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.