Atypical Myopathy is a frequently fatal condition caused by severe muscle damage. The condition, thought to be caused by ingestion of sycamore seeds seems to be associated with specific weather conditions particularly found in autumn.
Horses suffering from atypical myopathy have been found to have high levels of the toxin Hypoglycin A, found in the seeds of sycamore trees. The toxin prevents the normal use of fats as energy by muscle cells, causing build up within the cells and destruction of the normal muscle function resutling in complete dependence on carbohydrates as an energy source.
Unfortunately, once clinical signs are evident the disease is usually in the advanced stages and there is no curative treatment at present. Survival rates are between 20-30% with intensive care which usually requires transport to a hospital facility. However, in some cases transport is not recommended or possible due to the degree of muscle damage and you should be guided by your vet's advice in this instance.
Please see the attached fact sheet for more information on symptoms, treatment and preventative measures.