Horner's Syndrome

History. A 7-year-old male golden retriever is brought to your clinic for examination. The owner states that during the past 3 weeks, the dog has become progressively weaker in his left front leg and now cannot bear weight on that limb.

Clinical Examination. Physical examination abnormal­ities are limited to the nervous system. The dog is bright, alert, and responsive. Except for the left front leg, cranial nerve and spinal segmental reflexes and all intersegmental responses are within normal limits. The dog cannot bear weight on the left front leg, and the leg muscles are atrophied. No segmental reflexes (e.g., toe-pinch withdrawal) or intersegmental res­ponses (e.g., proprioceptive placing) can be elicited in the left front leg. The left upper eyelid droops lower than the right upper lid, and the left pupil is smaller than the right pupil. The left nictitating membrane (third eyelid) is prolapsed over part of the cornea, and the left eye seems more sunken into the orbit than the right eye.

Comment. This dog has a lesion of the left brachial plexus, probably a neoplasm. It has caused a lower motor neuron syndrome to the left front leg with atrophy, paralysis, and loss of reflexes. I he tumor has damaged the preganglionic neurons of the left sympathetic nervous system as they leave the first two thoracic segments on their way toward the eye. Loss of the sympathetic innervation to the region of the eye causes smallness of the pupil (miosis), drooping of the upper eyelid (ptosis), a sunken appearance in the eye (enophthalmos), and prolapse of the nictitating membrane. This constellation of clinical signs is called Horners syndrome.

Sympathetic preganglionic neurons pass through the brachial plexus (where they were damaged in this dog) and ascend in the vagosympathetic trunk to synapse with the postganglionic neurons in the cranial cervical ganglia. The postganglionic cell axons then go to the region of the eye, where they innervate the dilator smooth muscle cells of the iris. When paralyzed, the constrictor fibers of the iris are unopposed, resulting in miosis. The sympathetic nervous sys­tem also innervates several smooth muscle fibers that lift the upper eyelid and help position the nictitating membrane and the eye within the socket. Because the preganglionic fibers are relatively exposed in the neck, they are usually damaged.

Horner’s syndrome can also result from damage to either the postganglionic neurons or the neurons that descend from the hypothalamus to the rostral thoracic cord to control the preganglionic neurons.

Treatment. Treatment involves removing the cause of sympathetic nerve damage.