Characterization of sensory nerve conduction abnormalities at the time of ALS diagnosis

Although sensory nerve abnormalities have been reported in amyotrophic lateral sclerosis (ALS), their distribution at diagnosis, relative to motor involvement and clinical onset phenotype remains incompletely characterized. We aimed to systematically compare sensory and motor nerve conduction abnormalities at ALS diagnosis and determine whether sensory involvement follows an onset-dependent pattern similar to motor dysfunction.

In this prospective cross-sectional study, 40 newly diagnosed ALS patients enrolled in the Iran University ALS Registry (March 2022-March 2023) underwent standardized motor and sensory nerve conduction studies (NCS). For between-group comparisons, a matched ALS subgroup was compared with matched healthy controls, while the expanded ALS cohort was used for subgroup and correlation analyses.

SNAP amplitudes of the median, ulnar, and sural nerves were reduced by 45%, 34%, and 43%, respectively, compared with healthy controls, with similar reductions observed across upper- and lower-limb onset phenotypes and no significant onset-dependent differences. In contrast CMAP amplitudes were markedly reduced in ALS patients, most prominently in the peroneal (79%), median (47%), tibial (42%), and ulnar (37%) nerves. Motor abnormalities were most severe in the clinically affected limb but were also detectable in asymptomatic extremities, consistent with early subclinical spread.

At ALS diagnosis, sensory and motor nerve conduction abnormalities exhibited divergent spatial patterns, with asymmetric, onset-related motor involvement and relatively uniform sensory axonal dysfunction. These findings support the presence of measurable sensory nerve conduction abnormalities in a subset of ALS patients, while highlighting the need for cautious interpretation of sensory NCS changes in ALS.