Equine Polysaccharide Storage Myopathy (EPSM) is a common genetic muscle disorder primarily found in draft horses, and quarter horses, although it can also affect other breeds. It is characterised by the abnormal storage of glycogen (a form of sugar) in the muscle tissues, which leads to muscle damage and dysfunction. This can cause episodes of muscle stiffness and pain after exercise, also known as “tying up” or exertional rhabdomyolysis. Affected horses may be reluctant to move and may display sweating, lameness, and muscle tremors.
Equine Polysaccharide Storage Myopathy (EPSM) and Polysaccharide Storage Myopathy (PSSM) are both genetic muscle disorders in horses, but they have some differences. EPSM is a type of PSSM that was initially described in draft horses. PSSM is a broader term that includes both EPSM and other forms of glycogen storage disorders in horses.
There are two types of PSSM:
Type 1 PSSM: Caused by a genetic mutation in the GYS1 gene, leading to excessive glycogen production and storage in the muscles.
Type 2 PSSM: The exact cause is unknown, but it involves abnormal glycogen storage in the muscles without the GYS1 gene mutation.
It’s much more common in quarter horses and drafts than other breeds. You can read more about prevalence here:
There is no medical treatment for EPSM/PPSSM. The treatment is nutrition and movement, and is the same irrespective of whether your horse is diagnosed with EPSM or PSSM 1 or 2.
Recognising EPSM Symptoms
Chronic exertional rhabdomyolysis (tying-up)
Poor performance and lack of energy
Muscle wasting and weakness
Gait abnormalities
May show signs of back pain or whole body pain
Behavioural issues when saddled,
Reluctance to stand for the farrier
Blood samples can be run to confirm that your horse is experiencing a clinical case.
Treatment Strategies for EPSM
You’re going to be shocked. It’s 1 - 2% of the horse’s bodyweight in low-sugar hay. (If you’re new to our site, Sound Advice recommends 1 - 2%BW in low-sugar hay + minerals for pretty much all conditions).
Limit pasture to the mornings, when sugar is lower in the grass.
If you can only access higher-sugar hays, soak it.
Use a slow feeder net.
Horses with PSSM can have up to 20% of their caloric needs met by fat. Fat does not trigger insulin release and will not be stored as glycogen in muscle. Therefore, if your horse requires additional energy beyond what they are getting from their forage, fat is the preferred choice.
You’re looking at up to 500g of fat - depending on your horse’s condition. You can use flaxseed oil, whole flaxseeds/linseeds. Rice bran is about 20% fat. Copra meal is 10-15% fat. You’re looking for high omega 3 fats, rather than high omega 6 and 9s, which are inflammatory (i.e. not canola, cotton seed or sunflower seed oils).
If you are worried about laminitis from high-fat feeding, an alternative is the addition of a high calorie, high soluble fiber source such as:
Linseed meal cake
Beet pulp
Alfalfa / lucerne
Wheat bran
Muscle atrophy is a common symptom of EPSM/PSSM that can be partially mitigated by providing adequate levels of amino acids in the diet. (More on specific amino acids below).
For PSSM horses, ideally amino acids are supplied by adding high protein feeds (rather than pellets). This helps minimise starch and sugars which are normally high in pelleted feeds. Sources include:
Soybean meal
Hempseed meal
Flaxseed meal
Whey protein
You will need to supply minerals in your horse’s diet, which are not in adequate amounts in forage.
In addition to vitamin E (which is abundant in pasture, so not necessary if your horse has green pick), some of the minerals that are important for supporting metabolic health and muscle function include:
Zinc
Copper
Magnesium
selenium
These are all supplied at optimal levels in our Optimum Chia Trace Mix - chia is also high in omega 3 fatty acids.
Tying-up is usually addressed with magnesium supplementation and salt. Magnesium is super. Our magnesium supplement is Calm Mix. It's been a best seller for us for around 20 years. We also have loose Himalayan salt, actually from the Himalayas which is less processed than other forms of salt.
Carnitine
The studies into the role of carnitine in glycogen storage are in humans and rats - not horses.
Studies indicate that the addition of acetyl-l-carnitine redirects glucose away from glycogen and into energy pathways. It also supports the production of mitochondria for the more efficient aerobic metabolism.
Acetyl L Carnitine is not something we sell, but is readily available and supplement stores (the gym bro stores). You’re looking at about 10g per day. Anecdotally, owners of horses with EPSM have had success with it, and with no negative side effects, it's definitely worth a trial to see if it improves your horse.
Turn Out and Exercise
PSSM horses should not be confined (stabled).
Movement is important to the health of all horses, but especially for horses with metabolic concerns. Pasture turnout is highly beneficial for promoting movement and encouraging utilisation of glucose in energy pathways rather than for glycogen storage.
Reduce stress by ensuring they are with compatible companions during turnout.
Avoid extended stall-rest. They do not benefit from standing around. After a tying-up incident, reintroduce exercise slowly - uncollected work, maybe hand-walking or lunge-line work for 5 minutes, gradually increasing.
As you increase work, introduce rest periods of 2-5 minutes in your workout, with an opportunity to stretch.
Dietary management and appropriate exercise can have a significant positive impact on the overall health and comfort of EPSM/PSSM horses, ideally reducing your need for expensive vet visits.
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