Strabismus & Amblyopia
Ophthalmology · Orbit · lean revision notes
Strabismus & Amblyopia
Strabismus is a misalignment of the visual axes; amblyopia is a developmental reduction of best-corrected vision in an otherwise structurally normal eye. The two are tightly linked through the immature visual cortex, and together they form one of the highest-yield, easiest-to-score clusters in NEET PG Ophthalmology because the tests (Hirschberg, cover test), the named criteria, and the management cut-offs are all factual and reproducible.
High-yield: Strabismus is the misalignment; amblyopia is the vision loss that can result from it. You can have one without the other, but strabismic amblyopia (one cause of amblyopia) is the bridge between the two.
Definitions & basic terminology
| Term | Meaning |
|---|---|
| Orthophoria | Perfect ocular alignment even when fusion is disrupted (rare ideal) |
| Heterophoria (latent) | Tendency to deviate, controlled by fusion; manifests only on cover test |
| Heterotropia (manifest) | Constant/intermittent deviation present with both eyes open |
| Esodeviation (eso-) | Inward (convergent) deviation |
| Exodeviation (exo-) | Outward (divergent) deviation |
| Hyperdeviation | Upward deviation of one eye |
| Hypodeviation | Downward deviation of one eye |
| Comitant | Deviation equal in all directions of gaze (typically childhood/refractive) |
| Incomitant | Deviation varies with gaze direction (typically paralytic/restrictive) |
The suffix -phoria = latent; -tropia = manifest. So esophoria is a controlled inward tendency, while esotropia is a manifest inward squint.
High-yield: Comitant squint → cosmetic/sensory problem, usually no diplopia (suppression develops in children). Incomitant (paralytic) squint → diplopia and abnormal head posture, usually acquired in adults.
Classification of strabismus
A. By direction
- Esotropia (ET) – convergent
- Exotropia (XT) – divergent
- Hypertropia / Hypotropia – vertical
B. By comitance
- Comitant (concomitant) – angle constant; ocular muscles and nerves intact; classically refractive/childhood.
- Incomitant – paralytic (nerve palsy) or restrictive (thyroid eye disease, blow-out fracture, Duane, Brown syndrome).
Esotropia subtypes (very testable)
| Type | Key feature |
|---|---|
| Infantile (congenital) esotropia | Onset <6 months, large constant angle, may have latent nystagmus, DVD, inferior oblique overaction |
| Accommodative esotropia | Onset 2–3 yrs, due to uncorrected hypermetropia; corrects with glasses |
| Refractive accommodative | High hypermetropia, normal AC/A; treated with full plus correction |
| Non-refractive accommodative | High AC/A ratio; esotropia worse for near; treated with bifocals/miotics |
| Sensory esotropia | Secondary to monocular vision loss (e.g., cataract, corneal scar) |
Exotropia subtypes
- Intermittent exotropia – commonest childhood exodeviation; worse with fatigue, illness, distance fixation; child squints/closes one eye in bright sunlight.
- Constant exotropia – congenital or sensory (long-standing deprivation in older children/adults).
High-yield: Hypermetropia → accommodative esotropia. Myopia → exophoria/exodeviation tendency. (Hypermetrope over-accommodates → over-converges → eye turns in.)
Etiology & pathophysiology
The motor system depends on six extraocular muscles per eye and the three cranial nerves (III, IV, VI). Sensory fusion requires both foveas to receive matched images, which the cortex blends into single binocular vision (the Worth concept: simultaneous perception → fusion → stereopsis).
When alignment fails in a child, the developing brain protects itself from diplopia by two adaptations:
- Suppression – cortical "switching off" of the deviating eye's image.
- Abnormal retinal correspondence (ARC) – the fovea of the fixing eye pairs with a non-foveal point of the deviating eye.
Persistent suppression in the sensitive period leads to amblyopia. In adults, the mature cortex cannot suppress, so an acquired squint produces diplopia instead of amblyopia.
Flow of accommodative esotropia: Uncorrected hypermetropia → sustained accommodation to clear image → accommodative convergence (linked reflex) → eyes converge → esotropia → suppression of one eye → strabismic amblyopia.
High-yield: The near reflex triad = accommodation + convergence + miosis. The AC/A ratio quantifies the convergence produced per dioptre of accommodation.
