Polysaccharide Storage Myopathy (PSSM) is a disease in horses that causes glycogen (the stored form of glucose) to build up in the muscles. Usually, glycogen is stored in the body and converted to energy as and when it is required.

However, in a horse with PSSM, the glycogen doesn’t get used properly during exercise and therefore starts to build up. As well as storing too much glycogen, affected horses often make too much of it as well. 

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Why does it happen?

There are two types of PSSM – Type 1 and Type 2. Type 1 is caused by a genetic mutation and is most commonly seen in Quarter horses and some of the draft breeds. The cause of Type 2 is still unknown and can affect Quarter Horses, Arabs, and Thoroughbreds.

What are the signs of PSSM?

Often the classic signs of “tying-up” are seen, such as, hard and painful muscles, stiffness, sweating, and reluctance to move. The signs usually occur with exercise and dark coloured urine may also be seen. However, some cases can be more subtle, with a mild and intermittent hindlimb lameness being noticed.

How is PSSM diagnosed?

To confirm that a horse is “tying-up”, a blood sample will be taken to check the levels of the muscle enzymes. These are usually raised during an episode and can stay raised for several hours afterwards. If the horse is one of the common breeds affected by Type 1 PSSM then it is likely that a genetic test will be recommended, this can be carried out on a hair or blood sample.

If the horse is not one of the commonly affected breeds or genetic testing does not reveal a genetic mutation then a muscle biopsy will be advised. The horse will be sedated and then a small piece of muscle from the hindquarters will be removed and sent to the laboratory for examination. Performing this biopsy is necessary to diagnose PSSM and to distinguish the signs from other causes of “tying-up”. 

How is PSSM treated?

If you think your horse is showing signs of “tying-up”, you should call the vet immediately as it can be very painful and stressful for the horse. Exercise should be stopped straight away and the horse should be kept calm until the vet gets there.

The vet will give pain relief and may even sedate the horse, if dehydration is present then intravenous fluids may be required and therefore the horse may need to be hospitalised. The horse should be box-rested until the condition improves, after this, turnout in a small paddock can be allowed, which will encourage the breakdown of the glycogen in the muscle.

No cure, but it can be managed

Unfortunately, PSSM can not be cured, it can, however, be managed through careful control of diet and exercise. 

Changing diet

The aim of diet control is to prevent sudden increases in blood glucose and reduce the amount of glycogen that is stored in the muscle. Hard feed should be avoided, with good quality forage making up the majority of the diet. Where possible, this should have a low water-soluble carbohydrate level.

Any extra feed should be low in calories and sugar, aiming for feeds that release energy slowly. Controlled access to grazing and the use of a grazing muzzle may be required to reduce the amount of sugar that the horse consumes from the grass; especially if they are “good-doers”. Vitamins and minerals should be provided in the form of a balancer. Oils can be used as an alternative energy source. 

Increased exercise and preparation

Exercise helps to break up the glycogen that is stored in the muscle and therefore a horse with PSSM should have regular light exercise. Ideally, a small amount of exercise should be carried out every day, and consistency is key. Before undertaking hard or fast work the horse should be allowed to stretch and warm-up properly, frequent breaks from collected work are also recommended.

If careful attention is paid to an appropriate exercise regime and diet, the frequency and severity of the episodes can be reduced and many horses with PSSM can live long and happy lives.

Further information on genetic testing can be found here:

https://www.animalgenetics.eu/Equine/Genetic_Disease/PSSM.asp

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