We read with interest the case report of Sander et al. 9 and would like to comment on some points. The authors believe that the comparison of the compound muscle action potentials (CMAP) over the abductor pollicis brevis (APB), abductor digiti minimi (ADM), and first dorsal interosseous (FDI) muscles following stimulation of the median and ulnar nerves at the wrist and the elbow is not sufficient to screen for Martin-Gruber anastomosis (MGA). They emphasize the importance of a bifid waveform with thenar recording following median elbow stimulation (MES), and maintain that the collision technique can confirm the presence of a MGA.
To observe a bifid waveform on MES, a person must have carpal tunnel syndrome (CTS) with strongly prolonged motor terminal latency (e.g., >7 ms) and a MGA partially innervating the thenar. 3 In a group of 50 healthy persons, of which 23 had a MGA, a partial innervation of the thenar by MGA was found in only 3 (11.1%). 5 We did not detect the presence of a bifid waveform, which we were familiar with from observations in other studies 1,2 and from routine recordings in another unselected group of 50 persons, of which 16 had a MGA. 3 This group included 3 patients with CTS, but without MGA, 1 with cubital tunnel syndrome, 17 with polyneuropathy, and 29 of whom were healthy. These studies show that the majority of MGA cases must be proven by the amplitude criteria recommended in the literature, and only in rare cases by the bifid waveform favored by the authors.
Evidence of the bifid waveform is, however, always proof of MGA with partial innervation of the thenar, provided that stimulus spread to the ulnar nerve on MES has been ruled out. In their report, Sander et al. offer no details on stimulus parameters and precautions to prevent stimulus spread. Stimulus spread to the ulnar nerve during MES can imitate the presence of a MGA, and can occur quite easily. [3][4][5] Neither the collision technique 7,8 nor pharmacological blockade of the ulnar nerve at the wrist 6 can differentiate between stimulus spread and MGA 5 (Fig. ).