Horner's syndrome is a condition that can occur in dogs, cats, horses, and many other species. The symptoms generally include a sunken appearance to the eye (enophthalmia) small pupil (miosis), droopy upper eyelid (ptosis), and a prominent third eyelid. Horner's syndrome must be differentiated from the condition uveitis, which can produce similar signs.
Horner's syndrome is an intriguing and complex disorder than can be difficult to understand. A basic description of neurology is needed.
There are two major divisions within 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 your 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 over) 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 blocks the sympathetic impulses to the eye, there will be an overbalance of parasympathetic supply to the eye. The result is that the pupil will constrict and all of the muscles around the eye will relax. The eye will sink 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. It continues through the middle ear and eventually enters 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. Horner's syndrome may be associated with brain tumors, spinal cord injuries, chest tumors, injuries to the neck (fighting, choke collars), middle ear diseases, and a variety of neuropathies. The most common scenario is that no particular cause can be found.
Of all dogs diagnosed with Horner's syndrome, 90% or more are middle-aged to older Golden Retrievers. Cocker Spaniels are the second most commonly affected breed.
In most cases of idiopathic (no identifiable cause) Horner's syndrome, spontaneous recovery will occur in an average of 16 weeks but it can take as long as 6 months. A thorough physical examination is warranted to help rule out any of the specific causes of Horner's syndrome. If, for any reason, there is suspicion that a particular case is not idiopathic, then it is prudent to pursue a diagnostic evaluation that could include pharmacological testing on the eye, bloodwork, radiographs, or even an MRI.
If idiopathic Horner's syndrome is diagnosed, only time can potentially resolve the condition. In the meantime, signs can be masked by topical treatments of phenylephrine eye drops 2-3 times daily. It doesn't resolve anything, but can improve the appearance of the eye and vision if it is being affected by the elevated third eyelid.