Orthoptic Tests Made Easy: Cover Test, Hess Chart, Synoptophore, Worth 4-Dot and Stereopsis

Last updated: November 2026

Orthoptic tests answer three questions about a squint: how big is the deviation, how well do the two eyes work together, and is the problem paralytic or restrictive. Most of them are quick to do and easy to interpret once you know what the patient is looking at and which eye is behind which filter. This page goes through each test in the order you would use it in clinic, with how to read the result.

Measuring the deviation

Cover tests

Do the cover test with an accommodative target (letters or a picture), with and without glasses, and at both distances. Base-out prisms neutralise an esodeviation and base-in prisms an exodeviation: the apex of the prism points in the direction of the deviation.

Corneal reflex tests (when the cover test is not possible)

• Hirschberg test: the position of the corneal light reflex from a pen torch at 33 cm. Each 1 mm of decentration is roughly 7° (about 15Δ). Reflex at the pupil margin (3-mm pupil) is about 15°, mid-iris about 30°, and limbus about 45°. Remember angle kappa: a positive kappa (reflex slightly nasal) mimics exotropia.
• Krimsky test: place prisms in front of the fixing eye (or deviating eye, by your unit's method) until the reflex is centred in the deviating eye. Used in poor vision or uncooperative patients.
• Bruckner test: the red reflex is brighter in the deviating eye when both are viewed together with a direct ophthalmoscope; a quick screen in children.

Phorias and fusional reserves

Normal fusional amplitudes vary with the method and the target. As a guide: at distance, base-out break about 15–20Δ and base-in 6–8Δ; at near, base-out 25–35Δ and base-in 12–15Δ; vertical reserves only 2–4Δ, which is why a small vertical deviation causes symptoms. Sheard criterion: the opposing reserve should be at least twice the phoria. See convergence insufficiency and excess.

Tests of binocular function

Worth four-dot test

The patient wears red-green goggles: by convention red over the right eye, green over the left. The torch shows one red, two green and one white light. The red lens lets the right eye see only red, the green lens lets the left eye see only green, and the white light is seen by whichever eye is dominant (or by both, in fusion).

At distance (6 m) the dots subtend a small central area, testing central fusion; at near (33 cm) they fall on a larger, more peripheral area. A patient who fuses at near but suppresses at distance has peripheral fusion with a central suppression scotoma, typical of monofixation syndrome (microtropia).

Bagolini striated glasses

Plano lenses with fine striations at 45° and 135° turn a spot light into a streak for each eye. Because the glasses barely disturb normal viewing, it is the most natural test of correspondence. A cross with the light at its centre means fusion (or harmonious ARC with a manifest squint); a missing streak means suppression; a gap in one streak near the light means a small central suppression scotoma.

4Δ base-out test

A 4Δ base-out prism is put in front of one eye. If that eye has a central suppression scotoma (microtropia), the image moves within the scotoma and neither eye moves. If it does not, the eyes shift to refixate. Useful for picking up the deviating eye in monofixation.

Stereopsis

Normal adult stereoacuity is about 40–60″ or better. Random-dot tests have no monocular clues, so a pass is good evidence of binocular vision. Reduced stereoacuity with otherwise straight eyes is a clue to anisometropic amblyopia or microtropia.

The synoptophore (major amblyoscope)

Two tubes, one for each eye, each with a mirror and a +6.50 D lens so the eyes view slides at optical infinity. The arms can be moved horizontally, vertically and torsionally, and the scale reads the deviation in degrees.

• Grades of binocular single vision (Worth): grade 1, simultaneous perception (different slides, such as a lion and a cage, seen together); grade 2, fusion (similar slides with control marks fused into one, and a fusional range measured); grade 3, stereopsis (slightly dissimilar slides seen in depth).
• Objective angle: measured by flashing the lights alternately and moving the tubes until the eyes stop moving (like a cover test). Subjective angle: where the patient sees the lion inside the cage.
• Retinal correspondence: objective angle = subjective angle means normal retinal correspondence (NRC). If they differ, there is anomalous retinal correspondence (ARC); the difference is the angle of anomaly. Subjective angle zero with a manifest squint = harmonious ARC; subjective angle between zero and the objective angle = unharmonious ARC.
• Other uses: measuring deviation in nine gaze positions, torsion, fusional range before surgery (to predict postoperative diplopia), and orthoptic exercises.

