Diagnosis

Localizing which area of the sympathetic nerve system is affected goes a long way in determining the nature of the damage as different areas of the system are prone to different types of injury. Special eye drops can be used to stimulate different areas of the nervous system and determine if the lesion is in the first nerve segment or in the second nerve segment. Most are in the second nerve segment.

With second nerve segment involvement:
If ear infection is not obvious and disease of the eye beyond the Horner's syndrome itself is not obvious, then it is probably prudent to allow the syndrome to resolve on its own. This usually occurs within 6-8 weeks. Further diagnostics may be undertaken if new developments occur or if the syndrome persists beyond this time.

With first nerve segment involvement:
Involvement of the first nerve segment indicates a problem in the chest or spinal cord and is more significant. Chest radiographs should be taken to rule out cancer spread to the chest (the only sign of this may be the Horner's syndrome). The front leg should be carefully checked for evidence of function loss as a tumor or protruding intervertebral disc could be exerting pressure on the spinal cord. Trauma to the neck as with a strong jerk from a collar or straining against a leash can also produce Horner's syndrome from this section of the nerve. Generally more diagnostic work is needed for cases involving the first nerve segment as there is potential for more serious underlying causes. If the syndrome stemmed from pulling on the leash, it should resolve uneventfully depending on how badly damaged the nerve is.

Causes

In dogs, the most common causes are:

Idiopathic (cause unknown)

Car accidents with trauma to the head, neck, or chest

Bite wounds

Intervertebral (IV) disc disease in the neck area

Infections of the middle ear

Disease of the orbit (area behind the eye)

Cancer involving the brain or chest

As a result of a treatment (e.g.; ear cleaning) or medication