to accommodation ratio cases need bifocals; partially accommodative cases need surgery for the residual after full spectacle correction.
- Intermittent exotropia: divergent squint for distance or when tired, controlled near; management options are part-time occlusion, over-minusing lenses, convergence exercises and surgery for deteriorating control.
- Treatment principles: treat amblyopia before surgery (occlusion or atropine penalisation of the sound eye), correct refractive error fully, then bimedial recession or resection procedures to align the eyes; botulinum toxin injection is an alternative in selected cases.
- Special syndromes for exams: Duane retraction syndrome (co-contraction with globe retraction and palpebral fissure narrowing on adduction), Brown syndrome (restricted elevation in adduction from a tight superior oblique tendon), and superior oblique palsies; all restrictive or innervational anomalies rather than simple concomitant squints.
Common confusion
Students confuse pseudostrabismus with true squint: prominent epicanthic folds and a flat nasal bridge make eyes look convergent, but corneal reflexes are central and cover tests are negative. The second distinction is paralytic versus concomitant: in a paralytic squint the deviation increases towards the action of the palsied muscle, the secondary deviation exceeds the primary, and diplopia is prominent; in concomitant squint the angle is equal in all directions. Do not forget that the primary goal in children is the reversal of amblyopia and the development of binocularity, with cosmesis secondary.
Exam-focused takeaway
Examiners ask the cover test interpretations, the angle estimation by Hirschberg, the spectacle-first rule for accommodative esotropia, the timing of infantile esotropia surgery (early, within the first 2 years), and the direction and muscles operated for a given deviation (recession weakens, resection strengthens). Duane and Brown syndromes, the differential diagnosis of a head posture, and amblyopia treatment sequencing are recurrent themes in both MCQs and vivas.
Frequently asked questions
What is the difference between concomitant and incomitant squint?
In concomitant squint the angle of deviation is the same in every direction of gaze with full ocular movements, typical of childhood strabismus. In incomitant (paralytic or restrictive) squint the deviation varies with gaze, the secondary angle exceeds the primary, and diplopia is maximal in the field of action of the affected muscle.
How are the cover tests performed?
In the cover-uncover test, covering the fixating eye reveals a movement of the uncovered eye (a tropia); the alternate cover test dissociates fusion and shows the total deviation, latent plus manifest. The prism cover test quantifies the deviation in prism dioptres by neutralising the corrective movement.
What is accommodative esotropia and how is it treated?
It is a convergent squint, usually beginning at 2 to 3 years, driven by accommodation to overcome hyperopia, so that accommodative convergence overwhelms fusion. Full cycloplegic hyperopic spectacle correction usually abolishes it; a high accommodative convergence to accommodation ratio with near excess may need bifocal addition, and any residual angle after full correction is treated surgically.
When is surgery performed for infantile esotropia?
Infantile esotropia with a large constant angle is surgically aligned early, commonly within the first 1 to 2 years, since binocular potential declines with delay. Amblyopia is treated before or alongside surgery, and later inferior oblique overaction or dissociated vertical deviation may require further procedures.
What is Duane retraction syndrome?
It is a congenital innervational anomaly with miswiring of the sixth nerve, classically showing limitation of abduction with globe retraction and narrowing of the palpebral fissure on attempted adduction. It is managed conservatively when mild, with prism for compensatory head postures and surgery only for significant deviation or abnormal head position.