Hess and Lees charts: how to read them

The Hess (red-green) and Lees (two mirrored screens) tests plot the position of each eye in nine gaze directions while the other eye fixes. They separate the eyes by dissociation, so the chart shows each eye's motility, the primary and secondary deviations, and the sequelae of a palsy over time.

1. Find the smaller field. The eye with the smaller field has the underacting (paretic or restricted) muscle.
2. Find where it is most reduced. That position is the field of action of the affected muscle (for example, right lateral rectus: right eye field reduced on right gaze).
3. Look at the other eye. Its field is enlarged in the corresponding direction: overaction of the contralateral synergist (yoke muscle) by Hering's law. This is the secondary deviation, larger than the primary.
4. Look for sequelae: overaction of the yoke, contracture of the ipsilateral antagonist, then secondary underaction (inhibitional palsy) of the contralateral antagonist. With time the fields become more equal (spread of comitance).
5. Neurogenic or mechanical? In a neurogenic palsy the field is smaller in one direction with evenly spaced (proportional) squares and the yoke overacts. In a mechanical (restrictive) problem, such as a blowout fracture or thyroid eye disease, the field is compressed in opposite directions (for example up and down), the inner and outer fields are not proportional, the yoke overacts little, and forced duction is positive.

Repeating the chart over time shows recovery (fields expanding) and helps decide when to operate, usually after 6–12 months of stability.

Diplopia charting and the field of binocular single vision

• Diplopia charting: with red glass over the right eye, the patient looks at a light in the nine positions and describes the two images. The separation is greatest in the field of action of the paretic muscle, and the more peripheral image belongs to the affected eye. Crossed or uncrossed and the vertical position of the images point to the muscle.
• Field of binocular single vision (on a Goldmann or arc perimeter): the area in which the patient sees single. Normal is about 45–50° in all directions; at least the central 20° and the downgaze reading position matter most for function and DVLA-type standards.
• Parks–Bielschowsky three-step test for vertical deviations: which eye is higher in primary gaze, is it worse on right or left gaze, and is it worse on right or left head tilt. See cranial nerve palsies.

Torsion

• Double Maddox rod: a red and a white rod, one over each eye, in a trial frame. The patient rotates them until the lines are parallel; the reading shows the cyclodeviation. Excyclotorsion over about 10° suggests bilateral fourth nerve palsy.
• Fundus torsion: an objective sign. Normally the fovea lies between horizontal lines through the centre and the lower pole of the disc; in excyclotorsion it lies below the lower pole, and in incyclotorsion above the centre.
• Synoptophore and Maddox wing also measure cyclodeviation.

Test yourself: flip cards

Quick quiz

Related: Strabismus Measurement and Surgical Dose Tables: Quick Reference, Approach to Diplopia: Monocular vs Binocular and the Three-Step Test, Esotropia and Exotropia: Types, Tests and Surgery, Convergence Insufficiency and Convergence Excess: Diagnosis, Treatment and Exercises, Third, Fourth & Sixth Nerve Palsies.

Key sources

• American Academy of Ophthalmology. Basic and Clinical Science Course, Section 6: Pediatric Ophthalmology and Strabismus (current edition)
• Ansons AM, Davis H. Diagnosis and Management of Ocular Motility Disorders. 4th ed. Wiley-Blackwell; 2014
• von Noorden GK, Campos EC. Binocular Vision and Ocular Motility. 6th ed. Mosby; 2002
• Rowe FJ. Clinical Orthoptics. 3rd ed. Wiley-Blackwell; 2012