Dual innervation in first dorsal interosseous: Reply to Amoiridis and Ameridou's Letter to the Editor

We would like to thank the editors of Clinical Anatomy for the opportunity to address Drs. Amoiroidis and Ameridou, and welcome the comments of our peers. Amoiroidis and Ameridou have concerns about our interpretation of the potential recorded from the first dorsal interosseous (FDI) muscle with stimulation of the deep fibular nerve. They feel the FDI response is volume conducted from the extensor digitorum brevis (EDB) muscle. To assure that the potential was not a volume conducted response, we did record with both surface and monopolar needle electrodes in the FDI muscle with stimulation of the tibial and deep fibular nerves (DFN). We compared the needle and surface recorded FDI responses with each other and with that from the EDB. Only DFN-FDI responses that displayed a negative initial deflection with surface recording and also had a high degree of correlation with the needle recorded response were considered to be true motor responses. This only occurred in 5 of 30 subjects. The other 25 had clearly volume conducted responses. Amoiroidis and Ameridou have dismissed our interpretation of DFN-FDI responses for two primary reasons. The first is that the distal latency reported from the FDI with deep fibular stimulation (8.9 msec) is longer than the latency with LPN (5.2 msec) stimulation. Amoiroidis and Ameridou claim that the distance to the FDI recording site should be shorter with the DFN than the LPN. We did not standardize the distance from the recording electrode and the distal stimulation sites, however, as the purpose of this study was to determine if a response was present, and not to provide standardized normal values. The distal stimulation site for the tibial FDI recording was posterior to the medial malleolus. The distal stimulation site for the DFN EDB recording was 8 cm proximal to the recording electrode. By protocol, the EDB study was completed first, and the distal stimulation site marked. This same site was again used for DFN recording from the FDI (Galloway et al., 2004), and was therefore a much longer distance than that used for the LPN/FDI distal study. This would account for the longer latency with DFN stimulation. According to a previous anatomical study by Akita et al. (1999), the branch of the DFN supplying the FDI is small in size. The small diameter of this fiber may also account for a slower response time, as velocity decreases with a smaller diameter fiber (Arezzo and Zotova, 2002). The second reason that Amoiroidis and Ameridou give to question our interpretation of the FDI response is that the amplitude reported with DFN-FDI recording was 0.2 mV. This was an average of the five surface recorded responses that we deemed to have a negative initial deflection and to have a high degree of correlation with the needle latency. We did not record or report on amplitudes obtained from the needle recording, as these are often unreliable. Amoiroidis and Ameridou report that needle electrode motor unit potentials can be recorded with amplitudes up to 3 mV, and although this may be true, it has no correlation with the surface recording compound muscle action potential (CMAP) values reported in our study. The small diameter of the nerve found in Akita et al. (1999) anatomical study may also explain the size of the CMAP, as there is a direct correlation between axon size and the amplitude of the CMAP (Kimura, 1989). Normal amplitudes for the DFN with recording from the EDB can be as low as 2 mV (DeLisa et al., 1985) and the DFN branch to the EDB is a much larger diameter fiber, easily seen upon dissection (Hollinshead, 1985). A negative deflection of the CMAP may be observed, however, because of a spatial alignment of the foot intrinsic muscles’ motor point with the recording electrode. To verify that a volumeconducted potential is not present, a concentric needle electrode may be used as this type of needle electrode is less prone to recording volume-conducted potentials. (Dumitru, 2002). We thank Amoiridis and Ameridou for recommending the use of a concentric needle