[Physiopathology of amyotrophic lateral sclerosis: the contribution from transcranial magnetic stimulation]

The introduction of the technique of magnetic stimulation for the study of motor disorders has led to a better understanding of the pathophysiological mechanisms involved in amyotrophic lateral sclerosis (ALS).

Thirty two patients with ALS were studied in order to describe the behaviour of the variables: central conduction time (CCT), amplitude ratio (Ar) and motor threshold (MT) evaluated by means of motor evoked potentials (MEP).

Analysis of these variables showed the great sensitivity of MEP for the detection of abnormalities in the corticospinal tract with 95% of the results being abnormal. Subclinical abnormalities were detected in 22.4% of recordings. Axonal degeneration was the commonest kind of conduction disorder seen. A significant lineal relationship between the evolution time and the variables MT, Ar and CCT (p < 0.05) was demonstrated. This suggests that there is a progressive deterioration of the voluntary motor pathway which begins with an excitotoxic mechanism at the onset of the disease and ends with the death of motor neurones.

The electrophysiological results discussed, together with observations on isolated cases provide evidence in favour of the hypothesis of degeneration of the upper motor neurone. Although it is recognized that this alone does not explain the whole degenerative phenomenon of the motor system which occurs in ALS.