The clinical significance of type 1 fiber predominance.

In the course of investigating children with hypotonia, muscle biopsy of the vastus lateralis frequently demonstrates greater than 55% predominance of the aerobic type 1 fibers of "type 1 fiber predominance" (T1FP). The clinical significance of T1FP is not well known. T1FP can be associated with a variety of neurological disorders but a significant proportion has no apparent cause. We followed up 23 children with T1FP to establish whether a neurological disorder subsequently became apparent or whether a distinct clinical entity of T1FP could be identified. Sixty percent of the children were found to have a specific neurological disorder, while 40% did not and may represent a distinct clinical entity. The majority of children with the clinical entity of T1FP improved although some were left with mild motor disability. Such information is important for counseling when patients with T1FP are first assessed.