Open-biopsy electromyography. Direct correlation of a pattern of excessively recruited, pathologically small motor unit potentials with histologic evidence of neuropathy.

Open-biopsy electromyography (EMG) of two muscles of a 29-year-old man with slowly progressive proximal weakness demonstrated a striking pattern of excessively recruited, pathologically small motor unit potentials. This pattern is usually equated with myopathy. Histologic study of tissue enclosing the recording sites, however, yielded evidence of neurogenic disease alone. In muscle, this included isolated and small groups of atrophic type I, IIA, and IIB fibers, and in intramuscular nerve a loss of myelinated fibers with connective tissue and Schwann cell proliferation. The EMG pattern is considered to reflect a reduced number of activated muscle fibers within motor units due to random neurogenic involvement of terminal axons.