Clinical features
- Visible deviation (cosmetic concern is the commonest presenting complaint in children).
- Diplopia – in acquired/paralytic squint of adults (not in congenital comitant squint).
- Abnormal head posture (face turn/chin elevation) to use the field of single binocular vision – classic in IV nerve palsy and Duane syndrome.
- Asthenopia (eye strain, headache) in decompensating phorias.
- In intermittent exotropia: monocular eye-closure in bright light, transient diplopia, deteriorating control.
- Amblyopia: unilateral reduced acuity, crowding phenomenon (worse acuity for a line of letters than for a single isolated letter of the same size).
Diagnosis & investigations
1. Hirschberg (corneal reflex) test – screening
Shine a torch at 33 cm; observe symmetry of corneal light reflexes. A normal reflex sits slightly nasal to centre.
| Reflex position | Approx deviation |
|---|---|
| At pupil margin | ~15° (≈30 PD) |
| Mid-iris (between margin & limbus) | ~30° (≈45 PD) |
| At limbus | ~45° (≈90 PD) |
Rule of thumb: 1 mm of decentration ≈ 7° (≈15 prism dioptres).
High-yield: A reflex displaced temporally = the eye is turned in (esotropia). Reflex displaced nasally = eye turned out (exotropia). (The reflex deviates opposite to the eye's turn.)
Krimsky test – modification of Hirschberg using prisms to centre the reflex; quantifies the angle, useful when fixation is poor.
2. Cover tests – the gold standard for detecting and classifying squint
| Test | What it detects | Method & interpretation |
|---|---|---|
| Cover/uncover test | Manifest squint (tropia) | Cover the fixing eye; if the uncovered (other) eye moves to take up fixation, a manifest tropia is present. Movement outward → in → esotropia; inward movement → exotropia |
| Uncover test | Latent squint (phoria) | Cover one eye, then uncover it; if the just-uncovered eye moves to realign, a phoria is present (it deviated under cover) |
| Alternate cover (prism cover) test | Total deviation (phoria + tropia) | Rapidly alternate the cover; dissociates fusion. Neutralise the movement with prisms to measure total angle |
High-yield: Cover–uncover differentiates tropia vs phoria. Alternate (prism) cover measures the maximum/total deviation but cannot by itself separate phoria from tropia.
3. AC/A ratio (accommodative convergence per unit accommodation)
- Normal ≈ 3–5 prism dioptres per dioptre (3–5:1).
- High AC/A → esotropia greater for near than distance → non-refractive accommodative esotropia → treat with bifocals or miotics.
- Measured by gradient method (change deviation with +1.00 lens) or heterophoria method.
4. Tests of binocular sensory status
- Worth four-dot test – red-green dissociation; detects suppression/diplopia. 4 dots seen = fusion; 5 dots = diplopia; 2 or 3 dots = suppression of one eye.
- Synoptophore (major amblyoscope) – measures angle, assesses simultaneous perception/fusion/stereopsis, grades ARC, and is used for fusion training.
- Stereopsis tests – Titmus fly, TNO.
- Bagolini striated glasses – test for ARC/suppression under near-natural conditions.
5. Ductions & versions
Examine monocular movements (ductions) and binocular conjugate movements (versions) in the 9 cardinal positions to localise an underacting muscle (incomitant squint).
6. Diplopia charting / Hess charting
For paralytic squint: the smaller field belongs to the eye with the paretic muscle; the larger field is the contralateral eye with the overacting yoke muscle (secondary deviation > primary deviation in paralytic squint).
High-yield: In paralytic squint, secondary deviation > primary deviation (Hering's law – extra innervation overdrives the yoke muscle). In comitant squint, primary = secondary deviation.
Amblyopia ("lazy eye")
Definition: A unilateral (or, less commonly, bilateral) decrease in best-corrected visual acuity, not directly attributable to a structural ocular abnormality, due to abnormal visual experience during the critical/sensitive period of cortical development.
