Horner Syndrome Secondary to Suspected Internal Carotid Artery Dissection in a Golden Retriever.

A 5-year-old male golden retriever was presented after a subacute onset of left-sided Horner syndrome (HS). The dog had anisocoria with left-sided miosis, ptosis of the upper eyelid, and third eyelid protrusion in the left eye. Because of the absence of additional neurological abnormalities, clinical signs were suggestive of left isolated HS, and the lesion was localized at the level of either the preganglionic or postganglionic neuron of the sympathetic chain. Magnetic resonance imaging (MRI) of the head and total body computed tomography (CT) identified marked narrowing and irregularity of the left internal carotid artery (ICA) in addition to loss of normal vessel flow-void and T1-weighted hyperintensity in the lumen of the left ICA. Except for these abnormalities, MRI and CT results were normal. These findings were suggestive of left internal carotid artery dissection (ICAD), suggesting that ICAD should be considered as a possible differential diagnosis of HS in dogs.