When you type "Horner's syndrome" and "dogs" in Google's search engine, VetBlog shows up on the first page. Considering this syndrome is not uncommon, many people have posted questions on this blog in the comments section of my previous post on Horner's syndrome.
Due to its popularity, I have decided to post again on this subject.
Horner's syndrome (or Horner syndrome) is not a disease but rather a constellation of clinical signs resulting from a lesion to the sympathetic branch to the eye. The symptoms are 1) a droopy eyelid, 2) miosis (a constricted or smaller-than-normal pupil), 3) enophthalmos (a sunken eyeball) and 4) a protruding 3rd eyelid or nictitans.
The sympathetic branch (in this specific case) is a a part of the nervous system that courses down the spinal cord, does a U-turn at the upper thoracic spine, and then courses back to innervate the eye. It is responsible for doing the opposite of what we see in Horner's: bulging eyes, wide pupils, retracted 3rd eyelid, and eyelids that are kept up. You're correct if you've come to the conclusion that this occurs with either a "fight or flight" response. When an animal is frightened, its sympathetic nervous system kicks in.
Ok, enough anatomy!
Again, Horner's is not a disease but the manifestations of an underlying disease process, or its idiopathic.
Idiopathic Horner's syndrome means that no underlying disease can be found and the symptoms typically resolve in a few weeks or less.
Due to the coursing of the sympathetic branch through the middle ear, otitis media (or infection of the MIDDLE ear) often results in Horner's syndrome (and often accompanied by facial paralysis, from the 7th cranial nerve being concurrently affected). Middle ear infections often result from an outer ear infection that ruptures the eardrum, and then spreads to the middle ear. This needs to be diagnosed correctly so that proper long-term treatment can be instituted.
Horner's is rarely caused by tick-borne diseases and an underactive thyroid (hypothyroidism). Growths (especially polyps in cats) and tumors can also cause Horner's syndrome.
Protruded discs in the neck or upper thoracic spine can also cause Horner's syndrome.
A CT scan or MRI is not always necessary to make the diagnosis, but may be the most accurate. The prognosis depends on the underlying cause. It can range from excellent with middle ear infections, to grave with tumors.
Please keep posting on this subject - I hope I can continue to help you out.
If you see your pet with these symptoms, please contact your vet.
(note the droopy eyelid, the constricted pupil, the prolapsed 3rd eyelid - the sunken eyeball can't really be appreciated in this photo)