A serial muscle biopsy study in a case of congenital fiber-type disproportion associated with progressive respiratory failure.

The muscle fiber diameter and distribution were studied in serial muscle biopsy specimens taken at the ages of 21 months and 8 years in a girl with congenital fiber type disproportion. The patient was floppy from birth and showed delayed motor development. Progressive respiratory failure developed from the age of 8, which required artificial respiration during the night. The mean diameters of type 1 fibers at the ages of 21 m and 8 y were 10 mu and 21 mu, and those of type 2 fibers 32 mu and 61 mu, respectively. Whereas most type 1 fibers were hypotrophic in both biopsy specimens, a small number of type 1 fibers (approximately 15%) were normal-sized or hypertrophic, measuring 50 to 90 mu, in the second biopsy specimen, which were thought to have arisen on the maturation of normal-sized or hypertrophic type 1 fibers measuring 15 to 35 mu in the first biopsy specimen. Excessive hypotrophy of type 1 fibers was considered to be responsible for the progressive respiratory failure due to weakness of the diaphragm. The pattern of type 1 fiber evolution from the first to the second biopsy specimen suggested that dysmaturation of spinal motoneurons innervating type 1 muscle fibers would be involved in the pathophysiology of the fiber type disproportion in this case.