b'TYING-UP: TYPES OF EXERTIONAL MYOPATHIES BASED ON BREEDWhere Do We Stand?Sporadic exertional rhabdomyolysis due to a horse training or competing beyond its athletic ability, causing Of horses diagnosed with PSSM byANY BREED muscle damage, electrolyte deficitsmuscle biopsy, says Valberg, about 28%Dehydration, electrolyte abnormalities, otherof Quarter Horses have PSSM2, whereas 90% of Warmbloods and almost 100% ofRecurrent exertional rhabdomyolysis thought to be Arabians have PSSM2. Further, QuarterTHOROUGHBREDS,caused by abnormal calcium levels in muscle cells Horses with PSSM2 commonly experi- STANDARDBREDS during contractionence tying-up and have elevated circu-lating creatine kinase (CK, an enzyme indicating muscle damage). WarmbloodsWARMBLOODS Primarily PSSM2; only a small proportion of these (Hanoverian, Dutch/Swedish/ breeds appear to suffer from PSSM1with PSSM2, on the other hand, predomi-nantly exhibit an abnormal gait; reluc- Canadian/Danish Warmblood,Myofibrillar myopathy, a disruption of the alignment of Holsteiner, Oldenburg,the contractile proteins called myofibrils in affected tance to collect; a mild, shifting lameness;Trakehner, Rheinlander, others)horses muscle cells.and a less-consistent increase in CK. Points to Ponder When Diagnosing69% of Percherons and 39% of Belgians in North PSSM America and more than 75% of European Belgian DRAFT horses have the PSSM1 genetic mutationGenetic testing via blood or hair roots for PSSM1 is relatively inexpensive,Clydesdales and Shires rarely have PSSM1painless, and very specifically identifies PSSM1. In horses with chronic tying-upPrimarily PSSM1, also PSSM2 QUARTER HORSES that are among breeds known to carryAND RELATED BREEDSMalignant hyperthermia caused by a genetic mutation the PSSM1 mutation, genetic testing is a(Appaloosas, Paints) in the ryanodine receptor 1 (RYR1) genegood place to start, says Valberg.That said, many indicators of PSSM2poor performance, subtle lameness,ARABIANS AND ARABIAN- PSSM2 and myofibrillar myopathytopline atrophy (loss of muscling over theCROSSESback), difficulties changing leads and per-forming trot/canter transitions, changesMORGANS PSSM1 or PSSM2in behavior toward work, and sensitivity to groomingare common complaints that could be caused by a variety ofDiet alone will not fix these horses,to fuel the muscle cells, instead of storing conditions.she stresses.it, and to use fat as an alternate energy Owners are encouraged to workImplement a structured exercise pro- source. with their veterinarian to rule out moretocol, as well as turnout (using a grazingThe best diet for a horse with PSSM1 common causes for these clinical issuesmuzzle if pastures are lush). And as longdepends on his current clinical condition, before considering muscle biopsies toas your horse is not actively tying up, dobody condition score, and fitness level. evaluate horses for PSSM2, Valberg says.not give him time off from exercise.In general, says Valberg, there are five Only after a complete diagnosticIn terms of diet, the goal is to trick thekey nutritional goals when it comes to workup has exhausted more common ex- muscle into burning that excess glycogenmanaging horses with PSSM1:planations for the presenting complaints1. Offer hay thats <12% nonstructural (e.g., lameness, neurologic conditions)(water-soluble) carbohydrates, which should owners pursue testing.contain high levels of sugar. Recall that To further reinforce this, muscle biopsythe exuberant GYS enzyme (encoded results are, like many other diagnosticDID YOU KNOW? from the GYS1 gene) converts excess tests in veterinary medicine, subjective. Because the urine of horses withsugar to glycogen that accumulates in This means that without a propermuscle cells. workup, horses can be falsely diagnosedexertional rhabdomyolysis is dark,2. How much hay to offer depends on the with PSSM. This translates into institut- veterinarians in the 19th and earlyhorses body condition. It is important ing inappropriate or ineffective treat- 20th centuries thought kidney diseaseto achieve and maintain an optimal ments that could prolong time off from(which also causes dark urine) was tobody condition score (~5 or 6 out of 9) work or even (lead to) early retirement,blame. Researchers later discoveredin affected animals.Valberg says.that the dark urine is due to the3. If horses require additional energy abnormal presence of the protein(e.g., those still in moderate work), of-Managing the Disease myoglobin in the urine due to fer oil top-dressed on hay cubes or rice Valberg says horses with PSSM1 im- bran (naturally high in fat and low in prove markedly with a combination of amuscle breakdown. sugar). Generally, 13% of calories fed to customized diet and routine exercise. a horse daily in the form of fat provides 28TheHorse.comTHE HORSE March 2018'