# Pupillary Abnormalities and Horner Syndrome > Pupillary abnormalities and Horner syndrome for NEET-PG Medicine, covering afferent defects, Adie and Argyll Robertson pupils and the three-neuron pathway. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/pupillary-abnormalities-and-horner-syndrome - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-14 - Last updated: 2026-10-14 - Reference basis: Adams and Victor's Principles of Neurology; Bradley's Neurology in Clinical Practice - How to cite: "Pupillary Abnormalities and Horner Syndrome", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/pupillary-abnormalities-and-horner-syndrome ## Direct answer Horner syndrome is an interruption of the sympathetic supply to the eye, causing miosis, partial ptosis and ipsilateral facial anhidrosis. The sympathetic pathway has three neurons, and the level of the lesion decides which associated signs appear. Other pupillary abnormalities are separated by their light and accommodation responses, and topical cocaine confirms a Horner pupil while hydroxyamphetamine helps localise the lesion to preganglionic or postganglionic tissue. ## What you must remember - **First-order neuron:** Runs from the hypothalamus to the ciliospinal centre at C8 to T2; lesions here cause central signs alongside Horner features. - **Second-order neuron:** Preganglionic fibres from the cord, over the lung apex, to the superior cervical ganglion; a Pancoast tumour is the classic cause. - **Third-order neuron:** Postganglionic fibres that run along the internal carotid artery; carotid dissection causes painful Horner syndrome with headache. - **Afferent defect:** A relative afferent pupillary defect points to optic nerve or retinal disease, not to Horner syndrome, and is detected by the swinging flashlight test. - **Adie pupil:** Large, slow to react to light, with tonic accommodation, and supersensitive to dilute pilocarpine from denervation. - **Argyll Robertson pupil:** Small, irregular and light-near dissociated, the classic pupil of neurosyphilis. - **Third nerve palsy:** A dilated pupil with ptosis and a down-and-out eye, often from a posterior communicating aneurysm. ## Localising Horner syndrome step by step Horner syndrome is localised by working along its three neurons. Each step narrows the lesion using the associated signs and a pharmacological test. Pupil size alone is never enough, because the reflex pattern and the drug response carry most of the localising information. 1. **Confirm the Horner signs:** Check miosis, partial ptosis and anhidrosis, with enophthalmos as a minor feature. The anisocoria is most obvious in dim light. 2. **Exclude physiological anisocoria:** Physiological anisocoria is small and equal in bright and dim light, unlike the Horner pupil. 3. **Apply topical cocaine:** Cocaine dilates a normal pupil by blocking noradrenaline reuptake, but a Horner pupil does not dilate. 4. **Apply hydroxyamphetamine:** It releases stored noradrenaline, so a preganglionic lesion still dilates, whereas a postganglionic lesion does not. 5. **Localise the lesion:** Chest imaging is directed at the lung apex for second-order lesions, and carotid imaging is needed for painful Horner syndrome with headache. ## Where the exam tries to catch you Examiners mix up the pupil reflex patterns, so separate the afferent, efferent and accommodation pathways. - **Trap:** Assuming Horner syndrome causes a light reflex defect — **Correct:** Horner pupils still react to light, and the relative afferent defect belongs to optic nerve disease. - **Trap:** Attributing every Horner syndrome to a Pancoast tumour — **Correct:** a Pancoast tumour is only one second-order cause, and painful Horner syndrome with headache suggests carotid dissection. - **Trap:** Calling a large, slow pupil in a young woman a third nerve palsy — **Correct:** Adie pupil spares ptosis and eye movement, whereas third nerve palsy adds ptosis and a down-and-out eye. ## Frequently asked questions ### What are the signs of Horner syndrome? Horner syndrome causes ipsilateral miosis, partial ptosis, anhidrosis of the face and sometimes apparent enophthalmos. The affected pupil is smaller, and the difference is more obvious in dim light. Heterochromia may appear in congenital cases. ### Why does a Pancoast tumour cause Horner syndrome? A Pancoast tumour at the lung apex compresses the sympathetic fibres as they pass to the superior cervical ganglion. This second-order lesion causes Horner syndrome, often with shoulder pain and lower brachial plexus involvement. Pancoast tumour is a classic cause of second-order Horner syndrome. ### How does a Horner pupil respond to cocaine and hydroxyamphetamine? A Horner pupil does not dilate with topical cocaine, which dilates a normal pupil by blocking noradrenaline reuptake. Hydroxyamphetamine dilates the pupil in a preganglionic lesion but not in a postganglionic one. Cocaine therefore confirms Horner syndrome, and hydroxyamphetamine then helps localise the level of the lesion. ### What distinguishes an Adie pupil from a third nerve palsy? Adie pupil is a large, tonic pupil with a slow light reaction and better response to accommodation, usually without ptosis or eye movement loss. Third nerve palsy causes a dilated pupil with ptosis and a down-and-out eye, which can signal a compressive lesion such as a posterior communicating aneurysm. Dilute pilocarpine constricts an Adie pupil through denervation supersensitivity.