Strabismus Measurements: PBCT, Hess Chart and Squint Surgery Numbers

The prism bar cover test (PBCT) gives a deviation measured under stated conditions. It does not give a diagnosis by itself, and it certainly does not create a universal surgical prescription. In an exam, score well by reporting what was measured, how it was dissociated, at which fixation distance, with which correction, and which eye fixed before interpreting the number.

This article is for ophthalmology exam preparation. Measurements and surgical plans for a real patient require an orthoptic assessment, refraction, sensory evaluation, motility examination and a surgeon’s case-specific judgement.

The direct answer: preserve the conditions around the number

If a viva examiner asks, “What is the deviation?”, an answer such as “30 prism dioptres exotropia” is incomplete. A usable record is closer to: “Thirty prism dioptres of manifest exodeviation by alternate prism-cover test at distance, with full correction, fixing right eye,” plus near measurement and incomitance information where relevant.

The exact documentation convention may vary by unit. The exam principle does not: a naked number without test conditions is a weak answer.

Start with the correct cover manoeuvre

The cover test family asks different questions. Confusing them is a common OSCE failure because both tests involve an occluder and a fixation target.

Cover-uncover test: look for a tropia

Ask the patient to fixate a suitable target. Cover one eye while watching the uncovered eye. If the uncovered eye moves to take up fixation, a manifest deviation (tropia) is present. Repeat on the other side. The movement you observe is a refixation movement; verbalise that you are watching the eye that remains uncovered.

The test tells you that a manifest deviation is present and which eye is taking up fixation. It does not quantify the full deviation. A patient with intermittent control may show no movement at one moment and a tropia after dissociation, fatigue or removal of fusion. That is why you should not call “no movement on a single cover-uncover test” a complete exclusion of strabismus.

Alternate cover test: dissociate to reveal total deviation

With alternate cover, move the occluder back and forth without allowing binocular fusion to re-establish. Watch the eye as it is uncovered for a refixation movement. This dissociates the eyes and exposes the total deviation: tropia plus any phoria that was held by fusion.

For an MCQ, remember the direction of the observed refixation movement is often used to name the deviation. In actual examination, do not rely on a memorised arrow without checking which eye you were observing, which eye was covered and whether the description refers to the eye or the image. State the movement, then state the diagnosis. That is less elegant and more accurate.

PBCT: measure by neutralisation, not by guessing

The AAPOS overview describes the core idea: prisms bend light, and a cover test with increasing prism strength finds the amount needed to neutralise the refixation movement. In practical teaching, the end-point is described as neutralisation; some examiners teach a small reversal movement as confirmation. Use the local method you have been taught, but explain the underlying purpose: identify the prism power at which the eye no longer needs to refixate after alternate dissociation.

A PBCT sequence you can reproduce

1. Ensure an appropriate fixation target and the relevant refractive correction.
2. Measure at distance and at near, recording each separately.
3. Use alternate cover to break fusion before judging the movement.
4. Place a prism with the required base direction before one eye; increase strength until neutralisation.
5. Recheck with alternate cover and note whether a reversal movement occurs according to your unit’s convention.
6. Record magnitude, eso/exo/hyper/hypo component, distance/near, fixation preference and gaze dependence.

The phrase “prism bar” does not alter the logic. A prism dioptre is conventionally the prism that displaces a ray by 1 cm at 1 metre; numerical conversion questions sometimes test this definition. Do not confuse prism dioptres with degrees. In routine strabismus documentation, record the unit as prism dioptres (PD or Δ).

Base direction: use the neutralisation logic

Instead of trying to remember a long directional slogan, draw the deviated eye and ask which way the image needs to be displaced for that eye to stop refixating. The conventional shorthand is familiar: base-out prisms are used for esodeviations and base-in for exodeviations; vertical deviations use base-up or base-down depending on the measured eye movement and the convention being applied. But in a viva, a precise statement of observed movement and prism orientation is stronger than an unexamined mnemonic.

An examiner can deliberately make this difficult by switching fixation eye or adding a vertical component. Slow down, establish the horizontal and vertical components separately, and avoid calling a combined deviation “just an exotropia.”

Distance–near difference: write it, then explain it

Distance and near values may differ because accommodation, accommodative convergence, control, target quality and fusion vary. The mere fact that near is larger or smaller does not automatically settle the diagnosis. It becomes meaningful only after you know whether the patient is fully corrected, whether a +3.00 D lens test or occlusion test has been used in an appropriate context, and whether the deviation is comitant.

For examination purposes, the safe structure is:

“The deviation is X at distance and Y at near by alternate prism-cover test with correction. I would next determine control, comitance, fixation preference, sensory status and refractive/accommodative contribution before interpreting the difference.”

This sounds more clinically mature than applying a label from two numbers. It also keeps the article on the right side of its educational scope.

Primary and secondary deviation: the paralytic-strabismus hinge

Primary deviation is measured when the normal eye fixes. Secondary deviation is measured when the paretic eye fixes. In a paralytic deviation, secondary deviation is classically larger than primary deviation because extra innervation is required to make the weak muscle act; equal increased innervation reaches its yoke muscle in the fellow eye. This is the practical consequence of Hering’s law of equal innervation.

