A Case of Congenital Myopathy with the Pathologic Transformation from Fiber Type Disproportion to Type 1 Fiber Predominance Myopathy

Abstract

Many patients with a severe infantile form of congenital myopathies have respiratory and feeding difficulties from early infancy. We experienced a male patient who required an artificial ventilation soon after birth and showed marked generalized muscle weakness involving the facial muscles. He had skull deformity and mild mental retardation at the age of one year and 10 months.
The first muscle biopsy at the age of 1 month showed small caliber fibers and an increase in the number of undifferentiated type 2C fibers, indicating muscle fiber immaturity. Since type 1 fibers were smaller than type 2 fibers, he was tentatively diagnosed as having congenital fiber type disproportion myopathy. The muscle biopsied findings improved significantly in the second biopsy at 1 year of age which showed type 1 fiber predominance but no cytoarchitectural abnormalities. Accordingly, he was diagnosed as having congenital myopathy with type 1 fiber predominance.
A severe neonatal form of congenital myopathies may show striking muscle fiber immaturity in the early infancy. This may later develop into characteristic morphologic findings such as the formation of nemalines and cores, and abnormal fiber type distribution.