Atypical myopathy (AM) also known as seasonal pasture myopathy (SPM), is a frequently fatal disease in horses caused by eating sycamore seeds ("helicopers") or seedlings from the sycamore (Acer pseudoplatanus) and/or box elder (Acer negundo) trees. The seeds contain hypoglycin A, which is metabolized into a toxic substance known as MCPA (methylenecyclopropylacetic acid). MCPA prevents energy from being produced within the horses' muscle cells which result in damage to the muscles in the body. The severity of the disease varies from horse to horse, as it depends on the amount of toxin within the seeds and seedlings, amount eaten, and individual susceptibility of the horse. Affected horses may develop a wide range of clinical signs and may initially include lethargy, loss of appetite and reluctance to work.

AM affects full-time pastured horses and is more frequently reported in the autumn, immediately following inclement weather such as cold, humidity and rain. Horses that develop AM are usually kept in sparse pastures with an accumulation of dead leaves, dead wood and trees in or around the pasture and are often not fed any supplementary hay or feed.