Horner's Syndrome is not a painful condition. It is a neurological condition whereby one of the nerves to the eye and the area around the eye is not functioning normally. The result is a couple of or all of the following signs:
The nerve involved is the sympathetic nerve. This nerve has a very long pathway through the body and can be injured or affected anywhere along this path. The nerve starts in the brain, travels down the spine in the neck region, through the front of the chest, up the neck in the muscle, around and through the middle ear and finishes at the eye. In the eye, it supplies 4 areas:
the pupil (causing dilation)
Thus when there is lack of function we see a small pupil and lack of tone in the other areas. As this nerve is activated by adrenalin, the signs are often not as apparent when the animal is excited.
Some causes of damage to this nerve include neck trauma, tumours in the chest, and middle ear infections. Most of the time, no cause is found = idiopathic. Some breeds seem to be more prone to developing idiopathic Horner's syndrome than others, especially the Golden Retriever and Collie.
Most cases run a course of 6-8 weeks, and the signs gradually resolve over this time. No treatment is required. If signs are still very obvious at this time, further investigation is possible including neck x-rays or examining the ears under general anaesthetic.