Congenital myotonic dystrophy: variability in muscle involvement and histopathological process.

In order to understand the involvement of specific muscles in congenital myotonic dystrophy type 1 (DM1), we studied the clinical manifestations, and the genetic effects on various tissues in 2 siblings with congenital DM1. The distal leg muscles were more severely involved than the thigh muscles, as seen in the skeletal muscle magnetic resonance imaging. Molecular genetic analysis of the myotonic dystrophy type 1 protein kinase showed an elongation of the CTG triplet repeats between 850 and 1400 in the leukocytes, skin, fat, tendon, and muscles. Muscle biopsies showed a significant difference in the fiber type distribution between these two congenital DM1 patients. One revealed a prominent involvement of the tibialis anterior muscle with a predominance of type 1 fibers, similar to those muscle fiber distributions in older congenital or classic DM1 patients, suggesting a neurotrophic influence during muscle development. Another revealed a predominance of type 2 fibers in all muscle specimens, and dystrophic changes were observed in the peroneus longus muscle indicating a delayed differentiation or maturation of muscle fibers. We conclude that despite nearly the same number of CTG repeats in the leukocytes, highly individual variability of muscle differentiation may occur at teenagers of congenital DM1 in addition to different pathological findings in various skeletal muscles of patients with congenital DM1.