Horner's disease (more commonly known as Horner's syndrome) is a group of symptoms that occurs in dogs because of a neurological injury. It affects the muscles of the face by causing them to lose contact with facial nerves. The syndrome is not life-threatening or dangerous, though the appearance is alarming.
Horner's syndrome appears as a variety of conditions of the face. The most noticeable are constricted pupils and drooping eyelids. The eyes of dogs with Horner's syndrome may also appear sunken. Dilated blood vessels in the face result in increased pink colouration and warmth to the affected side of the face, although this is difficult to detect in very small dogs.
Horner's is caused by an injury to the sympathetic nerve system. The sympathetic system is the one responsible for the "fight or flight" response. It controls things like sweating and pupil dilation. This is why damage to the system results in pupil constriction. Two main nerve segments are involved in this system. The "first segment" is longer and runs along the spine to the base of the brain. The "second segment" is shorter and runs from the base of the brain to the eye.
Horner's is an indication of nerve damage that might require treatment. Many things can damage the nerves, including simple issues such as ear infections and more complicated and serious issues such as a neurological cancer. Physical trauma such as an injury sustained in a fight or a strong jerk on the neck from a leash can cause Horner's syndrome. Experts consider damage to the first nerve segment more serious than Horner's caused by an issue with the second nerve segment. A veterinarian conducts a full physiological exam to determine the cause of nerve damage. Veterinarians often are unable to determine the exact cause of Horner's syndrome. Veterinarians refer to these cases as idiopathic cases.
Horner's syndrome does not actually require treatment. It is not painful for the dog, nor does it interfere with vision. Eye drops can be used to relieve the visible symptoms, but these become less effective over time. Less serious causes of Horner's sometimes resolve themselves within a few weeks. This is the case about 50 per cent of the time. If these cases can be resolved, time is the only real cure. If the cause is something more serious, such as cancer, then appropriate treatment up to and including surgery must be undertaken.
Golden Retrievers display Horner's syndrome more often than other breeds. The reasons for this are not known, but about 80 per cent of all dogs with Horner's syndrome are golden retrievers in the middle to later stages of their lives. The next most frequently affected breed is the Cocker spaniel, owing to the tendency of this breed to develop ear infections.
Peteducation.com: What is Horner's Syndrome, and what causes it?