# Pupil Abnormalities > Pupil abnormalities for MBBS Ophthalmology: RAPD swinging flashlight test, Adie tonic pupil, Argyll Robertson, Horner syndrome and third nerve palsy. - Canonical URL: https://prepelephant.com/topics/mbbs/ophthalmology/pupil-abnormalities - Exam / course: MBBS · Subject: Ophthalmology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Pupil Abnormalities", PrepElephant, https://prepelephant.com/topics/mbbs/ophthalmology/pupil-abnormalities ## 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.