Polysaccharide Storage Myopathy Type 2 (PSSM2) & Myofibrillar Myopathy (MFM)

For years scientists have attempted to discover the causes of tying up in horses. One of the leading researchers in this field is Dr S Valberg. Dr Valbergs work has been immensely useful in understanding muscle myopathies in horses and more recently she’s also discovered a new form of exercise intolerance called Myofibrillar Myopathy (MFM).


What is PSSM Type 2?

PSSM is a chronic form of tying up and horses with this condition have abnormally high storage of glycogen in their muscle cell. PSSM and can be broken down in to type 1 and type 2. We know that in horses with PSSM type 1 it is caused a genetic mutation of the Glycogen Synthase 1 (GYS1) gene. However, in horses with PSSM type 2 we are still unsure what causes the abnormal appearance of glycogen in the muscle cells, although this may indeed be a result of defects in one or more genes.

What is MFM?

When developing new stains to evaluate the muscles of horses with PSSM type 2 Dr Valberg made a surprising discovery. Some of the horses diagnosed with PSSM type 2, particularly Arabians and Warmbloods, had abnormal clumping of a protein called desmin in their muscle.


In healthy horse’s desmin is found at very specific areas in the muscle fibres and its role is to create an orderly alignment of contractile proteins called sarcomere within the muscle fibres. In muscle biopsies from horses suffering from MFM the alignment of the protein filaments in the muscle fibres breaks down and desmin accumulates in abnormal clumps around these breaks. Muscle glycogen also accumulates in the gaps between the muscle fibres which explains why previously some horses with MFM have been diagnosed with type 2 PSSM. Now that we are able to stain muscle biopsies specifically for desmin we can distinguish horses with MFM, which is distinct from other forms of tying up including PSSM type 2.

Are PSSM Type 2 & MFM the same?

According to Dr Valberg it’s possible that PSSM Type 2 is an early stage of MFM, although until the exact cause or causes have been identified, it is not possible to say with certainty that the conditions are the same. Certainly, some horses with MFM have abnormal glycogen accumulations in their muscles, which obviously explains why, before the desmin stain was available, they were diagnosed with PSSM Type 2.

Signs of PSSM Type 2 & MFM

The clinical signs usually occur during or after exercise and resemble other types of tying up. However, the severity of clinical signs for horses suffering from MFM can be much milder and are often vague and unspecific (e.g. drop in energy level, poor performance, reluctance to engage hindquarters).

Signs of PSSM & MFM

  • Reluctance to move
  • Abnormal gait
  • Pain
  • Stiffness
  • Muscle tremors
  • Excessive sweating
  • Inability to stand
  • Muscle atrophy
  • Pigmenturia (brown coloured urine).
  • Elevated serum creatine kinase (CK) & aspartate aminotransferase (AST)


Diagnosis of PSSM Type 2 and MFM is possible by taking a muscle biopsy. Your vet will then apply stain to the biopsy sample, which highlight any abnormalities in glycogen storage and/ or clumps of desmin. Genetic tests are also available, but these are controversial as research has shown that genetic variant testing doesn’t always correspond to muscle biopsy results.

Supplements to support horses with PSSM Type 2 & MFM


Due to its role in mitochondrial function and as an antioxidant CoQ10 supplementation has been recommended for managing myopathies in several species. It is thought that alternations in mitochondrial function and the electron transport chain may be involved in several forms of tying up. Indeed, horses with MFM have been shown to have decreased expression of mitochondrial proteins and antioxidants in their muscle. Both vets and owners have reported that supplementation with For-Recovery, which contains a natural source of Ubiquinol (CoQ10), is immensely beneficial for horses prone to muscle myopathies, especially MFM.


Antioxidants such as vitamin E are crucial to the maintenance of muscle health due to the way in which they protect cell membranes from the damaging effects of oxidative stress. Horses diagnosed with a myopathy all seem to benefit from additional vitamin E supplementation. Indeed, some horses require relatively large amounts of vitamin E supplementation to maintain normal serum vitamin E levels. For horses prone to all forms of tying up we recommend Muscle Prep. Muscle Prep combines the well-recognised benefits of the powerful antioxidant vitamin E with key amino acids to promote muscle recovery.

Amino Acids

Good levels of amino acids in the diet of all horse prone to tying up is important for muscle repair. However, recent research has shown that one particular amino acid, cystine, may play a particular role in supporting horses with MFM. Cystine is a key component in many antioxidants and horses with MFM have been shown to have altered cysteine metabolism and a deficiency of cysteine-containing antioxidants.

To provide additional cystine to horses diagnosed with MFM we recommend feeding Muscle Prep. This highly palatable supplement contains hydrolysed protein and includes cystine as well as the branched-chain amino acids (BCAA’s) leucine, isoleucine, and valine, all three of which are essential amino acids that must be obtained from the diet. Research in other species has shown that these BCAA’s may also assist with protein synthesis, help delay the onset of fatigue and reduce loss of muscle mass after an episode.


Electrolytes are important in the maintenance of normal muscle function. If your horse is working regularly and sweating frequently a research-proven electrolyte should be added to their diet. For horses prone to muscle myopathies we recommend Equi-Lyte G. This powdered electrolyte supplement helps replace electrolytes lost through sweating and contains the powerful antioxidants vitamin E and C to help reduce cellular injury and aid muscle recovery.

Careful diet and exercise programmes are key factors to successful management of PSSM and MFM. If your horse is diagnosed with these conditions do not despair! There are many ways in which you can help your horse to lead a relatively normal life. The most appropriate diet for a horse suffering from any form of tying up will depend on a variety of factors such as body condition and workload. We recommend discussing how best to support your individual horse’s needs with our expert nutritional team

Related Products

Muscle Prep

Muscle Prep

Optimise muscle development and topline condition utilising readily absorbed protein

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Equi-Lyte G

Equi-Lyte G

The ultimate package of balanced electrolytes and antioxidants to support muscle function and recovery

View Product

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