Horner's Syndrome

Horner's syndrome is an intriguing disorder of the nervous system that is complex to explain. There are two major divisions of the nervous system. There is the part of your nervous system that you are aware of and have control over. For example you feel cold and in response you consciously initiate all the actions that result in you putting on a jacket. This is the voluntary nervous system. Additionally there is the part of your nervous system that is under automatic control. You cannot control this system and the actions occur without your input. Again you feel cold and start to shiver. That occurs without your control or input. Similarly, you do not have to think to make your heart beat. The automatic (doctor's call it autonomic) nervous system - the part you have no control of - itself has two divisions... the sympathetic and parasympathetic nervous systems. If you have just finished a large meal and are sitting around feeling "stuffed" and feel "snoozy" you are feeling the actions of the parasympathetic nervous system. During this time, your pupils will be small due to constriction. If someone attacks you with a knife, your pupils dilate, blood is shifted to your muscles and your heart beats faster. This is the 'fight or flight' reaction and is characteristic of the sympathetic nervous system. The eye, and the rest of the body as well, receives both sympathetic and parasympathetic innervation. Under normal conditions there is a fine balance between sympathetic and parasympathetic stimulation. If something were to block the sympathetic impulses into the eye, there would be an over balance of parasympathetic supply to the eye. This is what happens in Horner's syndrome. On its journey to the eye, the sympathetic nerve begins in the brain, travels down the spinal cord to the shoulders and exits from the spinal cord. After exiting, the sympathetic nerve joins an artery and vein and travels back toward the head. At the base of the ear, a nerve junction (ganglion) occurs. The portion of the nerve from the brain to the shoulders and back to the base of the ear is called the pre-ganglionic nerve. After the ganglion, the nerve goes to the eye. This is the post-ganglionic nerve. Around the eye are several muscles responsible for moving the eye which are under voluntary control. Surrounding these muscles is a smooth muscle cone which is controlled by the sympathetic nerve. The nerve causes the smooth muscle cone to become constricted and this in turn pushes the eye slightly forward to its normal position in the orbit. If the sympathetic nerve is not working well, the eye will sink somewhat into the orbit. When this occurs the inner or third eyelid will become prominent and the upper eyelid will become droopy. The sympathetic nerves surround blood vessels within the conjunctiva (white portion of the eye which we see) and cause an ever so slight constriction of these vessels. With the loss of the sympathetic nerves, these conjunctival blood vessels dilate and the conjunctiva therefore looks more red. Surrounding the pupil a sphincter (closing) muscle is present which is under parasympathetic control. Working against the sphincter muscle are radially arranged dilator muscles very much like spokes on a bicycle. These dilator muscles are under sympathetic control. If the sympathetic nerve is not working well, the pupil will be smaller than normal because the dilator muscles cannot work against the sphincter muscle. All the above describe Horner's syndrome. 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 anything from brain tumors; spinal cord injury in the neck; thoracic tumors such as lymphosarcoma or thyroid tumors; injuries to the neck from fighting or trying to draw blood; middle ear infections; and nerve abnormalities caused by viral, immune-mediated or other causes. Of all dogs diagnosed with Horner's syndrome 80% or more will be middle aged to older Golden Retrievers. Cocker Spaniels are the second most commonly affected breed because of chronic ear infections. Most cases of Horner's syndrome are idiopathic (of unknown cause) and spontaneously recover within an average of 16 weeks. A thorough physical examination is warranted to determine if any of the causes of Horner's syndrome are present. Blood tests and chest radiographs (x-rays) may be needed. A pharmacologic test should be performed by the veterinary ophthalmologists to further localize the site of the nerve abnormality, that is pre-or post-ganglionic Horner's syndrome. If Horner's syndrome is suspected to be due to unknown causes, only time will resolve the condition IF it can be resolved. The symptoms however may be reversed temporarily by the use of eye drops two or three times daily. Because the eyedrops lose effectiveness with continued use, they will have to be used more and more frequently. Thus, unless the patient is a show pet and drops can be used while they are in the show ring, drops are not effective for the long run. It is important to know that your pet is not in pain. The eye conditions are due to a nerve problem which is not painful. It just looks 'abnormal.'

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