Brief, transient Horner's syndrome can be the hallmark of a carotid artery dissection.

We describe a 41-year-old woman in whom the diagnosis of carotid artery dissection was suspected based on a recent history of anisocoria and ipsilateral ptosis that lasted 2 days. She had a normal neurologic examination, including no clinical evidence of anisocoria or ptosis. Subsequently, a cocaine test demonstrated pharmacologic Horner's syndrome. MRI confirmed the carotid dissection. This patient illustrates that a history of transient pupillary and eyelid abnormalities can lead to the diagnosis of a carotid dissection. Specific questioning about transient anisocoria and ptosis should be considered when a carotid artery dissection is suspected. Pharmacologic testing may be a useful tool in such instances.