But it's not all there

A 44-year-old man presented with acute onset headache and isolated anisocoria. Dilation lag was noted in the smaller pupil and Horner syndrome was suspected despite the lack of eyelid ptosis. Cocaine testing confirmed the clinical diagnosis, and urgent neuroimaging found an internal carotid artery dissection. The patient was managed with systemic anticoagulation with a good final outcome. Horner syndrome should be suspected in any patient with aniscoria and dilation lag, despite the absence of eyelid ptosis or other associated findings.