Check the pupil before you believe the visual field

A relative afferent pupillary defect takes fifteen seconds and settles a question that imaging often cannot: is this optic nerve asymmetrically damaged? It is the cheapest objective test in the clinic and it is skipped more than any other.

It earns its place in three specific situations. In apparently unilateral visual loss with a normal-looking disc, a defect points to the nerve and away from an explanation the patient's own account may be steering you towards. In glaucoma, a defect that is disproportionate to the discs and fields should make you look for a second cause rather than assume asymmetric glaucoma - compressive lesions present this way. And in a patient whose field test is unreliable - poor fixation, high false positives, language or literacy barriers, which is a large fraction of practice in India - the pupil is the finding you can trust.

Use a bright light, swing rhythmically, and watch the pupil you are not illuminating as well as the one you are. Record it as present or absent with an estimate of grade, because the useful comparison is against the last visit.

• Test for a relative afferent pupillary defect in every case of unexplained monocular visual loss
• Look for a second cause when the defect is disproportionate to the discs and fields
• Trust the pupil over an unreliable visual field, not the other way round
• Record presence, absence and grade each visit so change is visible
• Watch the non-illuminated pupil too - the escape is easier to see than the constriction

Why it matters

It is the only objective, equipment-free test of asymmetric optic nerve function, and an unreliable field is common enough that something objective is needed.

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