Enhanced spinal motoneuron excitability as a marker of motor neuron dysfunction in primary lateral sclerosis.

OBJECTIVE

To investigate F-wave parameters as markers of spinal motoneuron excitability in patients with primary lateral sclerosis (PLS).

METHODS

We prospectively evaluated F-waves and M-waves from the abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles. The study included three groups: 27 patients with PLS, 36 with amyotrophic lateral sclerosis with normal right-hand function (ALS-NH), and 30 healthy controls (HC). Recorded parameters included M-wave amplitude, F-wave frequency, latencies, amplitude, and the F/M-waves amplitude ratio. Upper motor neuron (UMN) clinical scores were also assessed. Statistical analysis was performed using One-way ANOVA or Kruskal-Wallis tests for group comparisons, and Spearman's rank test for correlations.

RESULTS

PLS had a significantly higher UMN score than ALS-NH (p < 0.001). F-wave amplitudes were significantly higher in PLS compared to both ALS-NM and HC across all muscles (APB, p = 0.01 and p = 0.03; FDI, p = 0.002 and p < 0.001; ADM, p = 0.03 and p < 0.001, respectively). F/M-waves amplitude ratio was also significantly higher in PLS vs. ALS-NH/HC (APB, p = 0.01 and p = 0.03; FDI, p < 0.001 and p < 0.001; ADM, p = 0.03 and p = 0.001, respectively). No significant differences were found between ALS-NM and HC for these parameters (p > 0.05). M-wave amplitudes and other F-wave parameters remained comparable across all groups (p > 0.05). No correlation was found between F-wave amplitude and UMN scores in PLS patients (p > 0.05).

CONCLUSIONS

Patients with PLS exhibit significantly increased F-wave amplitudes and F/M amplitudes ratios compared to ALS-NH and HC.

SIGNIFICANCE: Increased F-waves size serves as a neurophysiological marker of enhanced spinal motoneuron excitability in PLS.