Un­even sweat­ing?

EQUUS - - Consultants -

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.

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