HEALTH + NUTRITION CPD – APRIL 2026

Equine Muscle Disease & Disorders (Myopathies): RER to PSSM – which type is it?

Kate Hore - Head Nutritionist at NAF, BETA ENFAR Nutr, RNutr(Animal), R.Anim.Technol(Cert). 

On emerging topics of equine health, the greater awareness around the myriad of muscle conditions is definitely one attracting interest with customers. NAF’s own research found 83% of UK trade outlets questioned had recently been asked about PSSM alone. If it is that important to your customers, shouldn’t you know more about it? 

Unfortunately, start to do some reading and it can easily get confusing, quickly starting to feel like a game of ‘Acronym Bingo’! Not just Azoturia, there is now a range of different conditions affecting horses’ muscles. Most of them exhibit similar signs of tying-up, but with key differences in their physiology and cause. This article aims to help you know your RER from your MFM and how to advise customers on myopathy management. 

Clinical signs.

Signs vary depending on the exact myopathy, but the following are common to most. Not all horses will display all signs, and the extent and severity will depend on the stage of disease at the time. 

• Muscle pain
  • Short, choppy stride – particularly hindquarters
  • Tension or cramping in large muscle groups

• Muscle cramping / tying-up
  • Sudden stiffness during or after exercise
  • Excessive sweating
  • Horses may appear colicky

• Poor performance
  • Inconsistent gait
  • Poor transitions

• Reluctance to work
  • Refusal to go forward
  • Resistance to saddling or being ridden

• Exercise intolerance
  • Tires easily, particularly noticed in warm-up or light work
  • Can’t maintain intense exercise

• Dark / coffee coloured urine
  • Indicates acute issue; consult a vet immediately

• Abnormal gait or weakness
  • Muscle trembling or twitching
  • Stumbling or dragging hind limbs

Advise customers to work with their vet for a diagnosis, remembering that lines are not necessarily clear-cut, and we see some cross-over between conditions. For example, PSSM-I and MH can occur together, particularly in Quarter Horses; whilst PSSM-II and MFM can overlap in Warmbloods.

PSSM

PSSM (Polysaccharide Storage Myopathy), is muscle disease associated with disruption to normal glycogen within the muscles. To understand it, we firstly need to understand the normal function of glycogen. 

Glycogen is how energy is stored within muscle. Molecules of glucose (a polysaccharide) are linked into long chains by an enzyme (glycogen synthase) with additional branches of glucose being formed off that chain along its length. This glycogen is then stored in the muscles to be broken down back to glucose by an enzyme (amylase) for muscular energy. A disruption to this system causes a glycogen storage disease, and in horses we now recognise different subsets. 

PSSM Type 1

Type 1 is a genetic disease caused by a gene mutation that causes abnormally high levels of glycogen within the muscles. The glycogen is unusual in that it is made up of very long chains with few branches off. This form of glycogen is resistant to the action of amylase, meaning there is a significant slowing in the rate at which it can be broken down or metabolised by muscles. Two different genes have been recognised, and those are GSY1 and GBE1. 

First recognised in Quarter Horses, it is now found widely in European Draft breeds, and some Native UK Breeds, including Exmoor and Connemara ponies.

PSSM Type 2

Type 2 is a similar condition to Type 1. Again, causing abnormally high levels of glycogen in the muscles, but this time without any, yet identified, gene mutation. For this reason, unlike Type 1, there is no genetic test available (or at least, non-proven), and diagnosis is by muscle biopsy. The exact cause of this type is as yet unknown, though a genetic element is highly likely. 

The Type 2 variant is more likely to be found in Warmblood breeds, Arabs and Thoroughbreds. 

MFM

Emerging from PSSM II research is an entirely newly myopathy, MFM (Myofibrillar myopathy). This is distinctly different as although clinical signs are very similar the horses have normal glycogen levels and are less reactive to high starch diets. Diagnosis is by biopsy which shows abnormal clumping of proteins (typically desmin) within muscle fibres.

No gene is yet identified, though MFM is likely to have a genetic element and is typically most often seen in Arabs and Warmbloods. 

RER

RER (Recurrent Exertional Rhabdomyolysis) causes tying-up predominantly in racing breeds, such as Thoroughbreds, Arabs or Standardbreds. RER is associated with abnormal calcium regulation in cells causing pain or cramping during exercise.

Linked to high starch diets, RER has the traditional name of ‘Monday Morning disease’ reflecting the tradition of working horses having a day off on Sunday but still receiving a high grain diet. 

Unlike other myopathies, RER shows a distinct sex predilection, as it is more common in fillies compared to colts or geldings. Temperament is also key, as RER is more common in highly-strung and nervous individuals, unlike PSSM where the temperament is typically calm or even docile. 

Other rarer, but recognised, myopathies

• MH
  • Malignant Hyperthermia is a genetic myopathy caused by a mutation in the RYR1 gene. Signs include a very rapid rise in body temperature and muscle rigidity after exercise. 
• HYPP
  • Rarely seen outside of North America where it is related to a dominant gene traced back to a Quarter Horse stallion called ‘Impressive.’ HYPP affects sodium channels in muscle cells, and affected horses require a low potassium diet. 
• AM
  • Atypical Myopathy is distinctly different, although also causes signs of muscle disease which tend to be very severe. It is caused by ingestion of toxins (hypoglycin A) from sycamore seeds and seedlings. Occurring in autumn or spring, care should be taken to avoid grazing horses in affected pastures. Unfortunately, often fatal, if AM is suspected contact a vet immediately. 

Myopathies and breeds. Not limited to but commonly seen in these breeds.

Management

Management is key to successful outcomes in any myopathy affected horse. Particularly exercise and dietary management. 

Exercise 

For all myopathies it is important that work is consistent with a steady warm-up and cool-down, and any increase in intensity introduced very gradually. Sudden changes, particularly bouts of intense exercise, can trigger an attack. 

Turnout is key, and it is advised that horses spend as little time as possible stabled, allowing them to gently self-exercise whilst out. 

Research has found there are different ideal routines for different myopathies

• PSSM I, II, and RER
  • Very regular exercise. Walking out on rest days. 
• MFM
  • Benefit from rest days. Open to individual variation; three days’ exercise followed by two rest days works well for a lot of MFM horses.

Base Diet

The base diet is very significant for some myopathies, particularly PSSM-I and RER. They require a high-fibre, low-starch ration, increasing energy with additional oil rather than starchy cereals if needed. 

Although a low-starch diet is considered less crucial for PSSM-II and MFM, this diet naturally suits all equines and therefore is good advice for all myopathies regardless of diagnosis. 

Essential Supplementation: Antioxidants

Antioxidants are strongly advised in all myopathies for managing muscle health, particularly Vitamin E. 

Natural Vitamin E

• It is important to ensure that you choose ‘Natural’ Vitamin E, sometimes called ‘RRR’, as this is by far the most effective source. The more commonly used synthetic (all-rac) form is suitable for general health, but not the increased demands of muscle myopathies. 
• High level supplementation is advised with recommendations of around 2-3000 IU (International units) per day for muscle health, potentially at least doubling that for peak requirements. 

SOD (Superoxide dismutase)

• The body’s primary defense antioxidant. SOD, sourced from a specific melon variety, is relatively new to supplements but proving to be a useful addition in the antioxidant response to muscle stress. 

Conclusion

Research is ongoing and it is likely that further myopathies may still be identified. Whatever the exact issue, we should advise customers to work with their vet for a diagnosis, implement appropriate management and exercise regimes, and supplement with a targeted response based on Natural Vitamin E for an optimal outcome.