By far, my first post on Horner's has received the most comments. Horner's syndrome is not a disease but rather a constellation of clinical signs resulting from sympathetic (not empathetic!) pathology that courses circuitously from the brain to the eye after running down a portion of the upper spine. If I were to run down the street after you, brandishing a knife or other threatening weapon, your sympathetic nervous system would kick in: your pupils will dilate, your eyes would bulge out, your eyelids wide open, and you'd run like heck. In animals (and people) with Horner's syndrome, the sympathetic branch to the eye is disrupted. Therefore, they exhibit a droopy eyelid, miotic (constricted) pupil, a sunken eyeball, and a prolapsed third eyelid (we lost our third eyelid a long time ago so don't worry if you don't have one).
Most cases of Horner's are idiopathic - we don't know why it happens, not due to anything serious, and tends to disappear after a few weeks or months. That said, not all cases can be dismissed without concern. Causes of Horner's syndrome are myriad: middle ear infections, polyps, tumors/cancer, hypothyroidism, spinal disease, thoracic (chest) disease, and more. Even pulling too hard on a dog's leash and ear cleaning (especially in cats) can cause Horner's. Pets showing signs of Horner's syndrome should have a thorough physical exam done by a veterinarian. Keep posting and thanks for all the comments!
(in virtually all cases, unlike dogs, Horner's syndrome in cats has a primary cause. Photo from vision4pets.com.)