Which Type is Your Horse?
Understanding, managing and feeding the horse with muscle disease
If your vet has started discussing equine muscle disease or tying-up with you, it can quickly feel like a game of acronym bingo. RER, PSSM, MFM, MH… where do you even begin?
There are now several recognised equine myopathies, many of which present with similar clinical signs but differ significantly in their underlying cause, management and nutritional requirements. This guide will help you understand the key types of muscle disease in horses, recognise the warning signs, and work with your vet to manage your horse effectively.
What Is Equine Myopathy?
Myopathy is the term used to describe diseases affecting the muscle. In horses, these conditions commonly result in muscle pain, stiffness, weakness or tying-up, particularly during or after exercise.
Although the outward signs may look similar, the physiology behind each myopathy is different, making correct diagnosis essential.
Clinical Signs of Muscle Disease in Horses
Clinical signs vary depending on the type and severity of the myopathy, and not all horses will show all symptoms. Common signs include:
Muscle pain and stiffness
- Short, choppy stride (often behind)
- Muscle tension or cramping
- Sudden stiffness during or after exercise
- Muscle trembling or twitching
Performance and behaviour changes
- Poor or inconsistent performance
- Difficulty with transitions
- Reluctance to work or go forward
- Resistance to saddling or being ridden
- Exercise intolerance and early fatigue
More severe or acute signs
- Excessive sweating
- Dark or coffee-coloured urine (due to myoglobin release – contact your vet immediately)
- Appearing colicky
- Weakness, stumbling or dragging hind limbs
Does Your Horse Have a Myopathy?
Diagnosis should always be made in consultation with your vet. It is also important to recognise that these conditions are not always clear-cut, and overlap can occur.
For example:
- PSSM Type 1 and Malignant Hyperthermia (MH) may occur together in Quarter Horses
- PSSM Type 2 and Myofibrillar Myopathy (MFM) often overlap in Warmbloods
PSSM - Polysaccharide Storage Myopathy
PSSM is a group of muscle diseases involving abnormal storage and use of glycogen, the primary energy reserve in muscle cells.
In a healthy muscle:
- Glucose is stored as glycogen
- Glycogen is broken down to provide energy during exercise
In PSSM, this process is disrupted.
PSSM Type 1 (PSSM1)
PSSM1 is a genetic condition caused by mutations in the GYS1 or GBE1 genes. These mutations result in excessive amounts of abnormal glycogen being stored in the muscle.
Key features:
- Glycogen is poorly structured and resistant to breakdown
- Energy is not released efficiently during exercise
- Diagnosis via genetic testing or muscle biopsy
Breeds commonly affected:
- Quarter Horses
- European Draft breeds (approx. 36%)
- UK native breeds including Exmoor and Connemara ponies
PSSM Type 2 (PSSM2)
PSSM2 presents similarly to Type 1 but without an identified genetic mutation.
Key features:
- Abnormal glycogen accumulation
- No validated genetic test available
- Diagnosis by muscle biopsy
- Likely to have a genetic component, but research is ongoing
Commonly seen in:
- Warmbloods
- Thoroughbreds
- Arab horses
Myofibrillar Myopathy (MFM)
MFM is a distinct condition, although clinical signs may resemble PSSM.
Key differences:
- Normal glycogen levels
- Less sensitivity to dietary starch
- Muscle biopsy reveals abnormal clumping of structural proteins (commonly desmin)
Although no causative gene has yet been identified, MFM is thought to have a genetic basis and is most often seen in:
- Warmbloods
- Arab horses
Recurrent Exertional Rhabdomyolysis (RER)
RER is most commonly seen in racehorses and performance horses, including Thoroughbreds, Arabs and Standardbreds.
Key features:
- Abnormal regulation of calcium within muscle cells
- Muscle cramping and pain during exercise
- Strong association with high-starch diets
Historically known as “Monday Morning Disease”, RER was often triggered when horses had a rest day but continued to receive high grain rations.
Other risk factors:
- More common in fillies
- Linked to nervous or highly-strung temperaments
Other Recognised Equine Myopathies
Malignant Hyperthermia (MH)
- Genetic mutation of the RYR1 gene
- Rapid rise in body temperature after exercise
- Muscle rigidity
- Often seen in Quarter Horses and Appaloosas
Hyperkalaemic Periodic Paralysis (HYPP)
- Sodium channel disorder
- Associated with the Quarter Horse stallion Impressive
- Requires a low-potassium diet
- Rare outside North America
Atypical Myopathy (AM)
- Caused by ingestion of hypoglycin A from sycamore seeds or seedlings
- Occurs in autumn and spring
- Often severe and potentially fatal
- Immediate veterinary attention required
Managing a Horse With Muscle Disease
Exercise Management
Consistency is crucial for all myopathies:
- Regular exercise with steady warm-up and cool-down
- Avoid sudden increases in intensity
- Maximise turnout to encourage gentle self-exercise
Different conditions may respond differently:
- PSSM and RER: very regular work, including walking on rest days
- MFM: often benefit from planned rest days (e.g. 3 days work, 2 days rest)
Dietary Management
Diet plays a major role, particularly in PSSM1 and RER.
General principles:
- High-fibre, low-starch ration
- Energy supplied via oil rather than cereals
- Supports calm temperament and muscle metabolism
This approach is beneficial for all horses with muscle disease, regardless of diagnosis.
Vitamin E for Muscle Health
Supplementary natural vitamin E (RRR-alpha-tocopherol) is strongly recommended for horses with myopathy.
- Acts as a powerful antioxidant
- Supports muscle repair and function
- Synthetic (all-rac) vitamin E is not adequate for muscle disease
Typical supplementation:
- 2,000–3,000 IU per day minimum
- Higher levels may be required in active or severely affected horses
Always consult your vet for individual guidance.
Conclusion
Equine muscle disease is an evolving area of research, with new variants continuing to emerge. Accurate diagnosis, appropriate exercise management and targeted nutritional support are key to achieving the best long-term outcomes.
By working closely with your vet and tailoring management to your horse’s specific myopathy, many affected horses can enjoy comfortable, productive and athletic lives.
Written by: Kate Hore, Head Nutritionist. RNutr (Animal), R.Anim.Technol (Cert), BSc(Hons). – Find out more about Kate and out Technical Team HERE.
