Utility of Far‐Field Potentials as a Biomarker of Neurodegeneration in Spinal Muscular Atrophy

INTRODUCTION/AIMS: Far field potentials (FFP) have been proposed as a reliable neurophysiological prognostic biomarker in amyotrophic lateral sclerosis (ALS). This study evaluated the utility of ulnar nerve FFP as a robust research biomarker of lower motor neuron degeneration in spinal muscular atrophy (SMA). METHODS: Peripheral neurophysiological assessments were performed in 13 participants with SMA, 19 with amyotrophic lateral sclerosis (ALS), and 19 healthy controls. The ulnar nerve was stimulated at the wrist, and motor responses were recorded over the abductor digiti minimi (ADM) muscle. Recorded measures included compound muscle action potential (CMAP), FFP and near-field potential (NFP) amplitudes, and motor unit number index (MUNIX). RESULTS: The FFP amplitude was significantly lower in SMA participants compared to healthy volunteers (p < 0.001), but comparable to ALS (p = 0.11). The FFP amplitude showed strong correlations with the Revised Upper Limb Module (RULM) (ρ = 0.92), ALS Functional Rating Score-Revised (ρ = 0.85), upper limb MRC score (ρ = 0.89), CMAP amplitude (ρ = 0.97), NFP amplitude (ρ = 0.88), and MUNIX values (ρ = 0.84), all of which were highly statistically significant. Multiple linear regression indicated that FFP amplitude was an independent predictor of RULM (p < 0.001). DISCUSSION: FFP amplitude appears to be a promising neurophysiological biomarker for SMA, with potential utility for monitoring disease progression, particularly in a clinical trial setting.