The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
Issue link: https://thehorse.epubxp.com/i/936432
26 TheHorse.com THE HORSE March 2018 hyperthermia (MH) in Quarter Horses and recurrent exertional rhabdomyolysis (RER) in Thoroughbred racehorses. "PSSM1 and 2 are also classified as chronic causes of tying-up, yet seem to fall into a different category than MH and RER," says Valberg. "PSSMs appear to have abnormal muscle cell metabolism— the way cells process energy—rather than an abnormality in calcium levels, which are suspected in MH and RER." Regardless of the underlying cause, horses suffering from a bout of ER, in- cluding PSSM, typically show some or all of the following: ■ Excessive sweating; ■ Increased heart and respiratory rates; ■ Muscle twitching/fasciculations; ■ A reluctance or refusal to move; and ■ Firm, painful muscles over the back and croup (lumbar and gluteal muscles). Understanding PSSM As its name implies, PSSM's under- lying cause is an excessive storage/ accumulation of polysaccharides (poly = many, saccharide = sugar)—chains of sugar molecules linked together inside muscle cells. Cells, including muscle cells, take up these simple sugars such as glucose from the blood after a meal to produce energy. Cells that use a lot of energy store excess glucose for a rainy day in the form of glycogen. Thirty thousand glucose molecules (or more) can bond together to form glycogen inside cells as a backup energy source. While storing a small stash of sugar in muscle cells is a great idea in case of emergencies (the equivalent of your extra coffee pod at work on Mon- day morning), just a dab will do. Excess glycogen or other polysaccharides can be detrimental, as seen in PSSM-related cases of tying-up. Currently, we know of two forms of PSSM: The PSSM1 and PSSM2 Valberg mentioned. In both cases veterinarians can take muscle biopsies and apply a spe- cial stain to them to examine glycogen in the muscle cells. Horses with abnormally appearing glycogen have PSSM. The next step is to differentiate between types 1 and 2. PSSM1 Polysaccharide storage myopa- thy was recognized as a distinct disease in 1992. But it wasn't until 2008 that Molly McCue, DVM, MS, PhD, Dipl. ACVIM, Valberg, and colleagues at the Univer- sity of Minnesota (U of M) identified an underlying cause: a genetic defect in Quarter Horses. "Horses with type 1 PSSM have an excess of the polysaccharide glycogen within muscle tissue associated with an autosomal dominant gain-of-function mutation in the GYS1 gene," says McCue, director of U of M's Leatherdale Equine Center, in St. Paul. Let's break that information down: DNA contains genes that provide the framework for creating proteins, includ- ing enzymes such as glycogen synthase that helps make glycogen in muscle TYING-UP: Where Do We Stand? *Assess clinical signs and rule out lameness and neurologic diseases before pursuing additional testing. Diagnostic Decision Tree for Polysaccharide Storage Myopathy PSSM1: usually elevated Normal glycogen and amylase digestion Not PSSM Consider signalment (breed, age, sex) Is this horse likely to have exertional rhabdo- myolysis (ER) caused by PSSM? If yes, then: Measure creatine kinase and aspartate aminotransferase (enzymes indicative of muscle damage) levels in blood/serum during an episode of ER Muscle biopsy Genetic testing PSSM2: can be elevated in some cases (mostly Quarter Horses, rather than Warmbloods) No GYS1 mutation: PSSM2 if muscle biospy results support the diagnosis Excess glycogen and abnormal appearance of glycogen or resistance to amylase (an enzyme that digests starch and sugar) digestion PSSM GYS1 mutation: PSSM1