Malignant hyperthermia (MH) is a sudden and often fatal rise in body temperature which is mostly associated with induction of general anesthesia. It is characterized by sudden onset of extremely high body temperature, rapid and irregular heart beat, and rigidity of muscles. Malignant hyperthermia is not a disease. It is a pharmacogenetic, life-threatening hypermetabolic syndrome in genetically predisposed individuals exposed to certain anesthetic agents. Malignant hyperthermia affects humans, certain pig breeds, dogs, horses, and probably other animals. The underlying defect in susceptible individuals seems to be a faulty calcium metabolism in muscles which results in typical tetany symptoms. The burst of muscular activity produces tremendous quantities of heat which overwhelm the normal compensation mechanisms of the human body, and the central temperature and feedback control mechanisms may be damaged as well. In humans the syndrome is inherited in autosomal dominant pattern.
In dogs malignant hyperthermia is an inherited disorder of skeletal muscle characterized by hypercarbia (abnormally high level of carbon dioxide in the circulating blood), generalized skeletal muscle contracture, irregular heart beat, and renal failure, that develops on exposure to succinylcholine and other anesthetic agents. Autosomal dominant canine malignant hyperthermia is caused by a mutation in the gene encoding the skeletal muscle calcium release channel. This syndrome can also be caused by exposure to certain toxins or extreme heat. In 1995, The National Animal Poison Control Center at the University of Illinois in Urbana recorded 8 fatal cases of hops toxicity; the victims were seven greyhounds and one Labrador Retriever mix whose ingestion of spent hops from home beer-brewing kits resulted in malignant hyperthermia, an uncontrollable fever rising as fast as 2 degrees F every 5 minutes. If your dog is a Greyhound or other breed with particularly low body fat, be conscious of the potential of malignant hyperthermia which should be treated immediately.
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In one case, A 7-month-old male Siberian Husky affected by lower motor neuron tetraparesis was anesthetized for electrodiagnostic testing and collection of muscle and nerve biopsy specimens. The dog was anesthetized with isoflurane and developed hyperthermia. Injectable anesthetic agents were used to anesthetize the dog the second time, during which the dog developed severe malignant hyperthermia. The dog recovered from malignant hyperthermia and was discharged to the owner after 13 days of hospitalization. Dogs affected by genetic muscle disorders should be considered at risk for perianesthetic malignant hyperthermia, even in the absence of a gene mutation. 6
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