Uneven sweating?

Q: My 15-year-old Quarter Horse, Nikos Heart Garcia, seems fine and healthy. However I am curious about the fact that he sweats on only one side. I am sharing photos of him that were taken after a ride on a hot day in July. The image of his chest show a distinct line between the sweat on his right side and the dry coat on his left. Is there a medical reason for this? JoAnn Drinkwater

Coal City, Illinois

A: This is a very interesting case of unilateral sweating. Sweating is a horse’s primary mechanism for thermoregulation, and the process of sweating is controlled mainly by neural signaling. Receptors on the skin perceive environmental temperature changes and send that information to the hypothalamus. The hypothalamus, in return, signals the stimulus of the post-ganglionic neuron

fibers that innervate the sweat glands--thus provoking the sweat response.

Although at first glance this might look like a strange case of unilateral anhidrosis (non-sweating), your description and photographs are consistent with a more complex neurological condition called Horner’s syndrome.

In Horner’s syndrome, sympathetic denervation---that is, loss of the sympathetic nerves involved in the neural communication---is responsible for a collection of clinical signs, including sweating on only one side, as your horse does. The unilateral sweat pattern is related to a higher body temperature on the affected side. You would clearly see this effect with a thermography exam, in which a heat-sensing camera is used to detect differences in the surface temperature of the horse.

Your photos also show a difference between the size of your horse’s pupils (unilateral miosis) and “droopiness” of the upper eyelid (ptosis) between the sweating and the non-sweating sides--which are also consistent signs of Horner’s syndrome.

Although Horner’s syndrome is sometimes idiopathic---meaning no specific cause can be found---it is often triggered by injury to the sympathetic nerves. Potential causes of this nerve injury include fracture of the cranial bones or cervical or first thoracic vertebrae; localized infections; vaccination or intravenous/intracarotid medications (especially vitamin E/selenium); tumors; or guttural pouch lesions due to mycosis (fungal infection). Horner’s syndrome sometimes appears with laryngeal hemiplegia---paralysis of one side of the larynx commonly called “roaring”---although no specific connection has been established.

Naturally occurring Horner’s syndrome is usually temporary and does not affect a horse’s performance. However, a few horses with the condition develop hair loss, lameness or nasal passage obstructions, mucosal edema and other complications. There is no treatment specifically for this condition, but a veterinarian will look for underlying causes. If the source of the Horner’s syndrome is treated, the neurological signs will often go away.

I strongly recommend contacting a veterinary neurologist for a consultation and having a thermography performed. This professional will be able to evaluate your horse and provide a prognosis, or maybe even a different diagnosis, for this case.

Laura Patterson Rosa, DVM

University of Florida Brooks Equine Genetics Lab Gainesville, Florida

SUBTLE SIGNS: Although unilateral sweating is commonly associated with anhidrosis in horses, it may also signal a condition called Horner’s syndrome, which is usually triggered by damage to the sympathetic nerves. Another sign of Horner’s syndrome is ptosis, a slight droopiness of the upper eyelid (top photo) compared to the normal eye (left).

HOT OR NOT? In Horner’s syndrome, a unilateral sweat pattern is related to a higher body temperature on the affected side.