When should I refer a patient with Horner's syndrome for further investigation or specialist assessment?

Refer a patient with Horner's syndrome for further investigation or specialist assessment if:

• There is any suspicion of an underlying serious cause such as a stroke, tumour, or carotid artery dissection, given Horner's syndrome can be a sign of these conditions.
• Neurological examination reveals additional abnormalities or progressive neurological symptoms.
• The presentation is acute or associated with other red-flag neurological signs that require urgent neuroimaging or specialist evaluation.
• There is persistent or unexplained facial pain, numbness, or other cranial nerve involvement alongside Horner's syndrome.

In general, Horner's syndrome warrants prompt referral for neurological assessment to exclude serious underlying pathology, especially if the cause is not already known or if symptoms are new or worsening.