Reverse fiber type disproportion: A distinct metabolic myopathy

Introduction

In this investigation we characterized the physiological and metabolic responses to incremental exercise in 13 subjects with a predominance of type II fibers on muscle biopsy.

Methods

Subjects underwent incremental exercise testing with measures of maximum oxygen uptake ( O₂ max), maximum heart rate (f_c max), chronotropic index (f_c/ O₂ slope), maximum ventilation ( emax), blood lactate, ammonia, and creatine kinase (CK) levels. Muscle fiber type was determined by myosin ATPase histochemistry.

Results

Muscle biopsies showed more type II fibers (75%) in subjects compared with normal individuals (P < 0.01). Subjects exhibited normal O₂max and end-exercise lactate, whereas ammonia and CK levels at maximum exercise were significantly higher.

Conclusions

Subjects with type II muscle fiber predominance exhibited exaggerated increases in ammonia and elevated CK levels during exercise. Predominance of type II fibers on muscle biopsy is the opposite finding of congenital fiber type disproportion; we suggest these patients be referred to as having “reverse fiber type disproportion.” 

Muscle Nerve, 2016