Far‐field potentials in the compound muscle action potential

Abstract

It has long been believed that the compound muscle action potential (CMAP) in motor‐nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This “reference electrode potential” is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far‐field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle‐tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.

Proximal fibular nerve conduction studies to tibialis anterior: Optimal E2 (reference electrode) placement

“…Montages using distal AE2 sites (including TA‐Tibia) are expected to underestimate the TA amplitude by approximately one‐third, although we have no direct means of measuring the TA amplitude in isolation. This AE2 activity is virtually identical in amplitude, latency and morphology whether recorded from the tibia, ankle, or great toe, indicating this is an FFP 11 . It appears to be linearly related to the TA amplitude suggesting this arises from the TA or other antero‐lateral compartment muscles.…”

“…Several papers have drawn attention to the importance of E2 placement to achieve reliable CMAPs in motor NCS 3‐7 . Some E2 sites may be contaminated by near field or far field potentials (FFP) arising from adjacent muscles, or even the target muscle that detracts from the utility of those studies 5,8‐11 …”

Technische Grundlagen der Neurographie/Evozierten Potentiale Teil I Differenzverstärker und Elektroden

“…Diese Bezeichnung ist durchaus unglücklich, da die Referenz, wie wir in Abb. 1 gesehen haben, keineswegs inaktiv ist, sondern entscheidenden Anteil an der Messung des Potentials, und der Qualität, hat 4 5 . Im englisch sprachigen Raum ist oft nicht von Aktiv und Referenz/Inaktiv die Rede, sondern einfach von Elektrode 1 (E1) und Elektrode 2 (E2) 1 6 .…”

Clinical utility of far field motor potentials in amyotrophic lateral sclerosis

“…18 Far field potentials (FFP) are "stationary" potentials that are recorded by a reference electrode, commonly used in the conventional muscle belly-tendon montage, and previously thought to be inactive. 19 It has been established that FFP substantially contribute to the CMAP amplitude. 19 An advantage of FFP is that they are not influenced by positioning of recording electrodes, contrasting with the conventional CMAP amplitudes, and consequently could be of clinical utility in ALS research.…”

“…19 It has been established that FFP substantially contribute to the CMAP amplitude. 19 An advantage of FFP is that they are not influenced by positioning of recording electrodes, contrasting with the conventional CMAP amplitudes, and consequently could be of clinical utility in ALS research.…”