EXTENSOR DIGITORUM BREVIS INNERVATED BY THE TIBIAL NERVE (“ALL TIBIAL FOOT”): ANOMALOUS INNERVATION OR TECHNICAL PITFALL?

Abstract

EXTENSOR DIGITORUM BREVIS INNERVATED BY THE TIBIAL NERVE ("ALL TIBIAL FOOT"): ANOMALOUS INNERVATION OR TECHNICAL PITFALL? Linden and Be rlit4 have reported an anomalou innervation in a man in whom the extensor digitorum brevis (EDB) was apparently innervated exclusively by the tibial nerve. R centl y, C locker et al. 2 described the sa me anomaly, although in their patient, EDB was apparemly only partly innervated by the tibial nerve. rn his reply to this report, Linden uggests that, if xclusive tibial nerve supply of the EDB is probably rare, a coinnervation of EDB by the tibi al nerve may be freque nt. The latter a sumption is based on two channel surface reco rdings performed from EDB and abductor hallucis, in wh ich tibial nerve timu lation evokes a compound muscle ac tion potential, upposedly from EDB, in 17 out of 30 un elected ubjects. Both groups conclude that tl1is anomalous innerva tion must be con idered to avo id erron ous neurographi interpretation . There i a risk of overdiagnosis of thi anomaly, ince electrode placed over the EDB's surface a llow distam recording of the plan tar muscles when tim ulati ng the tibial nerve. T he potential, although of smaller amplitude and surface (on average 52%, SD 18%, in 24 healthy ubjects with 46 feet te ted), has a similar shape to that recorded from the plantar surfa e. Recording from remote motor poi n ts are usually po itive, or at least begin with a positive denection , so that they can be easily recognized. Due to the anatomy of the foot, stimulation of the tibial nerve causes a di tant recording from EDB's surface that i u ually negative. Thi probably occurs becau ea "virtual" motor point of the plan tar muscles li e below the elecu·ode placed over the EDB wh n record ing is performed "across the foot" (Fig. IA). The inver e situati on of a negative distant signal of EDB recorded from the plan tar surface in response to peroneal nerve stimulatio n i also observed, although attenuation of the signal is more pronounced (88%, SD 7%, in the same 46 normal feet) (Fig. 1 B) . The ele trophysiological data provided by Linden and Berlit4 and Clocker et a l. do not demonstrate anomalous inn ervation of EDB by the tibial nerve. The authors' hy-