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All tibial foot: an electrophysiological artifact
  1. GEORGIOS AMOIRIDIS
  1. Department of Neurology, University of Crete, PO Box 1393, 71 110 Heraklion, Crete, Greece. Telephone 0030 81 394651; emailGAmoiridis{at}compuserve.com

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    Yamashita et al 1 claim they have proved an “all tibial foot” for the motor innervation, an anomalous dual innervation of the tibialis anterior muscle by the deep peroneal and posterior tibial nerve, and a sensory coinnervation of the skin between the first and second toes by the tibial and deep peroneal nerve in a patient. To support their view they quote the letters of Linden and Berlit2 and of Glocker et al,3 ignoring our letter4 and that of Magistris and Truffert5, both considering the conclusions of Linden and Berlit and Glocker et al to be wrong. I point out that the mentioned letter of Linden and Berlit2 and our response to it were published in the same issue.

    We have recorded a compound muscle action potential (CMAP) with a negative initial deflection on tibial nerve stimulation in 83% of 50 subjects, using a surface electrode over the extensor digitorum brevis.4 In the same subjects no potential was recorded by means of a concentric needle electrode inserted in the extensor digitorum brevis.1 In our view, this proves that the CMAP recorded by surface electrode over the extensor digitorum brevis on tibial nerve stimulation is a remote potential originated in the plantar muscles (volume conducted potential). Furthermore, we consider that the CMAP recorded over the tibialis anterior muscle by surface electrode on tibial nerve stimulation in the popliteal fossa, as reported by Yamashita et al,1represents a volume conduction potential originating in the foot and toe flexors. The sensory nerve action potential recorded dorsally in the space between the first and the second toes on tibial nerve stimulation could also be a volume conducted potential originating in the first common plantar digital nerve, as the distance between this nerve and the recording electrode is short. Such volume conduction phenomena are known to occur on surface recordings from the median nerve at the wrist in severe carpal tunnel syndrome, when the forth finger is stimulated. It is unclear why Yamashitaet al could not record a CMAP over the extensor digitorum brevis bilaterally on deep peroneal nerve stimulation in their young patient who did not have neuropathy. A probable explanation is a bilateral aplasia of the extensor digitorum brevis, comparable with the known aplasia of the thenar.6The appropriate examination would have been a needle EMG of the extensor digitorum brevis.

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