A case of type 1 muscle fibre hypotrophy and internal nuclei.

A 14 year old boy was diagnosed as suffering from type 1 muscle fibre hypotrophy with internal nuclei. On histological examination of a biopsied muscle, there was selective hypotrophy of type 1 muscle fibre with internal nuclei, and focal degenerative changes were seen in a few type 1 fibres. The small type 1 fibres were arranged in small or large groups in one bundle. An EMG study of moderately weak muscles revealed low amplitude and short duration motor unit potentials as well as normal potentials and no spontaneous discharges. The H reflexes were abnormally low in amplitude comapred with the M response. The histological and electrophysiological findings suggested that the type 1 fibre involvement in the present case may have a neurogenic basis. It is likely that the clinical features of the reported cases are too variable for a single clinical entity.