Horner Syndrome
Robert J. MacKay
■ Definition and Etiology Horner syndrome results from interruption of ocular sympathetic pathways. Sympathetic pathways to the head originate as first-order neurons (upper motor neurons) in the tectum of the midbrain.
Axons descend to the first to third thoracic segments (T1 to T3) of the spinal cord, where they enter the gray matter of the dorsal horn, synapse on second-order (preganglionic) sympathetic motor neurons, and exit in the ventral spinal nerve roots. From there the nerves pass through the cervicothoracic and middle cervical ganglia (combined as the stellate ganglion) and ascend in the cervical sympathetic trunk (part of the vagosympathetic trunk) to the cranial cervical ganglion (CCG), where they synapse on the third-order (postganglionic) sympathetic nerve. In the horse, the CCG is part of a neurovascular bundle in the caudodorsal aspect of the guttural pouch; in ruminants, the CCG is located under the petrous temporal bone. The third-order fibers are distributed with the blood supply via canals through the petrous temporal bone to the sweat glands of the head, ciliary muscles, and periorbital smooth muscles, including an eyelid levator (superior tarsal muscle) and periarteriolar musculature. The sympathetic nerve supply of the head thus can be damaged within the brainstem or spinal cord (first order); in the cranial thoracic nerve roots, spinal nerves, or cervical sympathetic trunk (second order); or in the CCG, in the skull, or behind the eye (third order).Specific causes of Horner syndrome include mycotic guttural pouch infections; traumatic lesions of the basisphenoid area; otitis media; cervical trauma; abscesses, tumors, or spaceoccupying lesions in the anterior aspect of the thorax; periorbital abscesses or tumors; parotid duct obstruction and inflammation; esophageal rupture; and complications associated with surgical ligation of the carotid artery.1-7 Lesions involving first-order sympathetic neurons in the ipsilateral brainstem are rare but have been reported as a consequence of metastatic neoplasia or with EPM in horses.8,9 Horner syndrome has also occurred after intravenous injection of certain drugs, including xylazine, vitamin E or selenium, and phenylbutazone.10-12 The sympathetic nerve supply to the eye is rare cases involved in the inflammatory polyneuropathy characterizing horses with polyneuritis equi (cauda equina neuritis) syndrome.13,14 Tumors that have resulted in Horner syndrome include sclerosing respiratory epithelial carcinoma, squamous cell carcinoma, lymphoma, mesothelioma, and melanoma.2,4-6,10,15,16
■ Clinical Signs The clinical signs of Horner syndrome in horses vary but include miosis, ptosis, regional hyperthermia, excessive sweating on the ipsilateral side of the face, congested mucous membranes, inspiratory stridor, and dermatitis caused by chronic sweating.
Although enophthalmos is reported to be a reliable sign of equine Horner syndrome, many (possibly most) affected horses do not exhibit prolapse of the nictitans and thus are probably not enophthalmic. The presence of this sign is apparently variable among species: Enophthalmos is part of Horner syndrome in cats and dogs9 but not in humans and ruminants.15,17 Ptosis is caused by the combined effects of loss of the eyelid levator function of the superior tarsal muscle and of variable enophthalmos. It is most reliably detected by comparison of the eyelash angles on each side. On the affected side, the eyelashes slope at a relatively downward angle. The palpebral reflex and the menace response are normal. If the vertebral nerve or its cervical sympathetic branches are damaged concomitantly, sweating may also be observed on the skin of the neck. Regional hyperthermia results from cutaneous and mucosal vasodilation, which in turn results from removal of sympathetic vasomotor tone. Sweating is thought to be caused by vasodilation and increased delivery of epinephrine to sweat glands. Facial sweating may begin to decrease 6 to 14 days after sympathectomy.10,18,19In cattle, there is dryness of the ipsilateral planum nasale. This can be explained by the mediation of normal sweat gland secretion by postganglionic sympathetic fibers and α-adrenergic receptors in cattle.19 The other signs in cattle are similar to those in horses. The clinical signs of experimentally induced Horner syndrome in sheep and goats are limited to mild ptosis.19 Retrobulbar tumors may cause both exophthalmos and Horner syndrome.15 On cold days, while exhaled breath from the external nares of cattle normally contains a mist of condensed water vapor, breath from the relatively warm nostril on the affected side may be mist free.9
■ Diagnosis The level of sympathetic interruption can be determined by pharmacologic testing.9 Hydroxyamphetamine (1% solution) instilled into the eye will result in release of norepinephrine from intact postganglionic sympathetic neurons, causing pupillary dilation, but in no response when the postganglionic neurons are damaged.
A positive response to this test indicates a preganglionic sympathetic lesion, and a lack of response indicates a postganglionic lesion. In animals with a postganglionic lesion, topical administration of 0.1 mL of 1 mg/mL epinephrine solution directly activates the iris musculature and produces mydriasis within 20 minutes, whereas the onset of dilation occurs in approximately 40 minutes in animals with preganglionic lesions. Similarly, topical administration of 2.5% to 10% phenylephrine solution will produce pupillary dilation in an eye with a postganglionic sympathetic lesion but not in a normal eye. The sensitivity to these direct-acting sympathomimetics in animals with postganglionic sympathetic lesions is increased as a result of the phenomenon of denervation hypersensitivity, in which numbers and sensitivity of norepinephrine receptors in the iris muscle increase for days to weeks after postganglionic nerve injury. A positive response therefore is not observed until several days after the injury. Parenteral administration of 1 mL of 1 mg/mL epinephrine solution causes affected horses to sweat profusely over the affected side of the face.10 However, this test does not differentiate between preganglionic and postganglionic lesions.The guttural pouches of horses and the pharynx of all patients should be examined endoscopically to exclude the possibility of pharyngeal or laryngeal paralysis or guttural pouch disease. The jugular furrows should be palpated for swellings. Insertion of a nasogastric tube during palpation may be helpful for detecting subtle lesions on the left side of the neck. The thorax should be examined with auscultation, ultrasonography and, if indicated, radiography. Cranial ribs can be evaluated for lesions by scintigraphy.20 A neurologic examination should be performed to evaluate the function of the spinal cord. The skin temperature may be measured with thermography1,6,21; on the affected side, it is 1° C to 3.5° C higher than normal.
■ Treatment The treatment for Horner syndrome depends on the underlying cause of the denervation. Except for Horner syndrome related to IV injection of xylazine, the signs are often irreversible, even if the primary cause of the condition has been eliminated. With xylazine-associated Horner syndrome, the condition disappears spontaneously. The necrotizing effects of perivascular drug injections can be minimized by immediate dilution or irrigation, or both, with large volumes of saline infiltrated into the perivascular tissues. Antiinflammatory doses of flunixin and dexamethasone can be given systemically. Abscesses should be drained, and fungal infections of the guttural pouch should be treated as described in Chapter 31.