Measurement of Transcallosal Inhibition in Early Relapsing-Remitting Multiple Sclerosis

Early diagnosis of relapsing-remitting multiple sclerosis (RR-MS) is important to start immunmodulatory prophylaxis at an early stage in the course of the disease. According to the McDonald criteria [1], magnetic resonance imaging (MRI) is fundamental in detecting spatial and temporal dissemination. An evidenced-based review of the usefulness of evoked potentials in making the diagnosis of RR-MS did not test motor evoked potentials (MEP) [2]. We investigated whether the analysis of transcallosal inhibition (TI) using transcranial magnetic stimulation (TMS) is useful for the early diagnosis of RR-MS. 12 patients (9 female, 34±5 years) with suspected RR-MS were investigated. Inclusion criteria comprised a clinically isolated syndrome (CIS) not demonstrating spatial or temporal dissemination, MRI findings consistent with the diagnosis of MS, but not yet meeting Barkhof criteria [3], and occurrence of oligoclonal bands in the cerebrospinal fluid. TMS investigation included routine investigation and measurement of TI (latency, duration, persistence) from both first dorsal interosseus regions. Definite RR-MS was confirmed by further clinical relapses and MRI controls. Mean EDSS was 1±0.4. Clinical presentations were opticus neuritis (n=6) and sensory symptoms (n=6). Only 2 patients had motor deficits. A prolongation of central motor latency (CML) could be detected in these and in 2 further patients subclinically. A pathological TI was found in 9 of 12 patients (5 patients without CML prolongation, 11 of 18 hemispheres with TI loss, 1 prolongation of latency [+3 ms], 1 prolongation of duration [+4 ms]). Periventricular white matter lesions were found in 11 patients, a pericallosal localization only in 4 patients; 3 of the latter patients had a pathological TI. 6 patients with pathological TI had no pericallosal lesions. Measurement of TI by TMS may detect central callosal conduction deficits even in the absence of motor symptoms. TMS may be a useful tool for making the diagnosis of RR-MS in patients with CIS at an early stage of the disease. References: [1] McDonald WI. Recommended diagnostic criteria for multiple sclerosis. Ann Neurol 2001; 50: 121–127 [2] Gronseth GS. Practice parameter: the usefulness of evoked potentials in identifying clinically silent lesions in patients with suspected multiple sclerosis. Neurology 2000; 54: 1720–1725 [3] Barkhof F Comparison of MRI criteria at first presentation to predict conversion to clinically definite multiple sclerosis. Brain 1997; 120: 2059–2069