[A new familial muscular disorder demonstrated by the intra-sarcoplasmic accumulation of a granulo-filamentous material which is dense on electron microscopy (author's transl)].

: This report concerns a family in which several members presented an involvement of skeletal and velo-pharyngeal muscles, associated with hypertrophic cardiomyopathy, respiratory disturbances and lens opacities. The mode of transmission is autosomal dominant. The E.M.G. showed neither spontaneous activity nor myotonic discharges. In two patients, the muscle biopsies showed identical changes. By light microscopy it was seen that in numerous type I fibres, the intermyofibrillar network was "rubbed out" and this occurred along with splitting of such fibres. Electron microscopy revealed an intrasarcoplasmic accumulation of an electron-dense granulo-filamentous material: in some areas it formed a mesh of threads around the myofibrils and in others it was disposed regularly in small stacks facing the Z lines. Continuity or structural similarity of this material and the Z lines was not observed; its relationship with the dense strips of leptofibrils is suggested.