6.4 Pupil Measurement, Shape, Reactivity & Evaluating Anisocoria

Systematic pupil examination

The blueprint lists five pupil tasks: measure, compare, evaluate, shape, RAPD. A reproducible routine covers all five in under a minute.

1. Set the conditions. Dim the room. Have the patient fixate a distance target — this is the single most-missed step, because a near target triggers the near response and constricts both pupils, hiding the finding.
2. Measure in dim illumination. Use a pupil gauge card held beside the eye with tangential light from below, or an infrared pupillometer. Record both pupils to the nearest 0.5 mm.
3. Measure in bright illumination. Raise room lights or use a bright diffuse source, with the patient still fixating at distance. Record both pupils again.
4. Assess the direct response. Shine a bright light into one eye and grade the constriction of that pupil: brisk, sluggish or absent, and note the amplitude.
5. Assess the consensual response. Shine the light into one eye and watch the fellow pupil constrict.
6. Assess the near response if indicated: have the patient look from a distance target to an accommodative near target and observe the near triad — miosis, accommodation and convergence.
7. Swinging flashlight test for a relative afferent pupillary defect, quantified with neutral density filters where present.
8. Record shape and any irregularity.

A typical normal entry: "Pupils 4.5 mm dark / 2.5 mm light OU, round, briskly reactive, no RAPD."

Why light and dark both matter

The dilator is sympathetic and does its work in the dark; the sphincter is parasympathetic and does its work in the light. So the illumination in which the difference is greatest names the damaged pathway.

Physiologic anisocoria also has normal, brisk reactions in both eyes and no ptosis. Old photographs, including driving licences, are genuinely useful in establishing that it is long-standing.

Building the anisocoria differential

Anisocoria greater in the dark — sympathetic.

• Horner syndrome: miosis plus about 2 mm of ptosis from Müller's muscle denervation, sometimes anhidrosis, and dilation lag (the affected pupil takes 15 to 20 seconds to reach its dark size, so the anisocoria is greatest at 5 seconds and less at 15 seconds). An acute painful Horner syndrome with neck pain is a carotid dissection until proven otherwise and is an emergency.
• Pharmacologic miosis: pilocarpine, opioids, organophosphates.

Anisocoria greater in the light — parasympathetic.

• Third nerve palsy: the dilated pupil is accompanied by ptosis and an eye that is down and out. A painful third nerve palsy with pupil involvement is a possible posterior communicating artery aneurysm — emergency imaging.
• Adie tonic pupil: a large pupil with light-near dissociation (poor light reaction, better and tonic near reaction with slow redilation), vermiform segmental iris movements on slit lamp, most often in a young woman, frequently with reduced deep tendon reflexes (Holmes-Adie).
• Pharmacologic mydriasis: a very large pupil, typically 8 mm or more, that does not constrict at all, with no ptosis and no motility deficit. Classic in health-care workers, patients handling scopolamine patches, and after accidental contact with plant alkaloids.
• Traumatic mydriasis / sphincter tears: irregular pupil margin, history of blunt trauma, often with iris transillumination defects.
• Acute angle closure: mid-dilated (5 to 6 mm), oval, poorly reactive, with a red painful eye, corneal oedema and a hard globe.

Light-near dissociation — poor light reaction with preserved near reaction — is itself a localising sign: Adie pupil, Argyll Robertson pupil (small, irregular, bilateral, classically neurosyphilis), Parinaud dorsal midbrain syndrome, and severe afferent disease.

Pupil shape

A coloboma of the iris points inferonasally because it follows the closure line of the embryonic fissure — a reliable exam discriminator from a surgical iridectomy, which is usually superior.

Pitfalls

• Testing with a near target — constricts both pupils and hides the abnormality. Always fixate at distance.
• Using a light that is too dim for the direct response, producing spuriously sluggish reactions.
• Not dimming the room before measuring the dark pupil size.
• Measuring after dilating drops — always check whether drops were instilled, and by whom, before concluding a pupil is abnormal.
• Recording PERRLA by default when only the light reaction was tested.
• Failing to look for ptosis alongside anisocoria; ptosis plus miosis is Horner, ptosis plus mydriasis is third nerve.