b'TYING-UP: Understanding PSSM can take muscle biopsies and apply a spe-Where Do We Stand? As its name implies, PSSMs under- cial stain to them to examine glycogen in lying cause is an excessive storage/ the muscle cells. Horses with abnormally accumulation of polysaccharides (polyappearing glycogen have PSSM. The next hyperthermia (MH) in Quarter Horses= many, saccharide = sugar)chains ofstep is to differentiate between types 1 and recurrent exertional rhabdomyolysissugar molecules linked together insideand 2.(RER) in Thoroughbred racehorses.muscle cells.PSSM1Polysaccharide storage myopa-PSSM1 and 2 are also classified asCells, including muscle cells, takethy was recognized as a distinct disease in chronic causes of tying-up, yet seem toup these simple sugars such as glucose1992. But it wasnt until 2008 that Molly fall into a different category than MH andfrom the blood after a meal to produceMcCue, DVM, MS, PhD, Dipl. ACVIM, RER, says Valberg. PSSMs appear toenergy. Cells that use a lot of energy storeValberg, and colleagues at the Univer-have abnormal muscle cell metabolism excess glucose for a rainy day in the formsity of Minnesota (U of M) identified the way cells process energyrather thanof glycogen. Thirty thousand glucosean underlying cause: a genetic defect in an abnormality in calcium levels, whichmolecules (or more) can bond togetherQuarter Horses. are suspected in MH and RER.to form glycogen inside cells as a backupHorses with type 1 PSSM have an Regardless of the underlying cause,energy source. While storing a small stashexcess of the polysaccharide glycogen horses suffering from a bout of ER, in- of sugar in muscle cells is a great idea inwithin muscle tissue associated with an cluding PSSM, typically show some or allcase of emergencies (the equivalent ofautosomal dominant gain-of-function of the following: your extra coffee pod at work on Mon- mutation in the GYS1 gene, says McCue,Excessive sweating; day morning), just a dab will do. Excessdirector of U of Ms Leatherdale EquineIncreased heart and respiratory rates; glycogen or other polysaccharides canCenter, in St. Paul.Muscle twitching/fasciculations;be detrimental, as seen in PSSM-relatedLets break that information down:A reluctance or refusal to move; and cases of tying-up.DNA contains genes that provide theFirm, painful muscles over the backCurrently, we know of two forms offramework for creating proteins, includ-and croup (lumbar and glutealPSSM: The PSSM1 and PSSM2 Valberging enzymes such as glycogen synthase muscles).mentioned. In both cases veterinariansthat helps make glycogen in muscle Consider signalmentDiagnostic Decision(breed, age, sex)Tree for PolysaccharideIs this horse likely tohave exertional rhabdo-Storage Myopathy myolysis (ER) causedby PSSM? If yes, then:Measure creatinekinase and aspartate aminotransferase(enzymes indicative Muscle biopsy Genetic testingof muscle damage) levels in blood/serum during an episodeof ERPSSM2: can be Excess glycogenelevated in some and abnormalNo GYS1 mutation: PSSM1: usually cases (mostly appearance of Normal glycogen PSSM2 if muscle GYS1 mutation:elevated Quarter Horses, glycogen or resistance and amylase biospy results support PSSM1rather than to amylase (an enzymedigestion the diagnosisWarmbloods) that digests starchand sugar) digestion*Assess clinical signs and rule out PSSM Not PSSM lameness and neurologic diseases before pursuing additional testing.26TheHorse.comTHE HORSE March 2018'