Question of the Day

Horner syndrome is idiopathic in ~50% of dogs (45% of cats). It is caused by a lack of sympathetic innervation to the eye.

Look for four ocular signs on affected side with Horner:
1. MIOSIS (constricted pupil) causing anisocoria
2. 3rd eyelid PROTRUSION
3. ENOPHTHALMOS (sunken eye)
4. PTOSIS (drooped eyelid)

Rule out other Dx before making a diagnosis of idiopathic Dz, (e.g., guttural pouch mycosis in horses, brachial plexus avulsion, otitis media).

Note, horses also have ipsilateral facial sweating, and cattle have ipsilateral muzzle anhidrosis. 

To learn more, check out this Review of Horner’s Syndrome in Small Animals by Zwueste and Grahn in Can Vet J (2019) (full free article online).