Pupil Assessment

Pupil Assessment is 7% of the exam. It covers pupil size, shape, and reactivity; the direct and consensual responses; the swinging-flashlight test; and afferent pupillary defects.

Two iris muscles — the sphincter pupillae, a ring of muscle at the pupil margin, constricts the pupil (miosis); its nerve supply is parasympathetic, carried by CN III. The dilator pupillae, running outward like spokes, enlarges it (mydriasis); its supply is sympathetic. Light and near focus tighten the sphincter; in the dark the sphincter relaxes and the dilator widens the pupil.

The light reflex has two limbs — the afferent (incoming) limb carries light information from the retina along the optic nerve (CN II); the efferent (outgoing) limb returns the command to constrict through parasympathetic fibers riding CN III. An afferent fault in one eye, if large enough, causes a relative afferent pupillary defect (RAPD); an efferent fault, a poorly reacting, often larger pupil.

Pupils are examined BEFORE any drops, in a dim room, with the patient fixing a distant target (near fixation constricts pupils) and the light brought in from below or the side. Dim light lets the pupils show their range; off-axis light keeps the patient from focusing on it. Before starting, ask about recent drops, including dilation elsewhere today or yesterday and glaucoma miotics such as pilocarpine.

Why before dilation, every time — routine dilating drops leave both pupils wide and barely reactive for hours (days after atropine), so the light reflex, the near constriction and the swinging-flashlight test can no longer be judged, and an RAPD or a third-nerve pupil missed beforehand stays hidden until the drops wear off. The drops and the shallow-chamber screen are in the Pharmacology guide.

Size — measure both pupils in dim and bright light with a pupil gauge, in millimeters. The gauge is a card of printed circles or half-circles held beside the eye. Adult pupils typically measure about 2–4 mm in bright light and 4–8 mm in the dark, and they get smaller with age. Words such as small or medium cannot be compared across visits; millimeters can. A difference in size between the two pupils is called anisocoria.

Shape — a normal pupil is round. Note irregularity (surgical, traumatic, or synechiae from old iritis). Posterior synechiae are iris-to-lens adhesions left by inflammation.

Direct and consensual response — light in one eye should constrict both: the illuminated eye's constriction is the direct response, the fellow eye's is the consensual response. Both exist because each eye's signal is distributed to both CN III nuclei. Shine the light in the right eye, and the left pupil's constriction is the consensual response.

Reactivity — light each eye in turn for a few seconds, watching first the lit pupil (direct), then the other (consensual). Describe each reaction as brisk, sluggish, or absent (fixed), and compare the eyes.

Near response — convergence, accommodation, and miosis together: the eyes turn in, the lens focuses, and both pupils constrict. Test it by having the patient look from the distant target to a detailed near target, such as small print. A penlight is the wrong near target, because its own brightness triggers the light reflex.

Light-near dissociation — pupils that react better to near than to light, as Argyll Robertson and Adie pupils do. The near test matters most when the light reaction is poor.

Swinging-flashlight test — alternate the light between eyes every 2–3 seconds with equal illumination. Both pupils dilating when the light reaches one eye is a relative afferent pupillary defect (RAPD, Marcus Gunn pupil).

Technique discipline — equal brightness, equal duration (2–3 s), same angle and distance on each eye, brisk transit across the nose. Sloppy technique both creates false RAPDs and hides real ones.

What it means — the two eyes deliver unequal afferent signal: asymmetric optic-nerve disease (optic neuritis, ischemic optic neuropathy, advanced glaucoma) or widespread retinal dysfunction (large detachment, vascular occlusion).

What it does NOT come from — refractive error, amblyopia (usually trace at most), or cataract. Even a dense white cataract transmits enough light: an RAPD behind a cataract means something else is wrong — retina or nerve — and changes surgical counseling.

Why the pupils stay EQUAL — each eye's input drives both pupils, so an afferent defect never causes anisocoria. The RAPD is purely a dynamic finding in the swinging light; unequal pupils point to efferent or structural causes instead. Even an eye blinded by optic-nerve or retinal disease keeps a pupil the size of its partner's.

Relative means relative — equal disease in both eyes shows no RAPD, so a normal test cannot clear both nerves, and a bilateral RAPD is a contradiction in terms.

When one pupil cannot move (the reverse RAPD check) — a pupil fixed by injury, surgery, or a third-nerve palsy does not stop the test, since light in either eye also drives the working pupil. Swing as usual and watch only the working pupil: if it dilates as the light lands on an eye, that eye has the RAPD, even when it is the eye with the fixed pupil.

Grading — neutral-density filters over the better eye until the response balances quantify the defect in log units, letting the practice track optic-nerve disease over time. The number describes the eye without the filter. Many clinics instead grade the defect from 1+ (mild) to 4+ (marked); follow the clinic's method and always name the eye.

First check the drop history — a pharmacologically dilated or constricted pupil tells you about the drop, not the patient. Then use lighting to localize. Constriction is the light's job, so a pupil that cannot constrict stands out in bright light; dilation is the dark's job, so a pupil that cannot dilate falls behind in dim light. For example, pupils of 6 and 4 mm in dim light but 3 and 3 mm in bright light differ more in the dark, so the smaller pupil is abnormal.

Horner syndrome — a droopy lid (ptosis), a small pupil (miosis) and reduced sweating (anhidrosis) on one side; the small pupil is also slow to widen in the dark (dilation lag). A lesion anywhere along the sympathetic route, from the brainstem down to the top of the lung and back up along the carotid artery to the eye, can cause it; hence the workup.

Third-nerve palsy or a drug? — both leave a large pupil that reacts poorly. A third-nerve palsy brings company: a droopy lid and an eye that rests down and out (the Ocular Motility Testing guide covers the muscles). A drug-dilated pupil is usually very wide, with a normal lid and normal eye movements. Pilocarpine testing is done only on the physician's order.

A complete entry: "Pupils 4→2 mm OU, brisk, no APD." Sizes, reactivity, APD status — comparable at every future visit. Here 4→2 means 4 mm in dim light, constricting to 2 mm when lit.

PERRLA — pupils equal, round, reactive to light and accommodation. Only chart the 'A' in PERRLA if it was actually tested; many clinics prefer PERRL plus an explicit APD statement. PERRL itself says nothing about an RAPD; charts write it as APD or RAPD, the same finding.

Immediately — new anisocoria with ptosis, diplopia, or pain goes to the physician immediately. So does a large, poorly reactive pupil with a droopy lid or a down-and-out eye: a possible third-nerve palsy. So does a new RAPD with sudden vision loss, where minutes matter. A mid-dilated pupil in a painful red eye with halos means possible angle closure: measure IOP and tell the physician now.

At that visit — any other newly found RAPD or new anisocoria goes to the physician before the patient leaves, whatever its likely cause. An RAPD behind a cataract is flagged before any surgical counseling.

Be ready to pair each iris muscle with its nerve supply, name the reflex's two limbs, tell direct from consensual, list the near triad, run and grade the swinging-flashlight test, explain why an RAPD leaves the pupils equal and why cataract alone does not cause one, pick the abnormal pupil from light and dark sizes, recognize the classic pupils, chart PERRL and PERRLA correctly, and know which findings go to the physician immediately.