Augmented and prematurely occurring H-reflex recovery curve shows enhanced spinal network excitability in ALS patients when employing the double stimulation paradigm.

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disorder in which motoneuron loss in the brain and spinal cord induces complex neuroplastic changes. Although these alterations hold considerable potential for clinical diagnosis and disease monitoring, they remain underutilized due to the lack of sensitive and non-invasive assessment methods. The H-reflex is a monosynaptic spinal reflex arc and represents the neurophysiological analog of the Achilles tendon reflex. A modified H-reflex double-stimulation paradigm enables differentiation of the temporal dynamics of spinal inhibitory and excitatory mechanisms. In ALS, this approach may provide clinically relevant insights into motor system dysfunction. Furthermore, this approach may contribute to a better understanding of the pathophysiology of spinal network plasticity associated with this disease, reflecting the complex interplay between spinal and supraspinal pathways. We assessed H-reflex recovery in 15 ALS patients and 82 non-ALS subjects, including 12 age-matched healthy controls (HC). The protocol included 14 interstimulus intervals (ISI) within a timeframe of up to one second. In contrast to the HCs, ALS patients exhibited recovery at ISIs of 30 or 50 ms. At interstimulus intervals ranging from 50 to 200 ms, the extent of recovery was significantly elevated in the ALS group compared to the age-matched HCs. ALS patients thus demonstrate heightened spinal network excitability as evidenced by an augmented and prematurely occurring H-reflex recovery measurement. These alterations likely reflect changes in neuronal network activity and can be attributed to modifications in both segmental spinal circuits and supraspinal regulatory pathways.