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PSSM and Tying Up in Horses

Have you ever wondered what PSSM and tying is? 


The American Quarter Horse Association (AQHA) has funded research into this disease since 1995 and has provided us with the opportunity to learn much about the diagnosis, cause and treatment for this disease.

Prior to the development of a genetic test, PSSM was diagnosed by muscle biopsy. The muscle biopsy technique identified an apparent glycogen storage disease and the terms PSSM or EPSM and EPSSM were used to describe the condition. The variety of acronyms used are in part related to preferences of different laboratories, as well as to differences in the criteria used to diagnose polysaccharide storage myopathy. With progress into identifying the genetic basis for PSSM, we now recognize that there is more than one form of PSSM. Our laboratory now distinguishes type 1 PSSM (PSSM1) and type 2 PSSM (PSSM2). Type 1 PSSM is the form of PSSM caused by the genetic mutation in the glycogen synthase 1 () gene. Type 2 PSSM represents one or more other forms of a muscle disease that are characterized by abnormal staining for muscle glycogen in microscopic examination of muscle biopsies. 


The original research performed by Dr. Valberg on PSSM centered around Quarter Horses with clinical signs of tying-up and abnormal amylase-resistant polysaccharide in their muscle biopsies. This research determined that the horses had 1.8 fold more glycogen (storage form of sugar) in their muscles, a deficit in energy when they exercised and persistent elevations in serum CK activity with exercise unless fed low starch high fat diet. Continued research into PSSM in Quarter Horses resulted in the discovery of a genetic mutation in the glycogen synthase 1 gene in this herd of research horses. Testing of all of the muscle samples submitted to the NMDL that were diagnosed by muscle biopsy as having PSSM showed that not all horses diagnosed with PSSM by muscle biopsy have the mutation. The form of PSSM resulting from the mutation was termed type 1 PSSM.


In some breeds, horses with the genetic mutation for PSSM1 are asymptomatic. This may relate to differences in diet, exercise and impact of different genotypes in different breeds.

Horses with PSSM1 can have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move in conjunction with increased serum creatine kinase (CK) activity. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM1 can exhibit symptoms without exercise. 

During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM1 have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year. Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee colored, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse's kidneys if they become dehydrated.

Very young foals with PSSM1 occasionally show signs of severe muscle pain and weakness. This occurs more often if they have a concurrent infection such as pneumonia or diarrhea.

Follow the link below to keep reading! 


"TYPE 1 POLYSACCHARIDE STORAGE MYOPATHY." Michigan State University, cvm.msu.edu/research/