Origin and significance of small muscle fibres in neuromuscular disease.

Small muscle fibres, defined as those of less than 40 microns diameter in the male and 30 microns in the female were encountered in muscle biopsies of patients with spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), polymyositis (PM) and myopathy/dystrophy. Excessive reactivity with NADH-TR in small fibres did not discriminate between neurogenic and myopathic disorders. Quantification of perifascicular atrophic fibres, the number of nuclei in atrophic fibres, or the presence of isolated or grouped small fibres without histochemical kinship to their surrounding fibres did not aid recognition of the disease process in the groups studied. Small fibres which reacted strongly both with NADH-TR and ATPase at pH 9.4 (Type 3 fibres) constituted 38% of small fibres in the biopsies of SMA; 25% in ALS; but only 1% and 2.7% in PM and myopathy/dystrophy respectively. Thus, the presence of small Type 3 fibres in muscle biopsies may be a useful marker for neurogenic disorders in adults.