Myosin ATPase intermediate density fibers for diagnosis of reinnervation

Myosin ATPase (pH 9.4) differentiates two muscle fiber types in healthy human muscle, while diseased muscle often contains intermediate density fibers (IDFs). We evaluated the possibility that, since almost all IDFs in pathologic muscle biopsies are changing type after reinnervation by a motor axon of the opposite type, IDFs are useful in diagnosis. In a retrospective study of 208 muscle biopsies, IDFs were seen as often as were esterase-positive angular atrophic fibers (EPAAFs). In denervation identified by EMG and by histopathology, EPAAFs and IDFs were found much more often than were other indicators. Of biopsies diagnosed without use of IDFs as minimal histologic change or no pathologic diagnosis, 16% had IDFs with sparse EPAAFs and 21% had IDFs without EPAAFs, suggesting mild denervation with rapid reinnervation. IDFs correlate well with EPAAFs, identifying reinnervated versus denervated fibers. Type grouping reveals completed reinnervation change that may be many years old, while IDFs are changing type when biopsied and thus reveal recent reinnervation and preceding denervation. IDFs usefully belong with the histochemical indicators used to evaluate muscle disease.