Quantitative Spatiotemporal Analysis of Ultrasound Images of Fasciculations in ALS

Fasciculations are a hallmark of amyotrophic lateral sclerosis (ALS), yet quantitative description of individual events on muscle ultrasound (MUS) is limited. We characterized the spatiotemporal kinematics of individual fasciculations to determine whether they differ between ALS and other neurogenic conditions.

We retrospectively analyzed biceps brachii MUS recordings from 680 examinations (January 2020-June 2025), identifying 74 ALS and 40 non-ALS neurogenic recordings with fasciculations (167 and 62 segments). After propensity score matching for age and muscle strength, 62 matched pairs were analyzed. The Lucas-Kanade optical flow algorithm, which estimates frame-to-frame displacement vectors from local intensity gradients, was applied at 1-pixel intervals (57,600 points per 240 × 240 region; ≈60 μm) to quantify twitch durations, peak displacement velocity, and directional anisotropy as a measure of spatial movement coherence.

ALS fasciculations showed prolonged total duration (582.8 ± 112.8 ms vs. 489.2 ± 128.7 ms, p < 0.001), reduced directional anisotropy (0.534 ± 0.245 vs. 0.627 ± 0.215, p = 0.028), and lower peak displacement velocity (6.55 ± 6.56 vs. 9.53 ± 9.07 μm/ms, p = 0.039). MANOVA showed significant multivariate differences (Pillai's trace = 0.317 ± 0.030, p < 0.001) with moderate group separation (Mahalanobis distance = 1.10 ± 0.05).

ALS fasciculations showed spatially heterogeneous and temporally prolonged contraction patterns, suggesting motor units in a transitional state of incomplete reinnervation, distinct from the more stable architecture of chronic neurogenic disorders. This framework may complement existing ultrasound assessment and aid the study of motor unit pathology in ALS.