Atypical myopathy

These information sheets are provided for your interest. They should not replace veterinary advice from your veterinary surgeon.

Whilst every effort is taken to ensure the accuracy and completeness of the information provided, your specific circumstances must be discussed before advice can be given.

Fri, 20/05/2022 - 11:33 -- Ashley Stewart MRCVS

Atypical Myopathy (AM) is a potentially fatal disease seen in horses kept at pasture. Cases are mainly seen in the autumn, but may also occur in the spring. AM may affect either individual horses or groups, and can affect equines of all types and ages. First recognised in 1984, this disease is very similar to Seasonal Pasture Myopathy (SPM) seen in America. In recent years we have been seeing increasing numbers of confirmed cases of AM in the UK.


There has been considerable research into the potential causes of atypical myopathy, with the most likely culprit being a toxin caused Hypoglycin-A. This has been found in the seeds of the European sycamore (Acer pseudoplanatus). These seeds have a typical helicopter shape and are easy to identify. Hypoglycin-A prevents energy from being produced in the muscle tissues, leading to muscle damage resulting in significant muscular pain. The amount of toxin that horses need to ingest appears to be variable amongst horses, with some being more susceptible than others.

Clinical Signs

Atypical myopathy has a rapid onset and affected horses are often found at pasture unwilling to move, despite having been completely normal when last checked. Symptoms include:

  • Muscular stiffness and tremoring
  • Reluctance to walk, weakness and difficulty standing
  • Sweating, increased heart rate and breathing difficulties
  • Depression  to the point where the horse may appear sedated
  • Dark reddy brown urine

Often horses are found lying down at pasture, and sadly some may be found dead such is the nature of the disease. Once clinical signs are present the prognosis is guarded to poor depending on their severity and duration.


Atypical myopathy can be treated however treatment needs to be prompt. Treatment mainly relies on managing the symptoms of disease by providing extensive nursing care. It is very important that the horse receives fluid therapy in order to stay hydrated and protect the kidneys. This can be accomplished in the field by giving intravenous fluids via a jugular catheter, however ideally these cases require constant nursing and fluid therapy best achieved at a hospital. Other treatments include injectable vitamins and pain relief.


As treatment of atypical myopathy is so challenging, prevention is absolutely key. If you are aware of any European Sycamores which are close enough for seeds and leaves to drop on your grazing area then you should consider the following:

  • Checking fields for sycamore leaves and seeds
  • Removal of leaves and seeds if found
  • Fence off areas where leaves and seeds fall
  • Provide extra forage and reducing stocking density to ensure good quality grazing
  • Reduce turn-out time

If you suspect atypical myopathy in any of your horses, then all horses should be immediately removed from the pasture. We will often analyse muscle enzymes for damage and provide supportive care in these cases. Unfortunately outbreaks are common.