Brief, small, abundant motor-unit action potentials. A further critique of electromyographic interpretation

Abstract

The concept that there is a constellation of electromyographic (EMG) findings diagnostic of myopathy is criticized. The EMG pattern generally referred to as "myopathic" is one of brief-duration, small-amplitude overly-abundant (for the amount of power being exerted) motor-unit action potentials on slight voluntary effort. For it, the author prefers the descriptive and diagnostically noncommittal acronym "BSAP". In addition to myopathy, three hypothetical neurogenic mechanisms could equally well cause a BSAP EMG pattern: (1) Acquired functional or structural loss of a fraction of the axonal twigs in many individual motor-unit trees, (2) congenital motor-unit hypoplasia (too few muscle fibers per motor unit), and (3) acquired (atrophic) or congenital (hypotrophic) smallness of many muscle fibers. The term and concept of a diagnostic "myopathic EMG" thus is considered invalid.