Further evidence for corticomotor hyperexcitability in amyotrophic lateral sclerosis

A collision experiment has been used to investigate repetitive firing of first dorsal interosseous motoneurons following a single transcranial magnetic stimulus (TMS) in healthy subjects and patients with amyotrophic lateral sclerosis (ALS). An appropriately timed supramaximal peripheral nerve shock blocks the first descending impulses in the motor axons and allows the response due to repetitive firing to be quantified. Multiple firing of motoneurons in healthy subjects increases as TMS intensity rises and saturates at about 1.25 times threshold. Increasing background force also augments repetitive firing and saturates at force levels above 50% maximum. The ratio of the area of the response attributed to repetitive motoneuron firing to the area of the initial direct response to TMS was compared in 10 ALS patients and 10 healthy controls. In ALS patients, the ratio was significantly higher (P = 0. 0005), indicating a greater degree of repetitive firing of motoneurons. This suggests that, in ALS, there is corticomotor hyperexcitability either at the spinal motoneuron or motor cortex.