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Horner’s Syndrome
Rhea V. Morgan, DVM, DACVIM (Small Animal), DACVO
BASIC INFORMATION
Description
Horner’s syndrome arises when certain neurologic information
(sympathetic innervation or tone) to the eye is lost. Unlike sensory
(feeling) or motor (movement) innervation, sympathetic innervation to eye does several unique things. It keeps the eye in the front
part of the socket (orbit), the third eyelid recessed down in the
corner near the nose, the pupil partially open, and the upper eyelid
fully raised.
Sympathetic information begins in the brain, travels down
the spinal cord in the neck, and leaves the spinal cord on nerves
near the front of the chest. The nerve carrying the information
crosses the chest near the first rib, turns back toward the head,
and runs up the neck with the vagus nerve in the groove near the
jugular vein. The nerve then travels through the middle ear, back
into the floor of the brain cavity, and joins with the fifth cranial
nerve. This last nerve then travels to the ocular structures, carrying sympathetic innervation with it.
Causes
Horner’s syndrome can arise anywhere along the route that the
nervous information travels. Common causes include trauma to
the neck, such as from bite wounds, automobile accidents, surgery,
venipuncture, or insertion of an intravenous catheter. Problems in
the front of the chest, such as tumors, bleeding, fractured ribs, or
tearing of the nerves from trauma, may cause Horner’s syndrome.
Diseases, tumors, and surgery of the middle ear are potential causes. Head trauma, inflammation (meningoencephalitis),
tumors, and vascular diseases of the brain are uncommon causes
of Horner’s syndrome.
Approximately 50% of all cases in dogs arise for unknown
reasons and are called idiopathic. In cats, it is more common for
a cause to be identified. Horner’s syndrome typically affects just
one eye.
Clinical Signs
The four classic signs of Horner’s syndrome are a small pupil
(miosis), recession of the eye into the orbit (enophthalmos), protrusion of the third eyelid, and drooping of the upper eyelid (ptosis). Ptosis can be subtle and may not be noticeable in all cases.
Depending on the cause, other signs may also be present.
Diagnostic Tests
A tentative diagnosis is made by the presence of the four classic
signs. Other causes of these signs must be ruled out by a thorough
examination of the eye. Your pet may be referred to a veterinary
ophthalmologist if further evaluation of the eye is needed.
Once Horner’s syndrome is diagnosed, a search is undertaken for the cause. A complete physical examination is done,
with close scrutiny of the ears, neck, and chest. Neurologic evaluations are performed. Routine laboratory tests and chest x-rays
may be recommended. If other neurologic abnormalities are present, computed tomography (CT scan) or magnetic resonance
imaging (MRI) may be considered. In some cases, pharmacologic
testing (application of certain dilating drops on the eye) may be
performed to try and determine where in its course the nerve is
affected.
If all tests are negative, then the condition is considered
idiopathic.
TREATMENT AND FOLLOW-UP
Treatment Options
No specific treatment exists for Horner’s syndrome. Instead, treatment is directed at the underlying cause, if one can be found.
There is no evidence that topical eye or ear medications affect the
outcome.
Follow-up Care
Animals with Horner’s syndrome are usually monitored for
several weeks to months. The frequency of recheck visits and
whether certain tests are repeated depends on the underlying
cause. If any other neurological signs develop, notify your veterinarian immediately, as they may indicate the presence of brain
disease.
Prognosis
Prognosis for Horner’s syndrome varies, depending on the underlying cause. Horner’s syndrome associated with chest trauma and
tearing of the nerve, tumors of the chest, or brain diseases is often
permanent. Horner’s syndrome that accompanies trauma, surgery,
or irritation to the tissues in the neck may resolve with time. Most
cases that arise with inflammation or surgery of the middle ear
also improve or resolve.
Approximately 50% of idiopathic cases improve or resolve.
In some instances, the signs improve but do not completely disappear. Improvement usually starts within 6-8 weeks. Horner’s
syndrome is not painful, and most animals are oblivious to it.
If it persists, it does permanently change the appearance of the
animal.
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