Types of amblyopia (classic NEET list)
| Type | Cause | Notes |
|---|---|---|
| Strabismic | Constant unilateral squint (commonly esotropia) | Cortical suppression of the deviating eye |
| Anisometropic | Unequal refractive error between eyes | One blurred image is suppressed; commonest in hypermetropic anisometropia |
| Isoametropic | High symmetric bilateral refractive error | Bilateral amblyopia |
| Meridional | Uncorrected astigmatism | Selective meridian blur |
| Stimulus-deprivation (amblyopia ex anopsia) | Congenital cataract, complete ptosis, corneal opacity | Most dense / worst prognosis; needs earliest intervention |
High-yield: Deprivation amblyopia (e.g., congenital cataract) is the densest and most resistant, so congenital cataract must be operated early (within first weeks–2 months) to prevent irreversible amblyopia.
Critical period concept
The visual cortex is plastic up to ~7–8 years; amblyopia develops most easily and is most reversible within this window. The earlier the insult and the longer it persists, the deeper the amblyopia and the poorer the recovery. Treatment is most effective before age 7, though modern evidence shows partial benefit can still occur up to early adolescence.
Diagnostic clues
- Reduced acuity in one eye with normal fundus/media.
- Crowding phenomenon – worse acuity for linear vs single optotypes.
- Neutral density filter test – acuity drops little (or improves) in amblyopia, unlike organic disease where it worsens markedly.
- Often an afferent defect is absent (no RAPD) – helps exclude optic nerve pathology.
Management
Strabismus – general principles
- Correct refractive error first – full cycloplegic refraction; give full hypermetropic correction in accommodative esotropia. Glasses alone may fully correct the squint.
- Treat amblyopia before surgery (a straight but amblyopic eye is a poor functional result).
- Surgery for the residual non-accommodative angle or non-refractive squint.
High-yield: In fully accommodative esotropia, the drug/treatment of choice is the correct spectacle lens (full plus correction) — not surgery. Surgery is reserved for the non-accommodative residual deviation.
Surgical principles – recession vs resection
| Procedure | Effect on muscle | Effect on eye | Use |
|---|---|---|---|
| Recession | Muscle reattached behind original insertion → weakened | Moves eye away from that muscle's action | Weaken an overacting muscle |
| Resection | Segment of muscle excised and reattached → strengthened/tightened | Moves eye toward that muscle's action | Strengthen an underacting muscle |
Example – correcting a right esotropia: recess the (overacting) medial rectus and resect the lateral rectus of the same eye → eye rotates outward to align. Other weakening procedures include myectomy/marginal myotomy; strengthening includes tucking/advancement. Adjustable sutures help in adults.
Other modalities
- Botulinum toxin into an extraocular muscle – temporary chemodenervation, useful in small/acute VI palsy or as an adjunct.
- Prisms – relieve diplopia in adults with small incomitant deviations.
- Orthoptic (fusional) exercises – mainly for intermittent exotropia and convergence insufficiency (pencil push-ups).
- Miotics (echothiophate) – reduce accommodative demand in high AC/A esotropia (now rarely used).
Amblyopia – treatment
Stepwise approach:
- Refractive correction – prescribe full optical correction and allow refractive adaptation (acuity may improve with glasses alone over weeks).
- Occlusion (patching) of the better (sound) eye to force use of the amblyopic eye – the mainstay. Modern PEDIG regimens use 2 hours/day for moderate and up to 6 hours/day for severe amblyopia, rather than the older all-day patching.
- Pharmacological penalisation – atropine 1% drops in the sound eye blur near vision, encouraging use of the amblyopic eye; comparable to part-time patching for moderate amblyopia and improves compliance.
- Treat the underlying cause – e.g., remove congenital cataract, correct ptosis.
High-yield: Patch the good eye, not the lazy eye. Reverse amblyopia (occlusion amblyopia of the patched good eye) is the feared complication of over-patching → mandatory regular follow-up of both eyes.
Complications
- Amblyopia (the chief sensory complication of childhood strabismus).
- Loss of binocular single vision and stereopsis – occupational limitation (e.g., disqualifies from some careers).
- Suppression and abnormal retinal correspondence.
- Diplopia in acquired strabismus (and after surgery if alignment overcorrected in adults).
- Cosmetic and psychosocial impact.
- Surgical complications: under/overcorrection, scleral perforation, slipped/lost muscle, anterior segment ischaemia (with multiple muscle surgery).
- Occlusion (reverse) amblyopia from excessive patching.
