Short-interval intracortical inhibition and facilitation in amyotrophic lateral sclerosis related to disease phenotype.

Objective: To investigate the relationship between short-interval intracortical inhibition (SICI), short-interval intracortical facilitation (SICF) and amyotrophic lateral sclerosis (ALS) phenotype, using threshold-tracking transcranial magnetic stimulation (TMS).

Methods: A new paired-pulse TMS protocol was applied to 49 patients with ALS and 49 age-matched healthy controls. Motor evoked potentials (MEPs) were recorded from first dorsal interosseus muscle, while paired pulses were delivered at interstimulus intervals (ISI) of 1.0, 2.5 or 3.0 ms, with stimuli related to the resting motor threshold for a 200 µV MEP. For each ISI, 6 SICI and 3 SICF pulse pairs with different conditioning stimuli were randomised and interleaved with test-alone stimuli.

Results: ALS phenotypes were characterised as Pyramidal (n = 12, with prominent upper motor neuron signs), Classic (n = 20, with limb onset), or Bulbar (n = 17). Compared with healthy controls, Bulbar patients had significantly less inhibition at all ISIs, while SICI in Pyramidal patients was normal, and in Classic patients intermediate. The only SICF abnormalities independent of the changes in SICI were less facilitation in Pyramidal patients at ISIs 1 and 3 ms.

Conclusion: Changes in SICI and SICF depend on ALS phenotype.

Significance: ALS phenotypes should be matched between treatment and placebo arms of clinical trials.