Squint
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squint strabismus types classification treatment overview
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Squint (Strabismus)
Squint, medically termed strabismus, is any misalignment of the visual axes of the two eyes — also called "cross-eye" or lack of ocular fusion in the horizontal, vertical, or rotational coordinates.
Types (by direction of deviation)
| Type | Direction |
|---|---|
| Esotropia | One or both eyes turn inward toward the nose ("crossed eyes") |
| Exotropia | One or both eyes turn outward |
| Hypertropia | One or both eyes deviate upward |
| Hypotropia | One or both eyes deviate downward |
— Guyton and Hall Textbook of Medical Physiology, and Kanski's Clinical Ophthalmology, p. 751
Latent vs. Manifest
- Heterophoria (phoria) — latent deviation, controlled by fusion; only revealed by dissociation (e.g., cover test). Subtypes: esophoria, exophoria.
- Heterotropia (tropia) — manifest, constant or intermittent; fusion is broken.
Classification of Esotropia (most common form)
Kanski classifies concomitant esotropia as:
| Category | Key Features |
|---|---|
| Early-onset (infantile) | Develops within 6 months of life; large angle (>30Δ), alternating fixation, cross-fixation in side gaze |
| Fully accommodative | Due to uncorrected hypermetropia; glasses correct the deviation entirely |
| Partially accommodative | Glasses reduce but do not eliminate the deviation |
| Non-refractive (convergence excess) | Near deviation > distance; AC/A ratio high |
| Acute (late-onset) | Sudden diplopia ~5–6 years; must exclude sixth nerve palsy and CNS lesion |
| Sensory (secondary) | Caused by unilateral VA loss (cataract, optic atrophy, retinoblastoma) disrupting fusion |
| Consecutive | After surgical over-correction of an exodeviation |
| Sagging eye syndrome | Elderly; age-related ligament degeneration → bilateral inferior sagging of lateral rectus pulley |
— Kanski's Clinical Ophthalmology 10th, Table 18.1
Pathophysiology
- Neuromuscular abnormality — congenital or acquired disorder of extraocular muscles, their nerves (CN III, IV, VI), or the brainstem/cortical control centres.
- Abnormal fusion "set" in childhood — one eye fixates well while the other fails to do so simultaneously; conjugate movement patterns become abnormally fixed in neural pathways so the eyes never fuse.
- Sensory consequences at squint onset:
- Diplopia (pathological) — the same object projects to non-corresponding retinal areas.
- Confusion — different objects from each eye are projected to the same subjective space.
- Young children rapidly develop suppression of the deviating eye's image to eliminate these.
Amblyopia ("Lazy Eye")
Prolonged suppression of the squinting eye leads to amblyopia — reduced best-corrected VA that cannot be explained by structural abnormality. The neuronal connections in the visual cortex from the repressed eye are reduced in number. VA may fall to 20/400 or worse if untreated. Recovery is far better when treated early (within the sensitive period of visual development).
Concomitant vs. Incomitant
- Concomitant — angle of deviation is the same (<5Δ variation) in all positions of gaze. Most childhood esotropias are concomitant.
- Incomitant — angle varies with gaze direction, due to muscle restriction or abnormal innervation (e.g., paralytic strabismus from CN III/IV/VI palsy).
Key Investigations
| Test | Purpose |
|---|---|
| Cover-uncover test | Detects manifest deviation (tropia) |
| Alternate cover test | Uncovers latent + manifest deviation (total phoria + tropia) |
| Prism cover test | Measures angle of deviation in prism dioptres |
| Hirschberg corneal reflex | Estimates angle; 1 mm displacement ≈ 7Δ |
| Maddox rod | Dissociates the eyes to measure phorias |
| Synoptophore | Measures angle in all gaze positions; assesses grades of binocular vision (simultaneous perception, fusion, stereopsis) |
| Cycloplegic refraction | Essential to identify accommodative component |
| Fundus examination under mydriasis | Exclude retinoblastoma, optic atrophy, macular disease as cause |
Treatment Principles
- Correct refractive error — spectacles (especially for accommodative esotropia; full hypermetropic correction).
- Treat amblyopia — occlusion (patching) of the dominant eye to stimulate the lazy eye.
- Orthoptic exercises — for convergence insufficiency, phoria control, fusional amplitude training.
- Prisms — Fresnel or incorporated in spectacles (up to 10–12Δ per eye) to neutralise the deviation.
- Botulinum toxin — chemical weakening of an overacting muscle; useful in acute esotropia and as alternative to surgery.
- Surgery — recession (weakening) or resection (strengthening) of extraocular muscles; indicated for large angles, where glasses/prisms fail, or to restore binocular vision.
Sources:
- Kanski's Clinical Ophthalmology: A Systematic Approach, 10th edition, pp. 718–770
- Guyton and Hall Textbook of Medical Physiology, pp. 651–652
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