PSSM (POLYSACCHARIDE STORAGE MYOPATHY)

A GENETIC CONDITION IN NEED OF CLOSE MANAGEMENT

If you don't own a susceptible breed of horse, PSSM might be a term you've never heard before. However, even though this genetic condition is fairly breed-specific, it is worth every owner being aware of the signs. Fortunately, for those with horses with PSSM, the disease can be managed with a bit of effort. Here we look at this challenging condition and exactly how we can manage it in our horses.

What is it?

Polysaccharide Storage Myopathy (PSSM) is a disease resulting in an abnormal accumulation of glycogen. Glycogen is the form of sugar stored in the horse's muscles. This accumulation can result in stiffness, tying up or various degrees of pain in the horse's body. Two types of PSSM have been identified to date:

PSSM 1: PSSM 1 is caused by a genetic mutation in the glycogen synthase 1 (GYS1) gene. This mutation causes muscle cells to continuously produce glycogen. Due to this being an autosomal dominant trait, only one copy of the mutation is needed for a horse to be affected. Having said that, environmental factors such as diet and exercise play an important role in the development of clinical signs. PSSM1 has been discovered in more than 20 breeds, but the most commonly affected breeds are Quarter Horses, Paints, and Appaloosas, as well as draft breeds such as Percherons.

PSSM 2: PSSM 2 also results in abnormal glycogen storage in the muscles; however, the horses do not have the GYS1 mutation. The cause of PSSM2 remains unknown, and it is suspected that there may be multiple factors at play. It is thought that a condition called myofibrillar myopathy (MFM) which is characterised by exercise intolerance and intermittent exertional rhabdomyolysis (breakdown of muscles after exercise) may be an extreme subset of PSSM2 but further research is needed to confirm this. PSSM2, but not MFM, has been diagnosed in Quarter Horses. Cases of PSSM2/MFM have been reported in Warmbloods and Arabians.

Symptoms of PSSM

Clinical signs of PSSM can range from mild to severe and include:

• Excessive sweating

• Lameness

• Sore muscles

• Undiagnosed lameness

• Tying up

• Poor performance

• Muscle tremors

• Reluctance to move forward or collect

• Tripping

• Poor stamina

• Weakness or muscle loss

• Bucking

In some cases, no clinical signs will be seen.

Diagnosis

PSSM1 is diagnosed using a genetic test that checks for the presence of the GYS1 variant. The genetic test is performed using hair or blood samples. At present, there is no scientifically validated, peer-reviewed diagnostic test for PSSM2 or MFM. However, a muscle biopsy may be taken to evaluate muscle damage and measure the amount of glycogen stored in the muscle. The results of this biopsy may hint at PSSM2 or MFM. Still, horses can tie-up due to other causes such as malignant hypothermia, glycogen branching enzyme deficiency and myosin heavy chain myopathy. For this reason, it is essential to do this the biopsy, even thought it might not confirm PSSM2 or MFM, as it can rule out all these other conditions so that the horse is treated correctly and a different diagnosis is not missed.

Treatment and management

It is important to have your horse's feed checked or tested by a nutritionist so as to ensure that his non-structural carbohydrate (NSC) intake is less than 12%. This assists in keeping the blood insulin levels low and reduces glycogen storage in the muscles. Most horses with PSSM are easykeepers and do not usually require more than low-sugar hay and a custom mineral mix to maintain condition. If more energy is required, then a low NSC or high-fat feedshould be sufficient.

Keeping PSSM horses on pasture is not recommended unless the pasture has been tested to be consistently below 12% NSC. However, turnout is highly beneficial for PSSM horses, with 24/7 outdoor turnout in a large area being optimal. Because many of the horses with the condition cannot handle grazing on pasture, it is advised that owners consider a horse track system which allows the horse to keep moving over a large distance while staying off of grass entirely. In a track system, it is usual for the horse to have free access to low-sugar hay rather than grazing on pasture.

The diet should also have sufficient levels of quality protein in it, and some additional key elements should be carefully addressed in the horse's diet as shortages may significantly inhibit the horse's muscular comfort. These include salt, iodine, magnesium, selenium and vitamin E, as well as L-glutamine and L-leucine.

Regular exercise is highly beneficial for PSSM horses. Even just 10 minutes of activity has been demonstrated to be highly beneficial in reducing muscle damage. The better conditioned the horse is with a consistent exercise programme, the more likely they are to remain comfortable, and once they are fit and at a good point in their conditioning programme, they may be able to reduce the exercise to four times a week provided sufficient turnout is offered. For horses diagnosed with PSSM2, a lengthened, slow warmup with adequate stretching is recommended. It may also be of benefit to give the horse rest periods to relax and stretch their muscles between bursts of 2-5 minutes of collection under saddle.

During an acute episode of tying up, your vet should be contacted. They will likely administer sedatives, analgesics, muscle relaxants and non-steroidal anti-inflammatory drugs (NSAIDs) for pain relief. There isn't sufficient evidencebased data available with regard to treating horses diagnosed with PSSM2, but they are often treated much the same as horses with PSSM1.

A NOTE ON HIGH-FAT FEEDING

High-fat feeding, unfortunately, can come at a cost of its own. Highfat feeding can predispose to the development of Insulin Resistance, especially for 'good doers', which hugely increases their risk of developing laminitis. In fact, fat has been used by researchers to induce a state of Insulin Resistance in horses. For example, in a 2005 study by Dr Nicholas Frank and others, the researchers used the equivalent of just under a cup of rice bran oil to induce insulin resistance in Thoroughbreds.

Dr Eleanor Kellon VMD, an equine nutritionist specialist, recommends a different approach to the normal high-fat feeding recommendation. "I've lost track of how many horses suspected to have PSSM/EPSM and put on high-fat diets end up with me because they get Insulin Resistance and become laminitic. We don't know the safe dose of fat for an Insulin Resistant horse, but we sure don't want to be feeding as much fat as is already known to induce Insulin Resistance in a normal horse!"

Instead, Dr Kellon's approach is to limit carbohydrates and supplement with L-carnitine to assist in getting fat from body stores into the mitochondria to then be used for energy. She advises that this is done in conjunction with regular exercise and a mineral-balanced diet. With L-carnitine supplementation, it has been possible to greatly reduce or eliminate the need for high-fat feeding in these horses.

Final thoughts

Unfortunately, there is no cure for PSSM, but with careful management, through diet and exercise, many horses with the condition can live a comfortable life with minimal or no symptoms. Because the condition is genetic and only requires one parent to carry the gene, with at least a 50% chance of the parent passing on the causative mutation to its offspring, it is recommended that one tests for PSSM before using a horse for breeding, particularly if the breed is considered high-risk for PSSM. Despite the cause of PSSM2 being unknown, one should not breed with horses diagnosed with this condition via muscle biopsy.