Short Case: Squint — Cover Test to Diagnosis Without Fumbling

A squint short case is scored on what you make observable. Set fixation, look before you touch the patient, then report the test condition, eye movement and interpretation in that order. Cover–uncover looks for a manifest deviation. Alternate cover dissociates fusion and reveals the total alignment error. Neither test, by itself, gives you permission to invent a cause or a treatment plan.

This is an examination-rehearsal guide for ophthalmology postgraduates. It is not a patient-specific diagnostic or management pathway. In a real clinical encounter, incomplete history, acute symptoms or a new motility deficit require appropriate senior and local-protocol input.

What the examiner is actually testing

The examiner does not need a recital of every type of strabismus. They need to see whether you can create reliable conditions, observe a movement, and explain what that movement supports. If you cover an eye without stating fixation distance or target, the result becomes hard to interpret. If you say “exotropia” before describing the refixation movement, the examiner has no evidence chain to follow.

Think in this sequence:

1. Set the condition. Distance or near? With habitual correction if relevant? What is the accommodative target?
2. Observe alignment and fixation. Head posture, lids, corneal reflections, obvious deviation and whether each eye fixes.
3. Choose the test. Cover–uncover for a manifest deviation; alternate cover for dissociation and the total deviation.
4. Describe the eye that moves. Name the eye and its direction of movement before naming the deviation.
5. Measure and contextualise. Record the method, prism direction and magnitude if measurement is possible; then assess motility and any gaze dependence.

The AAPOS prism-and-cover-test reference supports this basic distinction between cover testing and prism quantification. Its terminology is useful for revision; it is not a substitute for the station brief in front of you.

Set up before the cover test

Start as you would in any short case: introduce yourself, confirm the patient identity, explain the examination and obtain consent. Ask whether the patient uses spectacles and inspect with the usual correction where appropriate. Then give yourself a quiet ten seconds of observation.

Check visual acuity in each eye before reducing the case to an angle measurement. In a child station, say that the method will be age-appropriate. Do not claim a visual-acuity value you have not measured. If a station is deliberately narrow, announce the omitted element rather than silently skipping it: “After this alignment task, I would complete acuity and the wider ocular examination as the station permits.”

Cover–uncover: decide whether there is a manifest component

For cover–uncover, have the patient fixate a target. Cover one eye and watch the uncovered eye. A refixation movement of the uncovered eye indicates that it was not fixing straight before its fellow eye was covered, supporting a manifest deviation in that test condition. Repeat on the other side and at the specified distance.

The useful spoken formula is:

“At [distance/near], with [correction], when I cover the [right/left] eye, the [other] eye moves [in/out/up/down] to take up fixation. This supports a [eso/exo/hyper/hypo] tropic component in this condition.”

Direction needs calm spatial reasoning. If the right eye moves inwards to take up fixation after the left eye is covered, it had been outwards before refixating: that supports a right exotropia in that condition. If it moves outwards to refixate, it had been inwards: an esotropic component is suggested. For a vertical movement, report the movement first and then use the station’s convention carefully rather than blurting out a laterality label.

Avoid “the squint switches” as an answer. If fixation appears to alternate, demonstrate it by repeating the manoeuvre and report what you actually saw. A short case rewards clean evidence, not a label chosen because it sounds familiar.

Alternate cover: dissociate, then describe the total deviation

Alternate cover is a different task. Move the cover from eye to eye without allowing binocular fusion to re-establish. Watch the eye uncovered after each transfer. The test can bring out a latent component and is used before prism quantification. The NCBI clinical overview of strabismus provides the wider terminology around phorias and tropias; in the examination, your job is to state what this manoeuvre changed.

Say the result in two parts:

1. “On alternate cover, there is a refixation movement of …”
2. “This suggests a dissociated/latent component; I would quantify the total deviation with prism alternate cover testing if cooperation permits.”

Do not describe alternate cover as “the test for phoria” and stop there. A patient may have a manifest deviation, a latent component, or both; the observation and the condition make the answer defensible. Also resist a common sequence error: do not start with a prism before you have shown that you can observe the movement it is intended to neutralise.

PBCT: neutralise the movement, document the condition

Prism alternate cover testing (PBCT) is the useful measurement step when the patient can cooperate. Place a prism in the appropriate orientation, continue alternate cover to maintain dissociation, and increase or adjust prism strength until the refixation movement is neutralised. Record prism dioptres, direction, distance or near, correction and fixation target. Repeat the relevant condition instead of carrying one number through the whole station.

