Test for defective afferent pupillary response
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* Alex Porzecanski

As an ophthalmologist, I have a question regarding the clinical history for the 16-year-old boy with progressive vision loss described by Shaun Morris and associates:1 Was the patient tested for a defective afferent pupillary response in the left eye? This is an important clinical sign of optic nerve function and in general is much more clinically significant than a colour vision test.

For patients presenting with unilateral loss of vision, the possible cause, be it retinal or related to the optic nerve, can be pinpointed by the results of the swinging flashlight test, also known as the Marcus–Gunn pupil test, which tests for defective afferent pupillary reflex.

The test consists of alternately illuminating the pupils with a penlight (or even the white light of an ophthalmoscope) in a dimly lit room, swinging the light from one eye to the other every 2 seconds or so. The observer checks for visible dilatation of the pupil in the eye with visual loss.

A positive result, in which the pupil exhibits visible dilatation, indicates optic nerve dysfunction or retinal “disconnection” (as in retinal detachment). A negative result, characterized by the maintenance of the initial pupillary constriction in both eyes equally, tends to indicate that the problem lies elsewhere, such as loss of vision associated with macular lesions, retinal vascular disease or cataracts.

**Alex Porzecanski** Ophthalmologist Victoria, BC

## Reference

1.  1. Morris S, Hiraki L, Muise A. A 16-year-old boy with progressive central vision loss in his left eye. CMAJ 2004;170(8):1228.
    
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