Synopsis
Recognise sympathetic pathway failure from the pupil and eyelid findings, distinguish it from physiological anisocoria and rapidly investigate painful or acute Horner syndrome for vascular, central, apical and neck causes.
- Horner syndrome reflects interruption of the three-neuron sympathetic pathway from hypothalamus through brainstem and cervical cord, over the lung apex and up the neck with the carotid arteries to the eye.
- The core signs are ipsilateral miosis and mild ptosis, often with apparent elevation of the lower lid; anisocoria is usually more obvious in darkness because the affected pupil dilates slowly.
- Anhidrosis varies with lesion level because facial sweating fibres separate along external and internal carotid routes; its absence does not exclude the syndrome.
Key red flags
New partial Horner syndrome with ipsilateral neck, face or orbital pain, headache or transient retinal or cerebral symptoms strongly suggests carotid dissection and requires emergency head-and-neck vascular imaging.
Investigation priorities
Determine which pupil is abnormal and whether anisocoria increases when sympathetic dilation is required.
Management branches
New miosis and mild ptosis accompanies ipsilateral neck, facial, orbital or head pain, with or without transient neurological symptoms.
- Establish last known well and trauma or manipulation history, perform ABCDE and a complete stroke and cranial examination and compare pupils in bright and dim light without delaying escalation.
- Contact the stroke or neurovascular service and obtain urgent head-and-neck CTA or the locally preferred arterial imaging for carotid dissection and associated infarction.