Q.  Last year my 18-year-old Quarter Horse mare became very sensitive to touch on her back and hindquarters. It progressed to a slight hind limb lameness. She would stand with her back feet close together until eventually she would lean and prop her haunches against the barn wall. When the lameness was most severe, she could not walk straight, although only the hindquarters seemed to be affected. Then, a day later, it began clearing up on its own and within a few days all symptoms were gone. Nothing showed up on standard blood tests and she tested negative for West Nile virus. Since then, every once in awhile the sensitivity to touch returns and I hold my breath to see if the other clinical signs do as well. So far they haven't, and within a few days she's back to normal. I haven't noticed any correlation between the problem and diet changes. She is HYPP (hyperkalemic periodic paralysis) negative. Someone recently mentioned polysaccharide storage myopathy (PSSM), but I'm not sure what that is. I am baffled and was wondering what to test for should this occur again.

Katie, via e-mail

A. Polysaccharide storage myopathy is a muscle disease affecting horses of several breeds. Horses with PSSM store excess abnormal glycogen (sugar) in their muscles. When these horses are asked to exercise they often have painful muscle cramping, sweating, and reluctance to move, which is often termed "tying-up." Other less common signs in Quarter Horses with PSSM include muscle wasting (atrophy), muscle tremors (fasciculations), muscle soreness, and other unexplained gait abnormalities or lameness.

The clinical signs you describe could be due to PSSM, although they are not typical. The signs could also be due to an orthopedic or neurologic problem. I suggest you have your mare evaluated to help determine if she has an orthopedic, neurologic, or muscle problem. A veterinarian should perform neurologic and lameness exams, including nerve blocks and radiographs (X rays) if necessary. If the these exams are normal, then evaluate for muscle disease.

This includes several diagnostic steps. First, your veterinarian should evaluate your mare's musculature, looking for any signs of atrophy or asymmetrical muscle mass, and identifying any sore areas with palpation. Second, your mare should have the concentration of two muscle enzymes measured in her blood, creatine kinase (CK) and aspartate aminotransferase (AST). These enzymes are indicators of muscle damage that can be routinely evaluated. Ideally, CK and AST levels would be measured, then your mare would be exercised for 15 minutes at a trot, and four hours after she completed the exercise, the CK value would be repeated. A rise in the CK value after exercise indicates muscle damage, even if we cannot see overt signs of tying-up when asked to trot. It is also important to evaluate your mare's blood levels of vitamin E and selenium. Horses deficient in either of these nutrients can have chronic muscle problems.

The next step is a muscle biopsy to detect any of several muscle diseases, including PSSM--which is indicated by abnormal sugar (glycogen) in the muscle cells.

There are two types of PSSM in Quarter Horses. Both are genetic diseases that are passed from sire or dam to their offspring. Type 1 PSSM is due to a mutation in a specific gene. The mutation in this gene results in abnormal sugar metabolism in the muscle, and it can be detected by testing blood or hair root samples. Type 2 PSSM is also a genetic disease; however, the mutation that causes this disease is currently unknown, and Type 2 PSSM must be diagnosed using a muscle biopsy.

More information about testing for these disorders can be found at www.cvm.umn.edu/umec/lab/home.html.

About the Author

Molly E. McCue, DVM, MS, PhD, Dipl. ACVIM

Molly E. McCue, DVM, MS, PhD, Dipl. ACVIM, is a post-doctoral associate at the University of Minnesota.

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