Following the direct M response, peripheral nerve stimulation frequently evokes a second wave which shows all characteristics of an ephaptic response. The antidromic impulse or back response, which is always associated, is detected by a double stimulation technique. The ephaptic response seems not to be related to any specific nerve or muscle pathology, as it is also observed in normal subjects. It is therefore important to differentiate the ephaptic response from a late potential or a double discharge, the latter two having pathological significance.
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Roth et al. (2008) studied this question.