Polysaccharide storage myopathy results in an abnormal accumulation of glycogen, the form of sugar stored in muscle. This can cause episodes of muscle stiffness and pain after exercise, also known as “tying up” or exertional rhabdomyolysis. Affected horses may be reluctant to move and may display sweating, lameness, and muscle tremors.
Clinical signs of PSSM range from mild to severe. They include sweating, lameness, sore muscles, undiagnosed lameness, poor performance, and muscle tremors (“tying up”). These may occur with or without exercise. Under saddle, affected horses may be reluctant to go forward or collect. Some affected horses, however, do not exhibit any clinical signs.
n/n ClearThis horse tested negative for P1 and does not carry the GYS1-R309H mutation identified in Quarter Horses. The horse will not pass on the defect to its offspring.
n/P1 AffectedBoth the normal and mutant alleles are present. This horse is positive for the GYS1-R309H (P1) mutation and may develop symptoms of exercise intolerance.
P1/P1 AffectedThis horse carries two copies of the GYS1-R309H (P1) mutation and may develop symptoms of exercise intolerance.