# Pupil Abnormalities > Pupil abnormalities for FMGE Ophthalmology: RAPD and swinging torch test, Horner syndrome, Adie tonic pupil, Argyll Robertson and anisocoria rules. - Canonical URL: https://prepelephant.com/topics/fmge/ophthalmology/pupil-abnormalities-fmge - Exam / course: FMGE · 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/fmge/ophthalmology/pupil-abnormalities-fmge ## Direct answer Swing a torch from one eye to the other and watch both pupils: when the light lands on a damaged optic nerve, both pupils dilate slightly instead of constricting — a relative afferent pupillary defect (Marcus Gunn pupil), the most informative pupil sign in neuro-ophthalmology. On the efferent side, unequal pupils (anisocoria) resolve with two questions: which pupil is abnormal, and does the difference grow in light or in dark? A difference worse in light indicts the parasympathetic pathway (third nerve palsy, Adie tonic pupil, Argyll Robertson pupil); worse in dark indicts the sympathetic chain (Horner syndrome — miosis, ptosis, anhidrosis and apparent enophthalmos, confirmed when apraclonidine reverses the anisocoria). Light-near dissociation — pupils that fail to light but constrict to near — belongs to Adie pupil, neurosyphilis and dorsal midbrain lesions, a trio the exam recycles endlessly. ## What you must remember - **Swinging torch test:** afferent disease shows paradoxical dilatation of both pupils when the torch swings to the affected eye; it is the hallmark of optic nerve lesions (optic neuritis, ischaemic optic neuropathy, ischaemic central retinal vein occlusion) and is characteristically absent in pure media opacities such as cataract. - **Physiological anisocoria:** difference under 1 mm, equal in light and dark, both pupils reacting normally. - **Horner syndrome:** miosis, mild ptosis (Mueller muscle), apparent enophthalmos, ipsilateral anhidrosis when the lesion is proximal to the carotid bifurcation; causes include Pancoast tumour, carotid dissection, neck surgery and birth injury (congenital cases with iris heterochromia). - **Horner pharmacology:** cocaine 10 per cent fails to dilate the affected pupil (classical confirmation); apraclonidine 0.5 to 1 per cent now preferred — it dilates the denervated pupil by supersensitivity and reverses the anisocoria. - **Adie tonic pupil:** ciliary ganglion damage, young women; slit-lamp sectoral palsy with vermiform writhing, light-near dissociation, slow redilation after near effort; dilute pilocarpine 0.125 per cent constricts it (denervation supersensitivity) but not a normal pupil. - **Argyll Robertson pupil:** small, irregular, both eyes, accommodates but does not react — neurosyphilis is the classical cause; screen accordingly. - **The fixed dilated pupil emergencies:** third nerve compression (posterior communicating aneurysm, uncal herniation) — especially with ptosis and down-and-out eye; and anticholinergic or drug-induced mydriasis, which lacks ptosis and motility loss. - **Light-near dissociation trio:** Adie tonic pupil, Argyll Robertson pupil, dorsal midbrain (Parinaud) syndrome — with convergence-retraction nystagmus and lid retraction completing the midbrain picture. ## A systematic look at unequal pupils A 35-year-old nurse notices her right pupil is larger than the left, spotted in a mirror. First confirm both pupils react and measure the difference in light and dark: the anisocoria is 2 mm and worse in the light, so the large right pupil is the abnormal one — a parasympathetic problem. Examine under the slit lamp: the right iris shows slow, wormlike segmental movements and constricts briskly to a near target but barely to light, redilating sluggishly afterwards — the vermiform movement and light-near dissociation of a tonic pupil. Dilute pilocarpine 0.125 per cent constricts the right pupil and leaves the left untouched, confirming denervation supersensitivity of the ciliary ganglion. Reassure her; the pupil often shrinks over years and reading glasses manage the near blur. Now contrast the man in the next bay with a small left pupil and drooping lid after neck radiotherapy: anisocoria worse in the dark, so the small pupil is abnormal — sympathetic failure. Apraclonidine testing confirms Horner syndrome, and imaging from chest apex to skull base hunts the lesion. Two unequal pupils, two nervous systems — the diagnosis flows from two bedside measurements. ## Where candidates slip The first error is calling every anisocoria pathological; the second is failing to use light-versus-dark to pick the culprit, which converts a two-step diagnosis into guesswork. The Adie versus Argyll Robertson discrimination is a favourite: both show light-near dissociation, but Adie is a large tonic pupil in a young woman while Argyll Robertson is small, irregular and bilateral with neurosyphilis — size alone answers it. Third, the painless fixed dilated pupil after surgery or aerosol drug exposure (anticholinergic mydriasis) gets over-investigated, while the dilated pupil with ptosis and headache gets under-investigated — the latter is the aneurysm until imaging says otherwise. Finally, dense media opacity does not produce a relative afferent pupillary defect — a "negative RAPD with poor vision" points away from the optic nerve. ## Frequently asked questions ### How is a relative afferent pupillary defect elicited? By the swinging torch test — both pupils constrict when light falls on the good eye but dilate paradoxically when it swings to the affected eye, indicating asymmetric optic nerve or severe retinal disease. ### Which drugs confirm and localise Horner syndrome? Apraclonidine reverses the anisocoria by supersensitivity (confirmation), while cocaine 10 per cent classically fails to dilate the affected pupil; hydroxyamphetamine helps localise the lesion to the third-order neuron by its release mechanism. ### What characterises the Adie tonic pupil? Sectoral iris palsy with vermiform movements, light-near dissociation, slow redilation, and constriction to dilute 0.125 per cent pilocarpine from denervation supersensitivity. ### Which three conditions cause light-near dissociation? Adie tonic pupil, Argyll Robertson pupil (neurosyphilis) and dorsal midbrain (Parinaud) syndrome. ### When is a dilated pupil a neurosurgical emergency? A fixed dilated pupil with ptosis, ophthalmoplegia or headache suggests compressive third nerve palsy — posterior communicating aneurysm or uncal herniation — demanding urgent imaging.