A concise report sounds like this:

“By alternate prism cover test, the deviation measures [x] prism dioptres base [in/out/up/down] at [distance/near], with [correction]. I would compare the relevant near, distance and gaze measurements before deciding whether it is concomitant.”

The word concomitant belongs to a pattern, not a single PBCT reading: the angle is broadly similar across gaze positions. If the angle varies by gaze, the deviation is incomitant. That should trigger a more careful motility description, not an immediate named aetiology. Exact surgical dose tables are outside this article because a measurement alone does not determine an individual operative plan.

Add motility without turning the station into a ritual

After alignment, assess versions and ductions as the station requires. Compare the nine positions of gaze, look for overaction or underaction, and relate a visible limitation to the alignment finding. Convergence, pupils, anterior segment and fundus may be relevant depending on the vignette. State the next examination item only when it helps answer the task.

Use this small reporting frame:

This is not an invitation to prescribe tests or management from a short-case prompt. It shows the examiner that you know the limits of one manoeuvre. The RCOphth curriculum is useful background on capability-based ophthalmic training, but it does not supply a universal marking scheme for every university or DNB practical examination.

A one-minute presentation that does not overclaim

Present only facts that the station supplied or you observed. Then name the narrowest conclusion they support.

“Visual acuity is [finding] in the right eye and [finding] in the left with [correction]. In primary position there is [head posture/corneal-reflex observation]. At [distance/near], cover–uncover shows [eye] moving [direction] to take fixation, supporting a [direction] tropic component. On alternate cover, [movement/finding] suggests [latent or total-deviation context]. PBCT measures [magnitude and base] under these conditions. Ocular movements are [finding], with [or without] gaze dependence. I would complete the relevant sensory, refraction and ocular assessment and correlate this with the history.”

That is better than “This is a right sixth-nerve palsy” unless the station has supplied enough history and examination evidence for such a conclusion. It also avoids the opposite mistake: listing every possible diagnosis without answering the alignment question.

Five examiner prompts: practise the change, not a fixed speech

Build drills that change one variable after your first answer. This prevents your cover-test routine becoming a memorised performance.

1. “There is no movement on cover–uncover but movement on alternate cover. What does that change?” Explain that a latent component has been revealed and you would quantify the total deviation.
2. “The distance and near measurements are different. What must be documented?” State both test conditions and measurements; do not force one number to represent both.
3. “The deviation changes in left gaze.” Describe the incomitance and complete targeted motility assessment before attaching a cause.
4. “The child does not fixate on the first target.” Adapt to an age-appropriate engaging target, document the limitation and return to the test rather than inventing a value.
5. “Give your conclusion in one sentence.” Use movement → interpretation → limitation: “The observed refixation movement supports … under these conditions; I would correlate it with motility and the wider examination.”

A 20-minute rehearsal loop

Use a fellow resident as examiner. Do not ask them to mark confidence; ask them to capture observable omissions.

Keep a small error log with four columns: condition omitted, movement misread, conclusion too broad, or station organisation. Next time, rehearse only the error type you logged. A resident who repeatedly forgets distance/near does not need another hour of passive strabismus reading; they need ten short reports that begin with the condition.

Common errors that cost clarity

• Calling Hirschberg a measurement when it was only a screen.
• Watching the covered eye during cover–uncover instead of the eye that is uncovered.
• Saying “phoria” merely because alternate cover was performed, without reporting a movement.
• Forgetting correction, target and distance/near in the measurement.
• Measuring once in primary position and declaring the deviation concomitant.
• Naming a nerve palsy, restrictive process or surgery from an alignment observation alone.
• Giving patient-specific management advice in a postgraduate examination answer when the task asks for examination findings.

For broader amblyopia and strabismus revision, use the paediatric ophthalmology study guide. When you need to practise the station flow with cases, instruments and viva-style prompts, the relevant site hub is the OSCE, Practical & Viva Voce Ready bundle. If measurement equipment or practical terminology is the gap, review sample pages for Instruments in Ophthalmology and check repeat terms in the ophthalmology glossary. These are OphthaMCQ resources, not professional-body materials or a substitute for supervised clinical training.

Sources

1. AAPOS: Prism and cover test — cover-test and prism-cover-test terminology; checked 18 August 2026.
2. NCBI Bookshelf: Strabismus — phoria/tropia and clinical-assessment context; checked 18 August 2026.
3. Royal College of Ophthalmologists curriculum — ophthalmic-training context only; checked 18 August 2026.