ATYPICAL myopathy, sycamore poisoning or seasonal pasture myopathy (SPM) is a severe and often fatal muscle disorder affecting equines and most frequently seen in the spring or autumn. It is caused by a toxin called Hypoglycin A (HGA) and results from ingestion of sycamore seeds, leaves or seedlings.

There are more than 25 species of Acer (sycamore and maple) tree, although not all species contain the HGA toxin and, for this reason, HGA concentrations can vary between plant samples derived from different trees or seedlings.