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Posted on 03-23-2017
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.
"TYPE 1 POLYSACCHARIDE STORAGE MYOPATHY." Michigan State University, cvm.msu.edu/research/
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