Correlates of disability in multiple sclerosis detected by transcranial magnetic stimulation.

OBJECTIVE

To study the usefulness of corticospinally mediated excitatory responses and transcallosal inhibition (TI) elicited by transcranial magnetic stimulation (TMS) as a surrogate marker of disability in patients with different courses of MS.

METHODS

Focal TMS of the motor cortex was performed in 118 patients with MS (96 with relapsing-remitting, 19 with primary progressive, and three with secondary progressive disease) who had an Expanded Disability Status Scale (EDSS) score between 0 and 6.5 and in 35 normal subjects. Central motor latencies (CML) and TI (onset latency, duration) were investigated. The Spearman rank correlation was used for statistical analysis.

RESULTS

TMS disclosed prolonged CML in 52.5% and abnormal TI in 61% of the patients. In all patients the EDSS correlated with the frequency of abnormal TI (r = 0.58, p < 0.01) and abnormal CML (r = 0.51, p < 0.01). In patients with primary progressive MS (EDSS 1.5 to 6.5) the frequency of TI abnormalities correlated with EDSS (r = 0.65, p < 0.01) whereas CML did not. Delayed corticospinal responses in hand muscles always led to abnormal TI.

CONCLUSIONS

The combination of central motor latencies and transcallosal inhibition evoked by transcranial magnetic stimulation yields objective data to estimate disease progression in MS as assessed by the EDSS.