Horner's Syndrome is a group of signs that occurs when specific muscles of the face lose their stimulation by certain nerves, specifically the sympathetic nerves. It is caused by some type of injury to, or lesion of, the nerves. The injury may occur at the level of the brain, upper spinal cord, or between the spinal cord and the face. In the dog, 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

For an unknown reason, Horner's Syndrome appears to be more common in Golden Retrievers. Approximately 40-50% of the cases of Horner's Syndrome in dogs are idiopathic.

Horner’s syndrome consists of five signs:

• Constricted Pupil
• Elevation of the Third Eyelid
• Retraction of the eyeball into the head
• Slight drooping of the Eyelid
• Increased pink color and warmth of the ear and nose on the affected side (very hard to detect in small animals)

All these signs are caused by damage to sympathetic nervous system as it supplies the eye on the affected side of the head.

Our bodies have numerous functions that are controlled by our nervous systems yet we are completely unaware of them. Our heart and respiratory rates, the amount of sweat and other secretions we produce, circulation to different body areas, pupil dilation and constriction are all regulated by our nervous systems automatically and without our knowledge or control. The part of our nervous system dedicated to these automatic systems is called the “Autonomic nervous system.”

The autonomic nervous system is divided into the “sympathetic nervous system” and the “parasympathetic nervous system.” The parasympathetic system maintains a normal state; whereas the sympathetic system prepares the body for a “fight or flight” situation. Some changes that might be stimulated by the sympathetic system include: increased sweating, dilated pupils, increased heart rate, and increased blood flow to muscles. Both systems coexist in balance in the healthy body.

When the sympathetic system controlling one of the eyes is damaged, only the parasympathetic nerves work and Horner’s syndrome is created.

The nerve carrying the tiny nerve fibers that provide sympathetic control to the eye have a long path and the damage may have occurred anywhere along this path.

The nerves originate in the spinal cord in the patient’s neck. They exit the spinal cord just inside the chest and travel up the neck to the head. These nerves then connect to new nerves just below the ear. The new nerves continue their journey to the eye. The damage can occur in the neck area, the ear area or the eye area. Damage can occur in the form of trauma, tumor involvement, infarction (abnormal blood clot), middle ear infection, or diseases of the eye itself.

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.

Horner's Syndrome is diagnosed by the presence of the signs listed above. What is more difficult, is the diagnosis of the cause of the condition. A complete physical and neurologic exam, radiographs (x-rays), a chemistry panel, complete blood count (CBC), and perhaps a CAT scan or MRI will help determine the cause. Epinephrine may be administered to the eye to assist in locating the site of the injury by measuring the time between administration and dilation of the pupil. When the injury to the nerves occurs outside of the brain and spinal cord, the epinephrine will cause the pupil to dilate within 20 minutes of administration. If the lesion is in the brain or spinal cord, pupil dilation generally does not occur until 30 or 40 minutes after the epinephrine is administered.

It is not necessary to treat Horner’s syndrome. The syndrome is not painful and does not interfere with vision. The significance of the syndrome is that it indicates nerve damage which must be recognized. If one wishes to treat the syndrome for cosmetic reasons, phenylephrine eye drops can be prescribed to relieve clinical signs. The underlying cause such as a bite wound or middle ear infection should be treated. In cases of idiopathic Horner's Syndrome, the condition often resolves after 6-8 weeks. Horner's Syndrome caused by injuries to nerves outside of the brain and spinal cord generally have a better prognosis.