Perimyositis with perineuritis and myofiber type grouping in the eosinophilia myalgia syndrome associated with tryptophan ingestion.

Skeletal muscle biopsies from five patients with severe myalgias, peripheral eosinophilia, and a recent history of L-tryptophan ingestion were analyzed. Perimysial inflammation, predominantly mononuclear with variable numbers of eosinophils was seen (in five of five patients), which was perineurial (in three of five) and perivascular (in five of five) in location. Grouping of the myofiber types was identified by enzyme histochemistry in two of four patients; fresh muscle for histochemical studies was unavailable from one patient. An occasional degenerating myofiber was seen in only one patient, who was still ingesting L-tryptophan at the time of biopsy. No vasculitis was seen. The focus of muscle injury in this syndrome appeared to be the perimysium and, in particular, the perineurial and perivascular connective tissue.