Squint and Amblyopia
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Direct answer
Squint and amblyopia are linked: squint is misalignment of the eyes, and amblyopia is reduced best-corrected vision, usually in one eye, without an organic cause, arising in the sensitive period of visual development. Occlusion therapy or atropine penalisation of the better eye works best when started before about seven to eight years of age. The Hirschberg test and the cover test are the key bedside tools for detecting misalignment.
What you must remember
- Hirschberg test: the corneal light reflex is displaced from the centre in a squint, with each millimetre roughly equal to 7 degrees of deviation.
- Cover test: covering one eye reveals a manifest squint when the other eye moves, while the alternate cover test uncovers a latent deviation.
- Commonest childhood squint: esotropia, with infantile esotropia appearing in the first six months of life.
- Amblyopia causes: strabismus, anisometropia and, most severely, deprivation from cataract or ptosis.
- Critical period: the sensitive window for treatment closes roughly around seven to eight years of age.
- Occlusion therapy: the better eye is patched to force use of the amblyopic eye.
- Atropine penalisation: an alternative to patching, applied to the better eye.
A clinic pathway for a child with a squint
Squint assessment combines acuity testing, light reflex and cover tests, and refraction. The steps below show the usual order in clinic, since amblyopia is often silent.
- Check visual acuity: test each eye separately with an age-appropriate method, such as fixation preference, because amblyopia rarely causes symptoms.
- Perform the Hirschberg test: shine a light on both corneas and note the reflex position, since each millimetre of displacement is roughly 7 degrees.
- Perform the cover test: cover one eye and watch the other for movement, which reveals a manifest squint. Repeat the test on the other eye.
- Perform the alternate cover test: alternate covering to uncover latent deviation, which the normal fusion mechanism may hide.
- Assess refraction: cycloplegic refraction detects hypermetropia or anisometropia, which need glasses alongside any patching.
- Start amblyopia treatment: patch the better eye for a prescribed number of hours daily, or use atropine penalisation, with regular acuity review.
- Refer for surgery if needed: strabismus surgery is considered when the deviation persists after amblyopia therapy or is large.
Where the exam tries to catch you
Squint and amblyopia questions often test the eye that is patched and the age limit for treatment.
- Trap: patching the squinting eye for amblyopia Correct: the better-seeing eye is patched to force use of the amblyopic eye.
- Trap: dismissing a squint as cosmetic only Correct: untreated squint causes amblyopia and loss of binocular vision, so early treatment matters.
- Trap: deferring treatment until school age Correct: treatment works best before about seven to eight years, while the visual pathway is still plastic.
- Trap: assuming a normal light reflex excludes a squint Correct: the cover test can detect latent or manifest deviations that the light reflex misses.
Frequently asked questions
What is the Hirschberg test in squint assessment?
The Hirschberg test shines a light on both corneas and observes the position of the corneal light reflex. A reflex displaced from the centre of the pupil indicates a squint, and each millimetre of displacement roughly corresponds to 7 degrees of deviation.
What is the difference between a manifest and a latent squint?
A manifest squint is a visible misalignment that appears when the eyes are open and is detected by the cover test. A latent squint is a tendency to deviate that fusion normally controls, revealed only when one eye is covered in the alternate cover test.
Why is the better eye patched in amblyopia treatment?
The better eye is patched in amblyopia treatment to force the child to use the amblyopic eye, stimulating its visual pathway during the sensitive period. Atropine penalisation of the better eye is an alternative when patching is not tolerated.
What is the critical period for amblyopia treatment?
The critical period for amblyopia treatment runs roughly through the first seven to eight years of life, while the visual pathway remains plastic. Treatment started early has the best chance of restoring acuity, although some children respond later.
Why is deprivation amblyopia the most severe form?
Deprivation amblyopia is the most severe form because a dense cataract or ptosis blocks form vision during the sensitive period. It needs urgent treatment, since it responds least well to later therapy.