The electrophysiological muscle scan
This study aims to assess the potential of the electrophysiological muscle scan or stimulus-response curve as a diagnostic instrument. If stimulus intensity is gradually increased from subthreshold to supramaximal values, all motor units in a muscle are successively activated. Thus, by plotting response size versus stimulus intensity, an impression (scan) of the entire muscle can be obtained. We recorded 54 detailed scans from 34 patients and 11 healthy subjects, and analyzed them visually and quantitatively. The scan summarized much diagnostic information in a single picture. Specific patterns in or properties of the scan (steps, maximum, variability, decrements, stimulus intensities used) provide clinically relevant information regarding motor unit number, size, and stability, and neuromuscular transmission and axonal excitability. The scan can be recorded noninvasively in about 5 minutes and is fairly easy to interpret. Because it is built up from contributions of all functioning motor units, the scan shows if and how many large motor units are present. There is no sample bias. For these reasons, further exploration and exploitation of this tool in the clinical setting are warranted.