Pupil Abnormalities
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Direct answer
The pupil examination divides every abnormality into afferent and efferent limbs: an afferent problem (optic nerve or severe retinal disease) leaves both pupils equal but produces a relative afferent pupillary defect on the swinging flashlight test — illumination of the bad eye causes both pupils to dilate rather than constrict — while an efferent problem makes the pupils unequal through third nerve palsy (large fixed pupil with ptosis and motility restriction), Adie tonic pupil (large, light-near dissociation, vermiform movement) or Horner syndrome (small pupil, mild ptosis, anhidrosis). Light-near dissociation — failure to light with preserved near constriction — indexes Adie pupil, Argyll Robertson pupils and dorsal midbrain (Parinaud) syndrome. The organising questions: is the problem anisocoria or an RAPD, and is the big or the small pupil abnormal?
What you must remember
- Swinging flashlight test: move a bright light between the two eyes; a dilating response when the light reaches the affected eye is a relative afferent pupillary defect (Marcus Gunn pupil), graded 1+ to 4+ — the most objective sign of optic nerve disease, absent in pure media opacity like cataract.
- Physiological anisocoria: up to 1 mm in roughly 20% of normal people, equal in light and dark — decide first whether the inequality is abnormal at all.
- Big pupil in the emergency department: a dilated sluggish or fixed pupil with ptosis and a down-and-out eye signals a compressive third nerve lesion — posterior communicating aneurysm or uncal herniation (the Hutchinson pupil) — and is a neurosurgical urgency; a pupil-sparing painful palsy in an older diabetic points to microvascular infarction.
- Adie tonic pupil: commonest in young women, usually unilateral; large pupil with slow tonic redilatation, light-near dissociation, vermiform segmental constriction, and absent tendon reflexes in Holmes-Adie syndrome; from ciliary ganglion damage, benign.
- Argyll Robertson pupils: bilateral small, irregular pupils that "accommodate but do not react", with light-near dissociation — the classic sign of neurosyphilis; diabetes can rarely mimic it, so serology settles it.
- Horner syndrome: miosis with mild ptosis, apparent enophthalmos and ipsilateral anhidrosis; confirm with apraclonidine and localise pharmacologically; causes include Pancoast tumour, carotid dissection, neck surgery and cluster headache — an acquired Horner in a child is neuroblastoma until imaged.
- Parinaud (dorsal midbrain) syndrome: light-near dissociation with mid-dilated pupils, convergence-retraction nystagmus, lid retraction (Collier sign) and upgaze paralysis from pineal region lesions or hydrocephalus.
- Pharmacological pupils: accidental atropine-like mydriasis (wide, fixed, no motility deficit); ask about drops, plants and inhalers before imaging a "third nerve palsy" that moves normally.
How to work through unequal pupils at the bedside
A 34-year-old woman attends because colleagues noticed her right pupil is bigger than the left. Run the algorithm: vision, motility and lids first — full movements, no ptosis: the blown third nerve exits early. Anisocoria is 2 mm, greater in light than dark, so the big pupil fails to constrict — the right is abnormal. At the slit lamp the right iris shows sectoral vermiform movement; light constricts it poorly, near effort slowly and excessively with tonic redilatation — the Adie pupil; test the ankle jerks, reassure her, and offer weak pilocarpine for glare. Now the reverse case for contrast: a 52-year-old smoker with a 1.5 mm smaller left pupil, mild left ptosis and worse anisocoria in the dark — the small pupil fails to dilate, so the left is abnormal: Horner syndrome; confirm with apraclonidine and image the chest apex and carotid, because a Pancoast tumour presenting as "cosmetic ptosis" is the miss this algorithm exists to prevent.
Where students slip
The swinging flashlight test is performed wrongly more often than rightly: the flashlight must dwell two to three seconds on each eye while both pupils are watched — judging only the illuminated pupil destroys the test's logic. The second slip is assigning the wrong pupil: anisocoria greater in the dark means the small pupil is abnormal (fails to dilate — Horner), greater in the light means the large one is abnormal (fails to constrict — Adie or third nerve); reversing this sends patients down the wrong imaging pathway. Third, a long-standing Adie pupil is often the smaller one — the "little old Adie" — and gets labelled Horner; the vermiform movement and tonic near response sort it out.
Frequently asked questions
How is a relative afferent pupillary defect elicited?
By swinging a bright light between the eyes with a two-to-three second dwell: both pupils dilate when the light reaches the affected eye, indicating reduced afferent input — graded 1+ to 4+.
What is light-near dissociation and which three syndromes cause it?
Failure to constrict to light with preserved near constriction — Adie pupil, Argyll Robertson pupils (neurosyphilis) and dorsal midbrain (Parinaud) syndrome.
Which pupil abnormality is a neurosurgical emergency?
A dilated fixed pupil with ptosis and impaired ocular motility — compressive third nerve palsy from posterior communicating aneurysm or uncal herniation, demanding urgent imaging.
How is Horner syndrome confirmed and localised?
Apraclonidine (reversal of anisocoria from denervation hypersensitivity) confirms it; anhidrosis pattern and imaging of the chest apex, neck and carotid localise the lesion.
What distinguishes an Adie pupil from an Argyll Robertson pupil?
Adie is unilateral (usually), large and tonic with vermiform segmental movement in a young woman; Argyll Robertson pupils are bilateral, small, irregular and associated with neurosyphilis requiring serology.