Morphological and histochemical changes of motor units in myasthenia.

Neuromuscular biopsies were obtained from 45 myasthenic patients. Motor innervation was studied in all specimens by vital staining with methylene blue. Quantitative data included the proportion of elongated motor endings, and the terminal innervation ratio (TIR) of motor axons. Quantitative histochemical data, obtained on 12 biopsies, included the atrophy factors of type I and II fibers, the I/II ratio, and the proportion of fibers strongly reacting to both ATPase and NADH diaphorase (type III fibers). Statistical analysis of the data led to the following conclusions: (1) elongated motor endings, found in 26 biopsies, were not related to denervation or to the severity of the disease, and were preferentially observed in younger patients; (2) increased TIR suggesting denervation was observed in 7 biopsies, only in patients over 50 years; and (3) various histochemical changes were found, mainly numeric reduction of type II fibers, having no demonstrable relationship with the incidence of elongated motor endings. The highest TIR was observed in a biopsy containing an increased proportion of type III and intermediate muscle fibers.