What is Horner's syndrome?

What is Horner's Syndrome

Horner's syndrome is a neurological condition resulting from disruption of the oculosympathetic nerve pathway anywhere from the hypothalamus to the eye, classically presenting with the triad of ptosis (drooping eyelid), miosis (constricted pupil), and facial anhidrosis (decreased sweating). 1, 2

Anatomical Basis

The sympathetic pathway consists of three neurons extending from the hypothalamus to the eye and face:

• First-order neurons originate in the hypothalamus and descend through the brainstem to the spinal cord at T1-T2 3
• Second-order neurons exit the spinal cord at T1-T2 and ascend in the sympathetic chain to the superior cervical ganglion 3
• Third-order neurons travel along the internal carotid artery through the cavernous sinus, where they run in close proximity to cranial nerves III, IV, and V1, making them vulnerable to damage from carotid artery aneurysms or inflammatory processes 3

Clinical Presentation

Classic Triad

• Ptosis (incomplete eyelid drooping) 1, 2
• Miosis (pupillary constriction on the affected side) 1, 2
• Facial anhidrosis (decreased sweating), though this is rarely prominent and virtually absent in postganglionic lesions 2

Associated Symptoms Based on Etiology

• Carotid artery dissection: Unilateral head or neck pain accompanied by Horner syndrome, with cerebral or retinal ischemia developing in 50-95% of cases 4
• Fibromuscular dysplasia: May present with stroke, TIA, cranial nerve palsies, or subarachnoid hemorrhage 4
• Cavernous sinus lesions: May cause concurrent third, fourth, or sixth nerve palsies 3
• Brainstem pathology: Neurological symptoms such as motor or sensory changes, ataxia, vertigo, dizziness, or nausea 3

Classification by Location

Central (First-Order)

• Associated with other central nervous system symptoms and signs 2
• Involves hypothalamic or brainstem lesions 2

Preganglionic (Second-Order)

• Most often caused by tumor (including Pancoast tumors) or trauma 2, 5

Postganglionic (Third-Order)

• Often associated with pain or headache 2
• Most frequently caused by carotid artery dissection or cluster headache 2
• Anhidrosis is virtually absent in this subtype 2

Diagnostic Approach

Clinical Examination

• Anisocoria greater than 1 millimeter warrants investigation for Horner syndrome, Adie tonic pupil, or third-nerve palsy 4
• Complete neurological and ophthalmological examination to identify cranial nerve palsies, nystagmus, or hearing loss 3
• Fundus examination for papilledema or optic atrophy, and visual field testing 3

Pharmacological Testing

• Apraclonidine is a good alternative to cocaine for confirming Horner syndrome 2

Imaging

• MRI is generally recommended for evaluation, with special sequences if necessary 2
• Carotid duplex ultrasonography may identify dissection flaps, but CTA or MRA is increasingly used to establish diagnosis 4

Etiology Spectrum

The pathological processes range from benign conditions (such as cluster headache) to life-threatening diseases (such as lung malignancy, carotid dissection, or Hodgkin lymphoma) 6, 5. Carotid dissection accounts for approximately 2% of all ischemic strokes and up to 15% of ischemic strokes in younger patients 4.

Important Clinical Pitfall

Despite its apparent lack of clinical severity, Horner syndrome can be the first or only manifestation of a serious or potentially fatal disorder 5. Prompt evaluation is required to rule out life-threatening conditions, particularly carotid artery dissection, malignancy, or brainstem pathology 7.