Binocular Vision
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Direct answer
Worth's grades give the vocabulary: binocular single vision is built of simultaneous perception, then fusion with amplitudes, then stereopsis — and it survives only while the eyes stay aligned within Panum's fusional area, so every heterophoria and heterotropia is a question about which grade has been lost or defended. The toolkit is short and logical: cover–uncover for the manifest deviation, alternate cover for the phoria it hides, prisms to measure, Worth four-dot to expose suppression or diplopia, and stereotests (Titmus, TNO) to grade the final payoff. The AC/A ratio (normal about 3–5 prism dioptres per dioptre) explains why some deviations flip between distance and near.
What you must remember
- Worth's three grades: simultaneous perception (superimposition of dissimilar images, e.g. lion in a cage on the synoptophore), fusion (merging similar images, with fusional amplitudes), and stereopsis (depth from retinal disparity).
- Prerequisites: clear images of similar size, aligned axes within Panum's fusional area, corresponding retinal points; failure produces diplopia, confusion or suppression.
- Cover–uncover test: occlude one eye — movement of the uncovered eye reveals a tropia; alternate cover breaks fusion and reveals the total deviation, measured with a prism bar.
- Worth four-dot (red over right eye): four lights = fusion (or harmonious anomalous correspondence); two lights = left suppression; three = right suppression; five = diplopia — crossed or uncrossed localises exotropia versus esotropia.
- AC/A ratio: normally 3–5 prism dioptres per dioptre; the gradient method measures phoria change with a −2.00 D lens, the heterophoria method compares distance and near deviations.
- Stereopsis: Titmus (polarised; fly about 3000 seconds of arc, circles to 40) carries monocular cues; TNO (red–green random dots, 480 down to 15 seconds of arc) has none and is the cleaner test; Lang needs no glasses.
- Suppression and anomalous retinal correspondence are the strabismic adaptations that abolish diplopia at the price of binocularity; amblyopia is the sensory consequence in the developing eye.
- Convergence insufficiency: near exophoria exceeding distance by roughly 10 prism dioptres, receded near point of convergence, low positive fusional vergence — treated with orthoptic exercises; convergence excess (high AC/A) patterns esodeviation worse at near, managed with bifocals in children.
A headache that is arithmetic, not mystery
A 19-year-old student gets brow ache and blurred print after two hours of reading, distance comfortable, refraction mildly hyperopic and corrected. The binocular work-up is arithmetic: distance cover test 2 Δ exophoria; near alternate cover 16 Δ exophoria — a near–distance gap of 14 prism dioptres. The near point of convergence is receded at 15 cm break; positive fusional vergence at near measures well below Sheard's criterion (vergence less than twice the phoria).
That constellation is convergence insufficiency, and the ladder is exercise-first: structured convergence therapy from pencil push-ups and dot cards to barrel cards and synoptophore fusion-range work, reviewed at six weeks against the same three numbers. The diagnosis the examiner probes here is decompensating heterophoria versus divergence insufficiency — but the latter is an esophoria worse at distance, the mirror image. Three numbers, two cover tests and a rule converted a vague headache into a measurable, treatable entity.
Where students slip
The failures are interpretation more than memory. Worth four-dot two-versus-three: with red over the right, seeing two red lights means the left eye is suppressed — candidates invert it because the white light's colour follows whichever eye perceives it. Cover test order: cover–uncover first (manifest deviation), then alternate cover (total); performing only alternate cover reports a huge "tropia" that is actually a phoria. The AC/A is asked as a number — 3 to 5 prism dioptres per dioptre — and the gradient method as a method: measure phoria, add −2.00 D, remeasure, divide the change by 2. On stereopsis, the TNO's advantage is specific: random dots carry no monocular cues, so it cannot be passed by a suppressor using one eye — the reason it ranks above Titmus for diagnosing true stereopsis.
Frequently asked questions
What are Worth's three grades of binocular single vision?
Simultaneous perception, fusion (with fusional amplitudes) and stereopsis — tested in sequence, classically on the synoptophore.
What does seeing five lights on the Worth four-dot indicate?
Diplopia — two images of each light; crossed diplopia suggests exotropia, uncrossed esotropia.
What is the normal AC/A ratio and how is it measured?
About 3–5 prism dioptres per dioptre; the gradient method compares phorias with and without a −2.00 D lens, the heterophoria method uses distance and near deviations.
Why is the TNO preferred to the Titmus test?
TNO uses random-dot red–green targets with no monocular cues, so it cannot be passed by suppression or monocular guessing — Titmus figures carry monocular depth cues.
What defines convergence insufficiency?
Near exophoria exceeding distance by about 10 Δ or more, a receded near point of convergence and low positive fusional vergence, with asthenopic symptoms — treated by orthoptic exercises.
What does alternate cover show that cover–uncover does not?
The total deviation — manifest tropia plus latent phoria — because sustained alternation breaks fusion; cover–uncover reveals only the tropia.