[Congenital desproportion of various types of muscle fiber, with relative small size of type I fibers. Morphological documents on muscle biopsies in 3 members of the same family].

In two sisters with a neo-natal hypotonia, muscle biopsies demonstrated as main pathological feature a disproportion in size between the two types of muscle fibers defined according to their myofibrillar ATPase activity. Type I fiber mean diameter was at the lower limit of the normal values, and type II fibers were larger than normal. Their father's biopsy also showed an abnormal smallness of the type I fibers, with a bimodal distribution. By electron microscopy, the small type I fibers did not reveal any significant abnormality in children's biopsies. In father's biopsy, there was an abnormal degree of filamentary interchange between contiguous myofibrils and a few stacks of rods in the type I fibers. These three cases demonstrate the familiar character of the disorder. The relationship of this new entity with the other congenital myopathies is controversial, as a similar congenital fiber type disproportion, has been found in association with different ultrastructural changes. Several data favour an insufficient development of the type I fibers rather than an atrophying process. The mechanism of this "hypotrophy" remains unknown.