Optimal stimulation settings for CMAP scan registrations

Background: The CMAP (Compound Muscle Action Potential) scan is a non-invasive electrodiagnostic tool, which provides a quick and visual assessment of motor unit potentials as electrophysiological components that together constitute the CMAP. The CMAP scan records the electrical activity of the muscle (CMAP) in response to transcutaneous stimulation of the motor nerve with gradual changes in stimulus intensity. Large MUs, including those that result from collateral reinnervation, appear in the CMAP scan as so-called steps, i.e., clearly visible jumps in CMAP amplitude. The CMAP scan also provides information on nerve excitability. This study aims to evaluate the influence of the stimulation protocol used on the CMAP scan and its quantification. Methods: The stimulus frequency (1, 2 and 3 Hz), duration (0.05, 0.1 and 0.3 ms), or number (300, 500 and 1000 stimuli) in CMAP scans of 23 subjects was systematically varied while the other two parameters were kept constant. Pain was measured by means of a visual analogue scale (VAS). Non-parametric paired tests were used to assess significant differences in excitability and step variables and VAS scores between the different stimulus parameter settings. Results: We found no effect of stimulus frequency on CMAP scan variables or VAS scores. Stimulus duration affected excitability variables significantly, with higher stimulus intensity values for shorter stimulus durations. Step variables showed a clear trend towards increasing values with decreasing stimulus number. Conclusions: A protocol delivering 500 stimuli at a frequency of 2 Hz with a 0.1 ms pulse duration optimized CMAP scan quantification with a minimum of subject discomfort, artefact and duration of the recording. CMAP scan variables were influenced by stimulus duration and number; hence, these need to be standardized in future studies

Waveforms and Artifacts

Artifacts can alter all of the variables used to describe the continuous and discrete waveforms recorded in clinical neurophysiology. Changes in amplitude, frequency, and distribution of waveforms occur in continuous waveforms. Frequency change may include the addition of new, abnormal frequencies, the loss of normal frequencies, and either an increase or a decrease in amplitude. Discrete events themselves may be abnormal. The configuration, distribution, size, and pattern of normally occurring discrete events may be changed by disease.</p

Quantitative Motor Unit Number Estimates

The all-or-none increment, the STA, DQEMG, MPS, and the STAT MUNE methods of obtaining MUNE give similar values in each of the muscles compared in normal subjects. Alternative MUNE methods continue to be developed, but none have been used sufficiently in a clinical setting to assess them. 64 In addition, Felice has shown that MUNE is more reliable than other measurements in documenting the course of ALS. 38 Thus each of these methods can be used whenever and wherever it is most feasible and that different methods may be appropriate in different settings. For example, when each of the motor units can be identified with small increments of stimuli in a severe neurogenic process, MUNE is defined most rapidly and accurately by actually counting the total number of increments. When the number of motor units is too large to do this or their size is too small for them to be identified accurately in the CMAP, the STAT MUNE or multipoint methods are appropriate. In muscles in which CMAPs cannot be obtained reliably, as in proximal muscles that are difficult to immobilize during stimulation of the motor nerve, the STA and DQEMG methods are most appropriate. The value of other methods of MUNE remains to be determined. 65–69 </p