Polysaccharide Storage Myopathy

This article is from the February 2021 Horse Deals magazine.

Polysaccharide Storage Myopathy (PSSM) is a relatively common disease in horses, particularly in Western bred horses such as Quarter Horses, Paints and Appaloosas. This article aims to describe what is PSSM, how is it diagnosed and what can we do to manage the condition.

What is PSSM?

PSSM causes the abnormal production of glycogen by muscle cells. Glycogen is how muscles store sugar to produce energy for muscle contraction and function. The excess accumulation of glycogen in muscles causes ‘tying up,’ also known as exertional rhabdomyolysis. This results in stiffness, muscle tremors, lameness and sweating.

Based on genetics, there are two types of PSSM - PSSM1 and PSSM2. PSSM1 is associated with a genetic mutation in the Glycogen Synthase 1. This mutation causes the muscle cells to constantly produce glycogen. When talking about genetics, this trait is an autosomal dominant gene, which means that only one copy of the trait by one of the parents is required to cause the mutation. PSSM1 is commonly associated with the Western bred horses as described earlier.

PSSM2 isn’t associated with the genetic mutation Glycogen Synthase 1, and it is still unclear what genetic mutation it is associated with. These horses still accumulate abnormal amounts of glycogen in the muscles, the same as PSSM1. PSSM2 has been reported in Warmbloods and Arabians.

How to diagnose PSSM?

Initially, a tentative diagnosis is made on clinical signs as previously described. A health profile blood test then may reveal an elevated Creatine Kinase (CK) which is an enzyme released by muscle when inflamed.

From there, PSSM1 can be diagnosed using genetic testing, performed on hair or blood samples. This will test for the Glycogen Synthase 1 gene mutation in DNA present within the sample. PSSM2 is much more difficult to diagnose, and may require a muscle biopsy to measure glycogen accumulation within the muscle.

How to manage a horse with PSSM?

Unfortunately there is no cure for PSSM. That being said, with the correct management, these horses can live a healthy life and even compete. As mentioned previously, glycogen is how muscles store sugar. Feeding a diet low in sugar (or non-structural carbohydrates NSC) will reduce the amount of glycogen produced in the muscle. This is very important - managing the diet will help these horses a great deal. Now managing diet is easier said than done. There are many variables when it comes to the horse’s diet, however broadly speaking they can be categorised into:

• Pasture
• Hay
• Hard feed

Buying a complete hard feed with the nutritional information is a good way to ensure the horse is being fed a low NSC diet. Buying tested low sugar hay from a reputable hay supplier is also important. Pasture on the other hand, can be quite difficult to manage. NSC levels in pasture can change from season to season, and even within a 24 hour period. In cases of moderate-severe PSSM, it is likely best to exclude pasture and just feed a low NSC hay, as well as a low NSC balanced hard feed. The NSC should not exceed 12% of daily intake.

As you now know, muscles accumulate glycogen as energy storage. One way to reduce the amount of glycogen in the muscles is to exercise the horse daily. If this is after an acute episode of PSSM you may need to start slowly (i.e. at a walk) and then build up the intensity over five to seven days.

If the horse has had an acute ‘tying up’ episode it is best to contact your veterinarian. Sedatives, muscle relaxants and non-steroidal anti-inflammatories can be prescribed to help the horse through the first 24-48 hours of inflammation. This should allow the horse to move, reducing muscle glycogen and reducing the effects of PSSM.

In conclusion, horses with PSSM can be managed successfully with low NSC diet and exercise programs. If you are buying a Western bred horse, it is important to understand the bloodlines, quiz the previous owner of any episodes of ‘tying up’ and even have the horse genetically tested if concerned.

Article by Dr. Luke Wells-Smith

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