Muscle Fiber-Type Disproportion

Abstract

• Two members of a family have slowly progressing proximal muscle weakness starting from ages 2 and 6 years. Muscle biopsy specimens from these patients, as well as two other asymptomatic members of the same family, demonstrate predominance of type I fibers (without fiber-type grouping) and minimally enlarged type II fibers. In all four cases there was an increase in serum creatine phosphokinase levels, but electromyographic studies were normal. To our knowledge, this family represents a previously undescribed syndrome of type I muscle fiber predominance without a history of "floppy" babies and includes asymptomatic members with similar pathologic changes.