Horner's Syndrome

Horner's syndrome describes a collection of changes to one eye and the surrounding area that occur when certain nerve signals are interrupted. The nerves involved help control the muscles around the eye, the eyelid position, and the size of the pupil. When these signals are disrupted, the eye on the affected side may appear smaller, the pupil may be more constricted than the other, and the third eyelid may become more visible. Owners often notice these changes quite suddenly, though in some cases they develop more gradually. The appearance can be subtle or quite obvious depending on the individual animal. The underlying cause of the nerve disruption varies widely—from ear infections to injuries to conditions affecting the chest or neck—and in many cases no specific cause is identified even after investigation. This page explores the patterns that may be observed, what happens along the nerve pathway to produce these signs, how the condition is investigated, and the range of approaches that may be considered depending on what is found.