PSSM (Polysaccharide Storage Myopathy)

PSSM is a hereditary muscle disease in horses with Quarter Horse bloodlines such as Quarter Horses, American Paint Horses and Appaloosas. Another form of PSSM also occurs in Draft, Draft crossbreeds, and warmbloods. Many of the clinical signs in these breeds differ. The signs include muscle soreness, reluctance to engage the hind quarters, muscle atrophy, weakness, difficulty in backing up, and picking up hind feet.

PSSM is characterized by the abnormal accumulation of the normal form of glycogen stored in muscle, as well as an abnormal form of polysaccharides in muscle tissue. To minimize these sugar problems carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, should be avoided. Extra calories can be provided in the form of fat.

Horses with PSSM have signs typically associated with tying-up. The most common signs are muscle stiffness, sweating, and reluctance to move. These signs are most often seen in horses when training begins or after a lay-up period when they have received little active turn-out.

HYPP (Hyperkalemic Periodic Paralysis)

HYPP is an inherited disease of the muscle which is caused by a genetic defect. The muscle of affected horses has a mutation in the sodium channel gene that is passed on to the offspring. Sodium channels control contraction of the muscle fibers. When this defective gene is present, the channel becomes "leaky" and makes the muscle overly excitable and susceptible to involuntarily contractions. The channel becomes "leaky" when potassium levels fluctuate in the blood. This fluctuation can occur with fasting followed by consumption of a high potassium feed such as alfalfa. Hyperkalemia, which is an excessive amount of potassium in the blood, causes the muscles in the horse to contract more readily than normal. This makes the horse susceptible to sporadic episodes of muscle tremors or paralysis.

RER (Recurrent Exertional Rhabdomyolysis)

RER is caused by an abnormality in intracellular calcium regulation. At present there is no specific test for RER. There are a number of mechanisms by which owners can manage horses with RER in order to reduce episodes of tying up. Any management program should be established by discussing your horse’s individual case with your veterinarian.

Tying-up is a term used to describe horses that develop firm hard muscles following exercise. Horses sweat profusely, breathe rapidly and become stiff and reluctant to move. A diagnosis of tying-up is based on a blood sample that measures the presence of muscle proteins such as CK and AST in the blood. For many owners this is a very frustrating condition as it occurs in talented horses often at a time when they are showing great potential.