Recent functional and imaging studies have substantially contributed to extend the concept of multiple sclerosis (MS), classically regarded as a disease limited to the myelin axonal sheath. Several findings, in fact, point to a parallel involvement of neuronal components of the central nervous system (CNS) in the course of MS. In the present study, therefore, we explored, in MS patients, some characteristics of central motor pathways related to changes of neuronal excitability as measured using transcranial magnetic stimulation (TMS).

Seventy-nine patients affected by relapsing-remitting (RR) MS were examined using single and paired TMS in order to assess excitability changes in the hand motor cortex occurring during relapse and/or remission of the disease. The analyzed parameters were: motor-evoked potential (MEP) threshold, silent period (SP), intracortical inhibition (ICI) with paired pulses from 1 to 6 ms interstimulus intervals (ISIs), and central motor conduction time (CMCT).

The analysis of variance exhibited a strong correlation (P<0.001) between the clinical phase and the type of excitability changes: ‘relapsing’ patients showed increased threshold and reduced SP duration. ‘Relapsing’ patients also displayed a significant lack of normal intracortical inhibition (ICI). By contrast, ‘remitting’ patients showed a significant SP prolongation with normal motor thresholds.

The present findings reveal changes in cortical excitability that might play a role in the pathophysiology of MS symptoms. In particular, the relapsing phase of MS has been found to be associated with cortical hyperexcitability irrespective of the site of clinical manifestation or new plaque formation. These results might help to explain the puzzling picture of neurological symptoms observed in MS patients during different phases of the disease.

Alterations of neuronal components of the CNS play a role in MS.