Polysaccharide storage myopathy (PSSM) has been recently characterized as a specific condition leading to muscle dysfunction. For many years, many horse breeds have been known to become stiff in their limbs (tie up) when exercised. This is also known as exertional rhabdomyolysis or azoturia. Although many different causes for tying up have been advocated, few have been characterized as well as PSSM. PSSM causes tying up in quarter horses and draft breeds.
For many years, draft horses and some other heavy warm blood breeds have been known to exhibit difficulty with muscle control of their hind limbs and tail when made to walk backwards such as is needed when being used to pull a load. When asked to walk backwards, affected draft/heavy horses exhibit marked trembling and fasciculation in the hind limb and tail. This trembling has been referred to as shivering (shivering syndrome)and has also recently been attributed to PSSM, and it has been estimated that it affects more than 50 percent of draft horses in the United States. The Belgian and Percheron breeds have been especially incriminated with this condition, but other breeds are also affected.
Affected quarter horses tend to develop stiffness, sweating and muscle rigidity shortly after mild-to-moderate exercise and become intolerant of work. Some horses experience shifting lameness affecting primarily the hind limbs. Following rest, these clinical signs generally resolve until the next exercise period. Less commonly, in severe cases, the affected horse may appear to be colicky and may pass brown-discolored urine (muscle tissue pigment is excreted in urine). On the other hand, PSSM has also been diagnosed in many quarter horses that have not exhibited recognizable clinical signs. Affected horses are often very quiet and appear sedate between episodes.
On the other hand, affected draft horses tend to exhibit signs of weakness and muscle trembling or shivering, especially in the hind limbs. Trembling of the tail (tail wringing) is also common when the affected horse moves backwards. Signs of PSSM may also be precipitated by picking up one of the hind limbs; affected horses exhibit difficulty when asked to pick up a hind foot, as when trimming the hoof. Affected horses tend to be lethargic and lose weight over time. Focal loss of muscle from the hind limbs is more commonly identified in draft breeds than in quarter horses affected with PSSM. During an attack, the draft horse may be so severely compromised that, following multiple whole body oscillations, the horse becomes recumbent and appears to be in pain or to be having a seizure.
As with quarter horses, PSSM has been identified in some draft horses that have not had any problems. Clinically evident PSSM typically appears by 3 to 12 years of age. As with any severe muscle disorder, muscle pigment (myoglobin) may appear in the urine causing it to be dark brown in color and, without appropriate treatment, may lead to kidney disease. In the most severe cases, the horse may die as a result of respiratory failure because the diaphragm muscle is involved.
A diagnosis of PSSM is based on recognition of signs of muscle dysfunction and ruling out other causes of muscle problems. As described above, the clinical symptoms of PSSM are somewhat different between different breeds. Episodes of PSSM may be precipitated by light exercise on a lunge line in the quarter horse breed.
The presence of darkly colored urine following exercise should alert one to the possibility of muscle disease. Confirmation of muscle disease is accomplished using a blood test. Blood and urine may also be tested for evidence of secondary kidney damage. The specific diagnosis of PSSM in all breeds is based on biopsy of the skeletal muscle. Skeletal muscle biopsy is usually easily accomplished under light tranquilization and local anesthetic. The veterinarian typically obtains a sample of the muscle at the gaskin. In some severely affected draft horses, the muscle biopsy procedure might precipitate an attack and should be undertaken with caution.
Treatment of PSSM includes the use of dietary changes to reduce the risk of further muscle damage. Further muscle damage tends to occur when recently-affected horses are exercised. Rest from enforced exercise should be instituted until the muscle enzyme tests (blood tests) have satisfactorily returned to normal. If kidney damage has occurred, the horse will be treated using IV fluids and diuretics. Other treatment strategies for muscle pain (cramping) include the use of NSAID (nonsteroidal anti-inflammatory agents), light tranquilization, and physical therapy (hot blankets).
Prevention of PSSM includes the use of a low carbohydrate – high fat diet. Enhanced fat content of the diet may be accomplished by adding 3 cups of vegetable oil to the ration daily. Grain, sweet feed, and molasses should be eliminated form the diet. A commercially available proprietary ration is available for horse owners to prevent PSSM based on these recommendations. At least 3 to 4 months are required for the fat enhanced diet to effect its maximum benefit; some treated horses will develop further bouts of muscle dysfunction after only 2 months of treatment with this diet. PSSM-affected horses should not be stall confined. PSSM-affected horses are less likely to develop signs of muscle dysfunction if they are allowed free choice exercise.
The prognosis for PSSM is generally favorable; some severely affected draft horses may die or be so compromised as to warrant consideration of euthanasia. The survival rate for severely affected draft horses has been reported to be approximately 50 percent. It also appears that PSSM may be inherited so the use of affected horses for breeding should be critically evaluated.