Key differentials
- Pseudostrabismus – apparent squint from prominent epicanthal folds or negative angle kappa (esp. pseudoesotropia in infants). Hirschberg reflexes are symmetrical and cover test shows no movement → reassurance only.
- Angle kappa – angle between visual and pupillary axes; positive (temporal reflex) mimics exotropia, negative mimics esotropia.
- Paralytic (incomitant) vs comitant squint – paralytic shows diplopia, abnormal head posture, secondary > primary deviation, and limitation in the field of the paretic muscle.
- Restrictive strabismus – thyroid eye disease (commonest cause of restriction; inferior rectus most affected), blow-out fracture (entrapped inferior rectus → impaired elevation), Brown syndrome (restricted elevation in adduction), Duane retraction syndrome (globe retraction + palpebral fissure narrowing on adduction, due to anomalous innervation of lateral rectus by III nerve).
- Sensory deprivation causes (cataract, corneal opacity, retinoblastoma) must be excluded in any squinting child — leukocoria with squint mandates fundus exam to rule out retinoblastoma.
High-yield: A child with strabismus + leukocoria = rule out retinoblastoma (squint is the 2nd commonest presenting sign after leukocoria). Never dismiss a childhood squint without a dilated fundus exam.
Eponyms & named items worth memorising
- Hirschberg test – corneal reflex screening.
- Krimsky test – prism over the fixing eye to centre reflex.
- Worth four-dot test – fusion/suppression/diplopia.
- Bagolini striated glasses – ARC under natural conditions.
- Hering's law – equal innervation to yoke muscles (explains secondary deviation).
- Sherrington's law – reciprocal innervation (agonist contracts, antagonist relaxes).
- Duane and Brown syndromes – restrictive/innervational anomalies.
- PEDIG – the trials defining modern patching/atropine regimens.
Mnemonics
- "LR₆ SO₄ AO₃" – Lateral Rectus by CN VI, Superior Oblique by CN IV, all Others by CN III.
- Recession = weakens & moves eye away; Resection = strengthens & moves eye toward. ("RecEssion wEakens.")
- RAT – "Resect the Antagonist, Tighten" vs recess the overactor.
Recently asked / exam angle
- Image-based Hirschberg: photo of corneal reflex displaced temporally in one eye → "Which deviation?" → Esotropia.
- Cover test logic: "On covering the fixing eye the other eye moves outward to fixate — diagnosis?" → manifest esotropia; differentiate phoria vs tropia by cover–uncover.
- AC/A ratio value (3–5:1) and its role in non-refractive accommodative esotropia → bifocals.
- Treatment of choice in fully accommodative esotropia → full hypermetropic glasses (not surgery).
- Amblyopia therapy: patch the sound eye; recognise occlusion (reverse) amblyopia as the complication; atropine penalisation as alternative.
- Densest amblyopia → deprivation (congenital cataract) → operate early.
- Recession vs resection direction-of-effect single-best-answer.
- Secondary > primary deviation ⇒ paralytic squint (Hering's law).
- Squint + leukocoria ⇒ exclude retinoblastoma.
- Pseudoesotropia from epicanthal folds / negative angle kappa → cover test normal → reassure.
Rapid revision
- -phoria = latent (only on cover test); -tropia = manifest deviation.
- Comitant squint = equal angle all gazes, no diplopia in kids; incomitant = varies with gaze, diplopia in adults.
- Hypermetropia → esotropia; myopia → exodeviation.
- Hirschberg: 1 mm displacement ≈ 7° ≈ 15 PD; reflex displaced temporally = esotropia.
- Cover–uncover distinguishes tropia vs phoria; prism alternate cover measures total deviation.
- Normal AC/A = 3–5:1; high AC/A esotropia → bifocals/miotics.
- Hering's law: paralytic squint → secondary deviation > primary.
- Amblyopia types: strabismic, anisometropic, isoametropic, meridional, deprivation — deprivation is densest.
- Critical period ≈ up to 7–8 years; treat amblyopia early.
- Amblyopia mainstay = occlusion of the good eye; alternative = atropine penalisation; danger = reverse amblyopia.
- Recession weakens (eye moves away); resection strengthens (eye moves toward).
- Accommodative esotropia DOC = full plus spectacle correction; squint + leukocoria = rule out retinoblastoma.