The defining feature of congenital fiber type disproportion (CFTD) is type 1 muscle fibers which are at least 12% smaller than type 2 muscle fibers.

CNS abnormalities and developmental delay may be present.

Severity of clinical weakness correlates with smallness of type 1 fibers.

Most cases are sporadic.

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.