Horner's Syndrome Horner’s syndrome is a disruption to the sympathetic nerve supply to the eye. It is sometimes seen after ear disease/ surgery, trau

Horner's Syndrome

Horner’s syndrome is a disruption to the sympathetic nerve supply to the eye. It is sometimes seen after ear disease/ surgery, trauma, neck or thoracic disease but is most commonly “idiopathic”, meaning that no underlying cause is found. It is named after Johann Horner, a Swiss ophthalmologist who described the condition in 1869.

The clinical signs are enophthalmos, miosis, ptosis (drooping) of the upper and lower lid) and prominence of the third eyelid. These 4 classic signs and a full eye examination to rule out uveitis allow the diagnosis of the syndrome. It is most commonly unilateral but can be bilateral.

A breed predilection has been reported in Golden Retrievers (Boydell, JSAP, 1995) and Rough Collies (Boydell, JSAP 2006 or so).

The pathway for sympathetic nerve supply to the eye (to the pupil, and smooth muscle behind the eye) starts in the brain and passes down the cervical spine. The preganglionic sympathetic cell bodies are located in T1-3. The fibres leave the spinal canal and terminate in the cranial cervical ganglion. The postganglionic fibres then run through the middle ear to the eye where they innervate the iris dilator muscle and muscles around the eye. This long and complex course makes the nerves vulnerable to trauma or compression over a large length. However, it is interesting to note that the commonest site for disease is believed to be post ganglionic nerve.

Most patients present with either a history of a prominent third eyelid, a sunken eye OR a history of a DILATED pupil. When investigating cases of anisocoria (uneven pupil size), it can sometimes be a dilemma as to which pupil is abnormal. Dogs with Horner’s syndrome do not have any vision loss, will still have a PLR (direct and indirect) however the resting pupil size will be smaller. Horner’s Syndrome can vary in intensity from profound miosis to moderate.


Image borrowed from Manual of Small Animal Ophthalmology - British Small Animal Veterinary Association.

The work up of a suspected case of Horner’s syndrome involves the use pharmacologic testing. Due to denervation hypersensitivity, if phenyephrine drops 2.5% (available as 1ml Minims) are placed topically the clinical signs will resolve over 10 minutes or so. Attempts have been made to correlate speed of return with where the lesion is situated (1st, 2nd or 3rd order) but there is considerable overlap. Ultradilution of the phenylephrine can also be performed to allow comparison with dilation in the normal eye but clinically this is rarely useful.

Management of Horner's Syndrome depends on the underlying cause.

The most common form of Horner’s Syndrome seen in our clinic (95% plus) is idiopathic. The majority of these cases will recover in 6-8 weeks with no treatment. Other signs present which will spur us to further investigation are: any other neurologic deficits (especially of the cranial nerves), neck pain, history of trauma, any thoracic disease, breeds other than Golden Retriever. Our standard work up would then include chest and cervical spine radiographs, rule out ear disease, +/- head CT/MRI as applicable.

The majority of our patients are therefore monitored for any progression of signs that would initiate further workup rather than launching into a costly and invasive workup list that is most likely to be unproductive.

Horner’s Syndrome is the most common neurologic disease of the eye and should be a differential in any protrusion of the third eyelid, sunken eye or anisocoria case.

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