Picornavirus-like inclusions in polymyositis--aggregation of glycogen particles of the same size.

We studied picornavirus-like inclusions in muscle of a 66-year-old man with chronic polymyositis. Inoculation of a muscle homogenate to suckling mice and VeRo cells failed to detect any virus. After treatment of ultrathin sections with 2% amylase, individual particles that constituted inclusions were digested, as were free glycogen particles in the same section. Particles of the inclusions were stained with periodic acid-thiocarbohydrazide-silver proteinate and alkaline-bismuth, as were glycogen particles. The picornavirus-like inclusions seemed to consist of glycogen particles than form a crystalline arrangement in degenerated muscle fibers.