Ophthalmology · Clinical photograph

Horner Syndrome

Mild drooping of one upper lid with a smaller pupil on the same side — Horner syndrome (oculosympathetic palsy). This infant has a right-sided Horner syndrome with a birth brachial plexus injury.

Face of an infant; the right upper eyelid droops slightly and the right pupil is smaller than the left
Right-sided Horner syndrome in a 2-month-old with complete right brachial plexus palsy.Image: Davplast, via Wikimedia Commons — CC BY-SA 4.0.

Written & reviewed by: Kinase Medical Team · Last reviewed: 30 September 2026 · Editorial policy

Key features

  • Ipsilateral ptosis, miosis and facial anhidrosis; enophthalmos is only apparent and is generally not part of the syndrome.
  • Heterochromia iridis (lighter iris on the affected side) suggests a congenital Horner syndrome.
  • Anisocoria is greater in dim light because the Horner pupil fails to dilate.
  • Three-neuron pathway: hypothalamus → ciliospinal centre (C8–T1) → superior cervical ganglion → along the internal carotid artery.
  • Causes: lateral medullary syndrome (1st order); Pancoast tumour, lower brachial plexus (Klumpke) injury (2nd order); carotid dissection, cavernous sinus lesions (3rd order); neuroblastoma in infants.
  • Apraclonidine drops reverse the anisocoria (the Horner pupil dilates) — a common confirmatory test.

Differential diagnosis

Differential diagnosis of Horner Syndrome
ConditionHow to tell it apart
Third (oculomotor) nerve palsyPtosis with a dilated pupil and the eye 'down and out'; anisocoria is greater in bright light.
Physiological anisocoriaSame degree of pupil asymmetry in dim and bright light, no ptosis, often variable from day to day.

How it appears in NEET PG

Shown as a face or eye photograph; questions ask for the lesion site, the associated tumour (Pancoast) or, in newborns, the associated brachial plexus injury.

Frequently asked questions

What is the triad of Horner syndrome?

Ptosis, miosis and anhidrosis on the same side as the lesion in the oculosympathetic pathway.

How is Horner syndrome linked to Klumpke palsy?

Injury to the lower brachial plexus roots (C8–T1) can also damage the sympathetic fibres leaving at T1, giving an ipsilateral Horner syndrome with the hand weakness.

Which eye drops confirm Horner syndrome?

Apraclonidine: because of denervation hypersensitivity, the small Horner pupil dilates and the anisocoria reverses. Cocaine drops were used traditionally.

Sources

  1. Radiopaedia — Horner syndrome
  2. Adams and Victor's Principles of Neurology (textbook)
  3. Nelson Essentials of Pediatrics (textbook)

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For exam preparation and medical education only — not a substitute for clinical judgement, diagnosis or treatment of any patient.