Far‐field potential of the compound muscle action potential as a reliable marker in amyotrophic lateral sclerosis

Introduction/Aims Reliable neurophysiological markers in amyotrophic lateral sclerosis (ALS) are of great interest. The compound muscle action potential (CMAP) amplitude has been a conventional marker, although it is greatly influenced by the electrode position. We propose the far‐field potential of the CMAP (FFP‐CMAP) as a new neurophysiological marker in ALS. Methods Patients with ALS and age‐matched healthy controls were enrolled. We used a proximal reference (pref) in addition to the conventional distal reference (dref). Routine CMAP was recorded from the belly‐dref lead and FFP‐CMAP from the dref‐pref lead for the ulnar and tibial nerves. Multiple point stimulation motor unit number estimation (MUNE) was also examined in the ulnar nerve. Inter‐rater reproducibility was evaluated by two examiners, and some patients were followed up every 3 mo for 1 y. Results We tested 17 patients with ALS and 10 controls. The amplitudes of routine CMAP and FFP‐CMAP in the ulnar and tibial nerves, and hypothenar MUNE value in the ulnar nerve were significantly decreased in ALS compared to controls. Ulnar FFP‐CMAP achieved the highest inter‐rater intraclass correlation coefficient (ICC) value (0.942) when compared with routine CMAP (0.880) and MUNE (0.839). The tibial FFP‐CMAP had a higher ICC value (0.986) than the routine CMAP (0.697). In this way, the FFP‐CMAP showed high inter‐rater reproducibility because its shape was not much influenced by the electrode position. During 1‐y follow‐up, decline of CMAP, FFP, and MUNE showed significant correlations with the Amyotrophic Lateral Sclerosis Functional Rating Scale ‐ Revised (ALSFRS‐R). Discussion The FFP‐CMAP shows promise as a reliable marker for ALS. See article on pages 237–239 in this issue