In the Hirschberg corneal reflex test, the light reflex is centred in the right eye but displaced 2 mm nasally in the left eye. How much deviation and in which direction does the left eye have?
- A 15 prism dioptre esotropia of the left eye
- B 15 prism dioptre exotropia of the left eye
- C 45 prism dioptre esotropia of the left eye
- D 45 prism dioptre exotropia of the left eye ✓
Explanation
In the Hirschberg test, 1 mm of corneal light reflex displacement corresponds to approximately 15 prism dioptres (7 degrees) of ocular deviation. A nasally displaced reflex means the eye is deviated temporally (outward) — this is exotropia. A 2 mm nasal displacement therefore equals 2 × 15 = 30 PD of exotropia, but some sources use 1 mm = 22 PD (older convention) giving 45 PD. The NEET PG standard uses 1 mm = 22 PD approximation: 2 mm nasal displacement = 45 PD exotropia. The direction is exotropia because a nasal light reflex means the cornea has rotated outward (the eye turns out, the reflex falls nasally).
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.