Horner Syndrome (Oculosympathetic Palsy)

Definition

Classic Triad

• Dilation lag — the affected pupil dilates more slowly than the contralateral pupil when lights are extinguished; this is often the most reliable sign
• Upside-down ptosis — lower lid elevation (elevation of the inferior tarsal muscle) causing a narrowed palpebral fissure
• Apparent enophthalmos — an illusion caused by the narrowed fissure, not true retraction
• Heterochromia iridis — in congenital cases, the affected iris is lighter (lack of pigmentation)
• Conjunctival injection — in acute cases

Key distinction: Anisocoria is more obvious in the dark than in ambient light (contrast with 3rd nerve palsy, where the large pupil is more apparent in light). The direct and consensual light reflexes are intact.

Left-sided ptosis and miosis (1a), and reversal following apraclonidine (1b) — diagnostic of Horner syndrome.

Anatomy: The Three-Neuron Arc

1st Neuron (Central) — Hypothalamus → Ciliospinal centre of Budge (C8–T2)
         ↓
2nd Neuron (Preganglionic) — Exit spinal cord → sympathetic chain → 
         around subclavian/carotid arteries → Superior cervical ganglion
         ↓
3rd Neuron (Postganglionic) — Along internal carotid → cavernous sinus → 
         V1 ophthalmic division → ciliary nerves → iris dilator & Müller's muscle

Causes by Lesion Location

First-order (Central) neuron

• Lateral medullary (Wallenberg) syndrome — most classic central cause
• Pontine or brainstem stroke/haemorrhage/tumour
• Syringomyelia
• High cervical cord trauma (bilateral Horner in high transection)
• Multiple sclerosis

Second-order (Preganglionic) neuron

• Pancoast tumour (apical lung tumour) — most important peripheral cause
• Cervical rib / apical pleural disease
• Thyroid or mediastinal tumours
• Aortic aneurysm
• Brachial plexus injury (e.g., birth trauma → congenital Horner)
• Anterior/posterior thoracotomy

Third-order (Postganglionic) neuron

• Carotid artery dissection — key vascular emergency; neck/face pain + Horner = dissection until proven otherwise
• Cavernous sinus lesion (thrombosis, aneurysm, tumour)
• Cluster headache (painful postganglionic Horner)
• Orbital lesion
• Nasopharyngeal carcinoma

Sweating pattern as localiser: Lesions at or proximal to the common carotid → anhidrosis of the entire face. Lesions distal to the carotid bifurcation → no sweating loss, or only medial forehead/nose (because facial sweating fibres travel with the external carotid, not the internal carotid). — Adams and Victor's Principles of Neurology, 12th Ed.

Pharmacological Testing

The apraclonidine test exploits denervation supersensitivity — the denervated dilator muscle upregulates α-1 receptors and responds excessively to even a weak agonist. Wait ≥24–48 h after acute onset before testing (supersensitivity takes time to develop). — Neuroanatomy through Clinical Cases, 3rd Ed.

Differential Diagnosis of Anisocoria

Workup / Imaging

• Acute painful Horner (neck pain, headache) → urgent MRI/MRA neck and brain to exclude carotid dissection
• Smoker / weight loss / shoulder pain → CT chest for Pancoast tumour
• Isolated, painless, chronic → MRI brain + neck (C8–T2 cord, mediastinum)
• Children → rule out neuroblastoma (urine catecholamines, chest/abdominal imaging)

Congenital Horner

• Usually caused by perinatal injury to the sympathetic chain in the neck (birth trauma)
• An autosomal dominant hereditary form also exists
• Key feature: heterochromia iridis — affected iris remains lighter because sympathetic innervation is needed for melanin deposition during development
• — Adams and Victor's Principles of Neurology, 12th Ed.

Key Associations to Remember