Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.
2 min synopsisUK scopeSources checked 7 Sept 2026Clinical review pending
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Painful new Horner syndrome may signal carotid dissection
New unilateral miosis and mild ptosis with ipsilateral neck, orbital or facial pain can precede retinal or cerebral ischaemia.
Action: Arrange immediate emergency-department assessment and urgent arterial imaging for suspected carotid dissection. Do not wait for apraclonidine testing, a routine eye appointment or the development of weakness.
Synopsis
Identify sympathetic denervation of the eye, distinguish urgent vascular presentations from longstanding anisocoria, and investigate the underlying lesion safely.
Horner syndrome produces a smaller pupil and usually mild upper-lid drooping on the affected side.
Anisocoria is more conspicuous in darkness because the affected pupil dilates poorly; absence of obvious facial anhidrosis does not exclude the diagnosis.
The sympathetic pathway extends from the brain through the spinal cord, chest and neck to the orbit, so investigation must follow the relevant anatomy.
Key red flags
New Horner syndrome with ipsilateral neck or orbital pain requires immediate assessment for carotid dissection.
Investigation priorities
01
Immediate arterial imaging in a painful acute presentationFirst step
Identify carotid dissection or another urgent lesion before ischaemic complications occur.
Management branches
EmergencyAcute pain or neurological involvement
New Horner signs occur with ipsilateral pain, trauma, visual ischaemia or focal neurological symptoms.
Send the patient for immediate emergency assessment, using emergency transport when stroke or other acute neurological compromise is suspected.
Record onset and any transient visual or neurological episode, because resolution does not remove vascular risk.
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 7 Sept 2026; clinical approval remains outstanding.