Segmental motoneuronal dysfunction is a feature of amyotrophic lateral sclerosis.

Objectives: There is accumulating evidence of dysfunction of spinal circuits in the pathogenesis of amyotrophic lateral sclerosis (ALS).

Methods: The present study was undertaken to characterise the pathophysiological changes in segmental motoneuronal excitability in 28 ALS patients, using recruitment curves of the soleus H-reflex and M-wave, compared with clinical assessments of upper motor neuron (UMN) and lower motor neuron dysfunction.

Results: H-reflex recruitment curves established that Hmax/Mmax and slope (Hθ/Mθ) ratios predicted clinical UMN dysfunction (p<0.001). Changes in Hθ/Mθ were driven by reduced Mθ. Assessment of Hmax/Mmax was similar in the ALS and control groups, and was affected by overlap of the H and M recruitment curves in ALS patients.

Conclusion: Changes in the slope ratio (Hθ/Mθ) in ALS suggested that alterations in peripheral motor nerve excitability following UMN damage may affect the recorded H-reflex. Increased collision of reflex discharges with antidromically-conducted motor impulses may be exacerbated in ALS due to preferential loss of large-caliber α-motoneurones, which may explain the similarities in Hmax/Mmax between groups.

Significance: Findings from the present study provide further insight into the pathophysiology of ALS, specifically the relative contributions of premotoneuronal and segmental motoneuronal dysfunction.