Ultrasound benign fasciculations

This is a real-time axial image of the abductor hallucis muscle in a person with benign fasciculations of the feet. Fasciculations are random focal twitching in muscle that contrast with the whole movement of muscle that occurs in normal voluntary contraction (refer to UpToDate topic on diagnostic ultrasound in neuromuscular disease for related video clips of normal muscle contraction).
A combined B-mode and M-mode recording of the same muscle accompanies the axial image. M-mode refers to motion, and in this image, one vertical line of the B-mode image, as indicated by the white line, is recorded in the M-mode image and followed over time. (The large tick marks at the bottom of the screen represent one second.)
Note that initially the bulk of fasciculations occur to the left of the white line, so the probe is moved to center the recording line over this area. A single fasciculation is then captured and measured. The M-mode image captures the contraction (110 ms) phase of the muscle twitch and its total contraction and relaxation phase (230 ms), which are much longer than the action potential duration of a motor unit. Ultrasound measures the mechanical activity of the muscle, rather than the change in membrane potential that is recorded by electromyography (EMG).

Courtesy of Francis O Walker, MD.

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