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Horner's Syndrome


Horner's syndrome is not uncommon and occurs in dogs, cats, horses and many other species. The symptoms generally include a sunken in eye (enophthalmia) with a small pupil (miosis), a droopy upper eyelid (ptosis) and a prominant third eyelid. Horner's syndrome must be differentiated from Uveitis which also produces a constricted pupil and a droopy looking eye.

Horner's syndrome is an intriguing disorder which is quite complex to explain. Here goes....

There are two major divisions with your nervous system. There is the part of your nervous system that you are aware of and have control over, and there is the part of your nervous system that is under automatic control. You can perceive cold, and in responding to that stimulus consciously initiate all the actions that result in you putting on a jacket. But the shiver occurs without your control. Similarly, you do not have to think to make your heart beat.

The autonomic nervous system - the part you have no control of - itself has two divisions... the sympathetic and parasympathetic nervous systems. Under normal conditions there is a fine balance between sympathetic and parasympathetic stimulation.

If someone attacks you with a knife, your pupils dilate, blood is shifted to your muscles, and your heart beats faster as you prepare to fight or flee. This is sympathetic stimulation.

The eye, has both sympathetic and parasympathetic innervation. If something were to block the sympathetic impulses into the eye, there would be an over balance of parasympathetic supply to the eye. The result is the pupil will constrict, and all of the muscles around the eye will relax. The eye will sink somewhat into the orbit, the third eyelid will become prominent and the upper eyelid will become droopy. This is Horner's syndrome.

The nerve that carries sympathetic innervation to the eye takes a remarkable course as it travels from the brain to the eye. The nerve travels down the spinal cord from the brain, emerges in the chest cavity and then finds its way up the neck along with the carotid artery and jugular vein through the middle ear and then into the eye.

Horner's syndrome is associated with damage to the sympathetic innervation to the eye. The damage may have numerous causes, and may occur anywhere along the course of the nerve's route from the brain to the eye. Thus Horner's syndrome may be associated with (strictly in anatomical sequence - not probability) brain tumours, spinal cord injury in the neck, thoracic tumours such as lymphosarcoma, injuries to the neck from fighting, choke collar injury or difficult venipuncture, middle ear infections, and viral, immune mediated or idiopathic neuropathies.

Of all dogs diagnosed with Horner's syndrome 90% or more will be middle aged to older Golden Retrievers. Cocker Spaniels are the second most commonly affected breed.

In most cases of idiopathic Horner's syndrome spontaneous recovery will occur in an average of 16 weeks. A thorough physical examination is warranted to rule out any of the other causes of Horner's syndrome. If the veterinarian has any suspicion that the cause may not be idiopathic, then it may be prudent to do blood work and a chest x-ray, and there is a pharmacologic test done by the ophthalmologist which may further localize the site of the lesion.

If the damage to the nerve is in the sympathetic ganglion (just behind the eye) or beyond we refer to it as post-ganglionic. Most cases of "idiopathic" Horner's syndrome are post-ganglionic, and these cases have a more favourable prognosis. If the damage to the sympathetic nerve is anywhere between the brainstem and the sympathetic ganglion we refer to it as pre-ganglionic Horner's which is prognostically more serious. Most veterinary ophthalmologists can tell the difference between pre and post ganglionic Horner's syndrome by doing a pharmacologic test which tests the response of the pupil to the application of different medications.

If idiopathic Horner's syndrome is diagnosed, only time will cure the condition. The symptoms however may be reversed temporarily by the use of 2.5% phenylephrine eye drops (Mydfrin - Alcon) two or three times daily.

Unlike dogs, cats do not develop "idiopathic" horner's syndrome - where the condition arises without tangible cause and resolves spontaneously in a number of weeks. Most cases of Horner's in cats have a tangible cause. In horses, a common additional symptom is noticable sweating on the side of the affected eye.