Squint Evaluation
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Direct answer
Evaluating a squint answers four questions in order: is it a true squint or pseudostrabismus from epicanthic folds or a wide interpupillary distance; is it concomitant (the same angle in every direction of gaze) or incomitant (varying with gaze, meaning a paretic or restrictive cause); is vision equal in both eyes; and is binocular single vision with stereopsis present? Corneal reflex tests (Hirschberg, then Krimsky with prisms) quantify roughly; cover tests reveal the tropia or phoria and prism alternate cover measures the deviation in prism dioptres; the synoptophore grades binocularity one to three and the Worth four-dot unmasks suppression and abnormal retinal correspondence. In every child, cycloplegic refraction and a dilated fundus examination precede all of it.
What you must remember
- Pseudostrabismus: prominent epicanthic folds covering the nasal sclera, wide interpupillary distance or abnormal angle kappa simulate a turn that cover tests refute; a positive angle kappa (commoner) simulates divergence — pseudoexotropia.
- Hirschberg test: a torch reflection decentred 1 mm from the pupil centre equals roughly 7 degrees, about 15 prism dioptres of deviation; a reflex at the pupillary margin, mid-iris and limbus corresponds to roughly 15, 30 and 45 degrees of deviation respectively.
- Krimsky test: prisms of increasing strength placed before the fixating eye until the corneal reflex recentres — the practical measure in uncooperative children and poor vision.
- Cover-uncover test detects a manifest deviation (tropia) and its direction; alternate cover dissociates fusion fully and reveals the total deviation (tropia plus phoria); prism alternate cover quantifies it for surgery.
- Concomitant versus incomitant: concomitant deviations measure the same in all gazes (typical childhood squint); incomitant deviations vary with gaze direction — paretic (diplopia maximal in the field of action of the weak muscle) or restrictive (thyroid eye disease, Duane syndrome, post-retinal detachment).
- Synoptophore grades of binocular single vision: grade one, simultaneous perception; grade two, fusion with amplitude; grade three, stereopsis — the ladder examined verbally and practically.
- Worth four-dot (with red filter over the right eye): four lights mean fusion or superimposition; two red lights mean left suppression; three green mean right suppression; five lights mean diplopia or unharmonious abnormal retinal correspondence.
- Bagolini striated glasses are the least dissociating, most physiological test of everyday binocularity; the 4-dioptre base-out prism test screens for fusion in infants.
- Mandatory paediatric workup: cycloplegic refraction (atropine 1 per cent in young children, cyclopentolate in older), dilated fundus examination — retinoblastoma, toxoplasmosis or other sensory causes hide behind a "simple" convergent squint — plus fixation behaviour and vision for the preliterate.
- Measure every deviation for distance and near, with and without glasses: the spectacle-wearing accommodative esotrope must be corrected fully with hypermetropic glasses (and bifocals for a high accommodative convergence-to-accommodation ratio) before anyone discusses surgery.
Cover tests demonstrated on two children
Sit a four-year-old with a small alternating esotropia in the chair. Cover the fixing right eye: the left eye swings out to take up fixation — a manifest left convergent squint; remove the cover and note whether control returns. Now alternate the cover rhythmically between the eyes: each eye moves outward as it is uncovered, and the movement is the same size in right, left and upgaze — concomitant. Put prisms base-in before one eye during alternating cover until the movement abolishes: that prism value, in prism dioptres, is your surgical number. Contrast an eight-year-old hit by a cricket ball: diplopia on right gaze, and alternate cover shows the deviation exploding in the field of action of the right lateral rectus — incomitant, paretic, needing neuro-ophthalmic follow-up rather than a simple muscle surgery. Between the two, the Worth four-dot at near on the younger child may report "three green lights" — suppression of the right eye, the sensory adaptation the brain used to escape double vision, and the reason amblyopia therapy starts before any cosmetic operation.
Where students slip
The leak with real consequences is skipping the fundus: a sensory squint from retinoblastoma or a toxoplasma macular scar sent straight to muscle surgery is the textbook disaster. Other slips are arithmetic (1 mm on Hirschberg equals about 15 prism dioptres), the angle kappa direction (positive simulates exotropia), and forgetting that accommodative esotropia is treated with full hypermetropic correction first — operating on an uncorrected accommodative squint wastes surgery on a child who needed glasses and bifocals.
Frequently asked questions
What is the Hirschberg test and its millimetre rule?
A torch shone from arm's length; each millimetre the corneal reflex is decentred from the pupil centre represents about 7 degrees, roughly 15 prism dioptres of deviation.
What does the alternate cover test add to cover-uncover?
Alternate cover fully dissociates fusion, exposing the total deviation — latent phoria plus manifest tropia — which prism alternate cover then quantifies for surgical planning.
In the Worth four-dot with red filter over the right eye, what does seeing only two red lights mean?
Suppression of the left eye, since only the red-filtered right eye contributes; three green lights indicate right suppression, five lights diplopia or unharmonious abnormal retinal correspondence.
Which instrument grades binocular vision, and what are its grades?
The synoptophore: grade one simultaneous perception, grade two fusion with convergent and divergent amplitudes, grade three stereopsis.
Why is cycloplegic refraction mandatory in childhood squint?
Because accommodation masks hypermetropia in children, and unmasked it drives accommodative esotropia — full plus correction frequently resolves the squint without surgery.
What is angle kappa and its squint mimic?
The angle between the visual and pupillary axes; a positive angle kappa displaces the corneal reflex nasally and simulates exotropia — pseudoexotropia — with full orthoptic normality.