Strabismus Assessment
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Direct answer
Strabismus is classified as pseudo (apparent, from epicanthic folds or angle kappa), latent (phoria, held by fusion) or manifest (tropia), and as comitant (same deviation in all gazes, typically congenital or accommodative) versus incomitant (varying with gaze, from palsy or restriction — where the secondary deviation exceeds the primary). The workhorse assessment is the cover–uncover test for tropia and the alternate cover test with prisms for the total deviation; Hirschberg's corneal reflex method estimates roughly 7 degrees, commonly quoted as about 12-15 prism dioptres, per millimetre of decentration when fixation is poor. Sensory status is probed with Worth 4-dot (suppression, diplopia), Bagolini lenses and stereotests such as Titmus (fly 3000 seconds of arc, circles down to 40). Management treats the cause first — full hyperopic correction for accommodative esotropia, prisms for small symptomatic deviations — and reserves surgery for residual misalignment.
What you must remember
- Classification skeleton: phoria versus tropia; eso/exo/hyper/cyclo; comitant versus incomitant; in incomitant deviations the secondary (deviated eye fixing) exceeds the primary — a palsy signature from Sherrington's law.
- Cover test grammar: cover–uncover reveals the manifest component; alternate cover dissociates fully and exposes phoria plus tropia; prism alternate cover quantifies the total deviation in prism dioptres.
- Hirschberg and Krimsky: each millimetre of corneal reflex displacement approximates 7 degrees (about 12-15 Δ by the common convention); Krimsky places prisms before the fixing eye until the reflex recentres — useful in poor fixation and young children.
- Direction measurement: esodeviations measured with base-out prisms, exodeviations with base-in, under full dissociation at distance and near, with and without correction.
- Sensory adaptations: suppression and abnormal retinal correspondence develop in childhood tropias; Worth 4-dot at distance and near, Bagolini striated lenses, and stereopsis grading (Titmus fly 3000″, animals, Wirt circles to 40″) map them.
- AC/A ratio: normal 3-5:1 by the gradient method; high AC/A explains near esotropia excess and justifies bifocal adds in children.
- Accommodative esotropia: prescribe the full cycloplegic hyperopia first — it corrects the deviation in wholly accommodative cases; bifocals (+2.50 to +3.00 executive-style) serve high-AC/A near excess; surgery only for residual.
- Incomitant quick keys: CN III palsy — ptosis, down-and-out eye, mydriasis; CN IV — hypertropia worse on contralateral gaze and ipsilateral head tilt; CN VI — abduction deficit with face turn toward the palsy; restrictive disease (thyroid, Brown syndrome) shows positive forced duction patterns and tight muscles.
Working through a vertical diplopia with the three-step test
A 52-year-old reports vertical double vision, worse reading and on stairs. Step one: which eye is hypertropic on alternate cover — the right. Step two: is the right hypertropia worse in right gaze (abduction — implicates superior rectus or inferior oblique pathways) or left gaze (adduction — inferior rectus or superior oblique)? She reports worse in left gaze, narrowing to the right inferior rectus or right superior oblique. Step three: Bielschowsky head tilt — tilting right (placing the right intorters, superior rectus and superior oblique, into action) worsens the right hypertropia; the weak intorter is the right superior oblique. Fourth nerve palsy, decompensated congenital or ischaemic-typical, confirmed by the larger secondary deviation when the left eye fixes.
Management ladder for a small symptomatic residual: vertical prisms (base-down before the hypertropic right eye), split between eyes; larger or gaze-evolving deviations refer for imaging when new-onset, and for surgery when stable. The same three-step applied carelessly — skipping the head tilt or testing gaze with poor targets — collapses into a wrong muscle, so the exam version demands narrated logic.
Where marks are lost
The recurring errors: calling every inward deviation "comitant" without comparing distance and near (accommodative esotropia is near-biased) or gaze positions; measuring the phoria with a simple cover–uncover (it only catches manifest components); forgetting that alternate cover converts a phoria into a tropia and therefore measures the total, not the tropia alone. Candidates also confuse pseudostrabismus with true: epicanthic folds look crossed yet cover-test orthophoric — the reflex and cover test, not the parent's photograph, decide. And in every paediatric eso, the first prescription question is cycloplegic plus, not surgical timing.
Frequently asked questions
What does the alternate cover test measure?
The total deviation — manifest tropia plus latent phoria — under full dissociation, quantified by holding prisms (base-out for eso, base-in for exo) until movement ceases.
How is the Hirschberg estimate converted into prism dioptres?
Each millimetre of corneal reflex decentration approximates 7 degrees, conventionally quoted as about 12-15 prism dioptres, with the reflex temporal to the pupil indicating esotropia.
What distinguishes incomitant from comitant strabismus?
Incomitant deviations change size with gaze direction and show secondary deviation greater than primary (paresis or restriction), while comitant deviations stay similar across gazes.
How is a superior oblique palsy identified by the three-step test?
Ipsilateral hypertropia, worse in contralateral (adducted) gaze, and worse on ipsilateral head tilt — the classic Parks-Bielschowsky sequence.
What is the first treatment for accommodative esotropia?
Full cycloplegic hyperopic correction; the glasses themselves straighten wholly accommodative deviations, with bifocals added for high AC/A near excess before any surgery is considered.