Patterns of muscle fiber-type disproportion in hypotonic infants.

We studied the histochemical characteristics of muscle in five hypotonic infants. A number of different patterns of disproportion in the sizes and numbers of type 1 and type 2 fibers were found. In three cases, type 1 fibers were smaller than type 2 fibers and type 2b or 2c fibers were largest. In one case, type 2 fibers were smaller than type 1 fibers and were reduced in number, while in a case of Prader-Willi syndrome there was a preponderance of type 2 fibers that were smaller than type 1 fibers. Type 2c fibers were increased in number in all but one case. We postulate that these various patterns of fiber-type disproportion are the result of altered neural influences leading to impaired maturation of type 1 or type 2 motor units.