Horner syndrome stems from nerve damage to parts of the autonomous nervous system and provokes a number of distinct symptoms, seen usually in one eye only.
Horner syndrome is defined by the following three main symptoms: 1. Ptosis: A drooping lower eyelid. 2. Miosis: An excessively narrowed pupil. 3. Enophthalmus: A sunken-in eyeball Additionally, the vessels of the white of the eye are usually incised making the eye appear bloodshot. Symptoms develop suddenly and without any impairment of the animal's overall health. In some cases symptoms may regress spontaneously after a period of time, usually ranging from a couple of weeks to a month. Changes may be permanent if the optic nerve has been damaged.
The nerve cell bodies of the autonomous nervous system are located inside the spinal cord of the thoracic (chest) and lumbar (abdominal) segment of the vertebral column and extend from there into other routes via the thoracic trunk towards the head. The respective nerves pass both the brachial plexus of the front limbs and the middle and inner ear. Injuries or inflammation of these structures can damage the nerve and thus impair nerve support of the eye. As a result, the above-mentioned symptoms develop. Possible causes are road traffic accidents in which a forelimb is overextended. This results in a bruising or stretching of the plexus. Another common cause is inflammation of the middle ear, which often develops in dogs as a complication of an infection of the ear canal. However, in many cases the underlying cause cannot be determined. When this occurs, the disease is referred to as idiopathic Horner syndrome.
By applying a certain medicine, in this case, phenylephrine, to the affected eye may reverse symptoms for a short period of time. One way the condition can be diagnosed is to see if symptoms do reverse with the application of phenylephrine. Treatment is aimed at the underlying cause, provided it can be determined. Accidents or infections may severely damage the nerve, possibly causing permanent alterations. In case of an idiopathic Horner syndrome treatment is often not necessary, and symptoms usually regress by themselves.
The potential health risks related to Horner syndrome are considered low. However, as some other conditions affecting the eye may produce similar symptoms, it is still advisable to consult a vet upon the next opportunity.