Amoiridis et al. support this suggestion. Specifically, the thenar-recorded median elbow stimulation (MES)-evoked response has a clear increase in area, but only a minimal concomitant amplitude increase, compared with the median wrist stimulation (MWS)-evoked response.
Supramaximal stimulus parameters were determined using incremental intensity increases, while observing waveform morphology. In our patient, needle stimulation or recording was not necessary, because there was a low likelihood of MES stimulus spread to the ulnar nerve given that: (1) submaximal intensity MES slightly above threshold evoked both bifid waveform components; (2) the supramaximal stimulus duration was relatively low (0.1 ms) and the patient had a large arm diameter; (3) a very high MES intensity (0.5-ms duration), deliberately applied to cause stimulus spread, evoked a different morphology waveform than supramaximal MES.
MGA electrodiagnosis is not based solely on MESevoked responses. A greater ulnar wrist stimulation (UWS) than ulnar elbow stimulation evoked response, with hypothenar/FDI recording, will confirm a MGA. 1 In most cases, including ours, this confirmation, which is MES independent, obviates the necessity for needle techniques. In some cases, the collision technique may aid in establishing a diagnosis of MGA. Needle techniques may be helpful in occasional instances in which there is an increased risk of MES stimulus spread, for instance: a small-diameter arm; a high stimulus intensity; or when supramaximal, but not submaximal stimulation, evokes the feature suggesting an anomaly.
Wrist stimulation parameters during evaluation of suspected anomalous innervation should also be optimized. Simultaneous thenar and hypothenar recording during stimulus adjustment may aid in selective median-or ulnarnerve activation. Repositioning the MWS electrode slightly laterally or the UWS electrode slightly medially is sometimes helpful.
Incidentally, although case (b) of Amoiridis et al. does indeed seem to have a MGA, the FDI-and ADM-recorded collision technique (UWS, MES) waveforms in their figure are difficult to interpret. The display gains are tenfold or fivefold lower than the gains of the respective MES recordings provided for comparison. In fact, small waveform rises appear to occur at the locations in which the MES-evoked components (which the collision technique is attempting to abolish) would be expected to occur.