What is the role of EMG in the workup of spinal muscular atrophy (SMA)?

Updated: May 29, 2019
  • Author: Jeffrey Rosenfeld, MD, PhD, FAAN; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP  more...
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Compound motor action potentials (CMAPs) are low normal or reduced, depending on the severity of disease. In chronically weak muscles, CMAPs may be in the near-normal because of reinnervation and collateral sprouting. Motor velocities are normal. Modest slowing of motor conduction, when present, may accompany severe motor axon loss because of the loss of the fastest-conducting motor fibers.

In affected muscles, needle-electrode examination reveals widespread broad and polyphasic motor unit potentials (MUPs) firing in a reduced or rapid neurogenic recruitment pattern. Superimposed low-amplitude, short-duration, and polyphasic MUPs may be present. These configuration changes may resemble myopathic MUPs, but in the case of spinal muscular atrophy are instead due to early MUP reinnervation. Fibrillation potentials may be seen in limb and paraspinal muscles and are most striking in early or progressive spinal muscular atrophy. In late-juvenile and adult-onset forms, active motor axon loss is sparse. Fasciculation potentials are uncommon, but spontaneously firing motor unit action potentials (MUAPs) at 5-15 Hz have been described as a unique feature of SMA I and II.

Mild pseudomyotonic discharges have been observed in patients older than 6 years. However, these discharges are not specific for etiology and may be seen in chronic neurogenic disorders.

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