Variable innervation of the intrinsic foot muscles. An electrophysiological study.

We performed motor nerve conduction studies of the peroneal and tibial nerve with simultaneous 2-channel recordings (surface and needle recordings) in order to evaluate the innervation of the extensor digitorum brevis (EDB) and the abductor hallucis (AH) muscle. In 11 of 30 subjects (36.7%) compound muscle action potentials (CMAP's) of the EDB and AH were only elicited by peroneal and tibial nerve stimulation (PNS and TNS), respectively. In 19 subjects (63.3%) CMAP's could be evoked by stimulation of the "wrong" nerve. Co-innervation was only stated if needle recordings could also elicit a CMAP which was the case in 15/30 individuals (50%). A co-innervated EDB was found in 15 subjects (50%) with an even greater CMAP after TNS than PNS in 3 subjects. Co-innervation of the AH occurred in 23.3% (7/30) of the subjects with always low CMAP's (< 1 mV). Co-innervation of the small foot muscles by the "wrong" nerve is common and reaches a relevant degree regarding additional tibial nerve supply of the EDB. A low CMAP after peroneal nerve stimulation may confuse the electromyographer by erroneously suggesting axonal damage of the nerve.