In the Hirschberg corneal light reflex test, 1 mm of corneal reflex displacement from the pupillary centre corresponds approximately to how many prism dioptres of deviation?

• A 22 prism dioptres ✓
• B 7 prism dioptres
• C 15 prism dioptres
• D 30 prism dioptres

Explanation

In the Hirschberg test, approximately 1 mm of corneal reflex displacement from the centre of the pupil corresponds to about 22 prism dioptres (or approximately 15 degrees) of ocular deviation. This is based on the geometry of the reduced eye model. A reflex at the pupil margin (~2.5 mm from centre) thus represents about 45–50 PD. This rule allows quick estimation of large angles of strabismus without formal prism-cover testing, though it is a rough guide subject to kappa angle variation.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.