Different patterns of I-waves summation in ALS patients according to the central conduction time

Objectives: To study facilitatory I-waves interaction, using two near threshold stimuli, to test both excitability and conductivity changes related to cortico-motoneuronal involvement in amyotrophic lateral sclerosis (ALS) patients in different stages of the disease. Methods: Pairs of threshold magnetic stimuli were applied over the motor cortex at inter-stimulus intervals (ISI) ranging from 1–1.5 to 2.5–3 ms and from 4 to 4.5 ms. The electromyogram responses were recorded from relaxed first dorsal interosseus (FDI). Results: The data of I-waves summation were distributed according to the central conduction time (CCT) and all 3 peaks of facilitation were considered for statistical analysis. Patients with normal CCT showed a normal I-waves summation for the first peak, whilst patients with abnormal CCT had a significant reduction in facilitation (P<0.02). Six out of 11 patients with normal CCT had facilitation in the first peak, which exceeded 2 SD of normal values. Conclusions: In conclusion ALS patients showed two different and opposite patterns of I-waves summation which could be related to different stages of the disease.