Demyelination and axonal degeneration in corpus callosum assessed by analysis of transcallosally mediated inhibition in multiple sclerosis

Objective: Following focal transcranial magnetic cortex stimulation (fTMS), inhibition of voluntary EMG activity in the ipsilateral first dorsal inter osseus (FDI) muscle was studied, in order to assess the functional integrit y of the corpus callosum in patients with multiple sclerosis (MS). Methods and results: Thirty-four patients suffering from definite MS and 12 healthy, age-matched normal subjects were examined. In mid-sagittal slices , 29 patients showed lesions within the truncus corporis callosi in T2-weig hted MRI. In 20 patients, all areas (anterior, middle and posterior parts), in one both the anterior and posterior part, in 3 exclusively the anterior , in 4 the middle and in one the posterior area were affected. In 5 patient s, lesions of corpus callosum were lacking. In normal subjects, fTMS elicit ed a transient inhibition (TI) of preactivated (50% of maximal force) isome tric voluntary ipsilateral FDI muscle activity. Mean onset latencies of TI were 35.5 +/- 5.4 ms in right and 36.1 +/- 4.2 ms in left FDI. Mean duratio n of TI amounted to 23.0 +/- 8.4 ms for right and 24.6 +/- 8.4 ms for left FDI. In the MS group, TI latencies were significantly increased in 23 and T I durations in 16 cases, whereas a lack of TI was found in 5 patients bilat erally and in 6 unilaterally. In patients, mean onset latencies of TI were 40.4 +/- 13.8 ms in right and 43.3 +/- 14.4 ms in left FDI, TI duration amo unted to 30.5 +/- 17.4 ms for right and 31.0 +/- 25.2 ms for left FDI. Incr ease of onset latencies and durations of TI were positively correlated with the summed area of lesions of corpus callosum in representative mid-sagitt al MRI slices. Significant correlations between TI onset latencies and dura tion on the one hand, and central motor conduction latencies along corticos pinal tracts (CML) on the other hand, were not found. Conclusion: The present investigation indicates that measurement of TI elic ited by fTMS seems to be a sensitive method for an assessment of demyelinat ion and axonal degeneration within corpus callosum in MS patients. (C) 1999 Elsevier Science Ireland Ltd. AU rights reserved.