Sherrington’s law describes reciprocal innervation of agonist and antagonist muscles within an eye. Hering’s law explains paired yoke-muscle action across the eyes. Both are regularly tested, and both become easier if you link the law to a movement instead of reciting its wording.

Do not use “secondary greater than primary” to label every incomitant squint as a simple nerve palsy. Restrictive and mechanical causes, long-standing adaptations and poor fixation can complicate measurement.

Read a Hess chart from the smaller field outward

A Hess chart is a field-of-action plot used in evaluating incomitant ocular-motility disorders. It is not a generic “sixth-nerve chart.” The useful reading sequence begins with the smaller field. A reduced field indicates the eye with reduced excursion; within it, the greatest restriction points towards the underacting muscle. Then look to the opposite chart for the overaction of the contralateral yoke muscle.

Seven steps for a Hess-chart question

1. Check that you know which plot belongs to which eye and which colour/filter convention the figure uses.
2. Identify the smaller field. That eye is usually the affected eye in a paretic pattern.
3. Find the direction of maximum restriction within that field.
4. Identify the muscle whose field of action corresponds to that direction.
5. Look at the fellow-eye chart for the expanded field of the yoke muscle.
6. Check whether the pattern is isolated, multiple, restrictive-looking or inconsistent with a simple palsy.
7. Correlate with head posture, diplopia direction, versions, ductions and the rest of the stem.

The first two steps prevent the classic reversal error: selecting the overacting yoke as the primary weak muscle. If the left-eye field is smaller in left abduction, think first about a left lateral-rectus action deficit. If the right-eye plot is expanded in the corresponding gaze, that supports right medial-rectus yoke overaction. The chart should be read as a paired system.

Why Hess charts can mislead in isolation

Hess plots depend on patient cooperation, fixation and the test protocol. Chronic deviations can show secondary adaptations. Restriction and paresis may require additional testing to distinguish them. A chart cannot replace forced duction assessment, imaging or the rest of ocular-motility examination. In a written exam, however, the direction of underaction plus the yoke pattern is usually the intended route.

Common pattern errors worth rehearsing

“Largest deviation equals surgery amount”

False. A measured deviation is only one input into surgical planning. The target, stability, comitance, previous surgery, restriction, age, sensory status, amblyopia, pattern deviations, torsion, refractive state, adjustable-suture plan and surgeon-specific tables can change the plan. An exam may ask an approximate institutional rule or a calculation based on a table supplied in the stem. Use the table given by the examiner; do not invent a universal millimetres-per-PD rule.

“A normal primary-position measurement excludes a motility disorder”

False. An incomitant deviation may be small or controlled in primary position and larger in a particular gaze. Versions, ductions, head posture and diplopia map can be more revealing than one primary-position PBCT value.

“A Hess chart diagnoses the nerve palsy”

Incomplete. A Hess pattern supports a muscle-action deficit. The diagnosis still depends on history, exam, forced-duction considerations where relevant and other anatomical clues. In a clinical setting, new diplopia or motility limitation needs appropriate supervision.

How to answer ‘squint surgery numbers’ in an exam

Some written papers legitimately test definitions, prism conversions, the general relationship between deviation and a recession/resection plan, or a named table already provided in a station. The safe and accurate answer has three parts.

1. State the measurement. “The largest stable, relevant angle is … PD under these conditions.”
2. State the plan category. “A comitant horizontal deviation may be planned with a horizontal-muscle procedure according to the unit’s surgical table and full assessment.”
3. State the limitation. “Exact millimetres are case- and surgeon-specific; they are not derived from a PBCT value alone.”

For an OSCE, this earns more than confidently quoting a dose table from memory. It shows you understand why preoperative assessment exists. If a station hands you its own dosage chart, follow it meticulously, quote the row and explain which measurement it applies to. A generic online table is not a substitute.

A six-minute practical script

1. Introduce yourself, confirm identity and explain that you are assessing alignment and eye movements.
2. Ensure spectacles are worn if required; inspect head posture, lids, pupils and obvious deviation.
3. Check fixation at distance and near with cover-uncover, then alternate cover.
4. Quantify with PBCT, recording each condition rather than one shorthand number.
5. Assess versions and ductions; note incomitance and ask appropriately about diplopia.
6. If shown a Hess chart, identify smaller field, maximal restriction and yoke overaction.
7. Summarise the pattern and say what further orthoptic/sensory/refraction or supervised assessment would be needed.

Practise that sequence with another resident using a timer. It protects you against the common practical problem: performing a technically acceptable cover test but failing to document what you did.

Revision plan: convert every measurement into a sentence

Do ten PBCT examples. For each, write a one-line report containing distance/near, correction, method, direction and magnitude. Then take five Hess charts and force yourself to name the smaller field before naming any muscle. Finally, take a set of mixed general ophthalmology MCQs so that strabismus is not always signposted by the question stem. The pediatric ophthalmology guide is the parent topic page for wider amblyopia, squint and paediatric revision; practical-station preparation is available through the site’s OSCE and viva resources.

Sources

• AAPOS: Prism and cover test
• PubMed: Quantitative analysis of binocular alignment by Hess screen testing
• NCBI Bookshelf: Strabismus