Shared problems

Your informative article “Sorting Out Muscle Disorders” (EQUUS 496) mentioned malignant hyperthermia (MH) in Quarter Horses.

In people, MH presents as a dangerous hypermetabolic response to the halogenated volatile anesthetic gases halothane (which is, I believe, no longer made), sevoflurane, desflurane and isoflurane. Untreated, MH has a high rate of morbidity and mortality. The most potent trigger of human MH is the depolarizing muscle relaxant succinylcholine. Rarely, in humans, MH may be induced by heat and/or vigorous exercise.

Since the article mentioned equine MH only in passing, I’d like to know whether the same triggers apply to horses as well as to people and whether the same high risk of morbidity and mortality occurs in horses if they present with an MH code or crisis. Charese Pelham, MD Williamsburg, Virginia

Stephanie Valberg, DVM, PhD, responds: It is interesting to see the similarities and, often, differences in muscle diseases found in horses and humans!

Malignant hyperthermia (MH) is one disease that has a similar clinical picture in horses as in humans. In horses, it is caused by an autosomal dominant mutation in the skeletal muscle ryanodine receptor 1 gene (RYR1) that occurs in less than 1 percent of Quarter Horses and Paint Horses. Horses with MH can be asymptomatic, have fatal anesthetic hyperthermia or intermittently show signs of exertional rhabdomyolysis (ER) and elevated body temperature similar to humans.

As in humans, the RYR1 mutation lowers the activation and heightens the deactivation threshold of the ryanodine receptor, which intermittently can result in a drastic efflux of calcium from the sarcoplasmic reticulum, producing muscle contracture. Anaerobic glycogen metabolism is activated, lactate is produced, excessive heat is generated and massive muscle necrosis ensues. Some MH-affected horses have died suddenly after an episode of ER or under general anesthesia.