The role of transcranial magnetic stimulation and motor evoked potentials in the investigation of central motor pathways in multiple sclerosis

摘要

Multiple sclerosis (MS) is characterized by multiple foci of demyeli-nation scattered in the white substance of the central nervous system. The prime consequence of demyelination is a slowing of nerve conduction, which leads to increased temporal dispersion or even blockage of signals in the central nervous pathways. The symptoms of a lesion depend on which pathway is afflicted. Many lesions give rise to subtle symptoms which may pass undetected by the patient, or even give no symptoms at all. Besides a history compatible with MS, a diagnosis depends on the detection of more than one CNS lesion with different locations. During the early seventies the methods of evoked potentials were introduced for the detection of silent lesions. These potentials were evoked by stimulation of afferent pathways of different modalities. Besides the detection of lesions, the methods allowed of suggestions about the nature and severity of the lesion. Ten years later the method of magnetic resonance imaging revolutionized the diagnosis of MS and made in vivo studies of the disease process possible. However, a method for objective, functional assessment of the central motor pathways was still missing. In 1985 Barker et al. introduced a magnetic stimulator by which transcranial depolarization of the motor cortex was possible. In essence the magnetic stimulator is a capacitor, which after loading can be discharged through a flat copper coil. If the coil is held close to nervous tissue the resulting magnetic pulse induces a secondary current in the tissue, which may be sufficient to excite the neurons. In this way cortical motor neurons as well as peripheral motor nerve fibers may be excited giving rise to a contraction of the muscles supplied by these neurons. By means of surface electrodes placed over the contracting muscles a compound muscle action potential, in this context called a motor evoked potential (MEP), is measured. In 1988 we started a series of studies aiming at a standardized technique of MEP recording using transcranial magnetic stimulation (IMS), primarily for the study of MS. The first four studies addressed methodological aspects of magnetic stimulation including the safety of the technique and confidence intevals of MEP parameters in healthy volunteers. In the next study the standardized method was applied to patients suspected of having MS to evaluete the diagnostic value of the method. In the final study consecutive examinations of a group of MS patients were performed in the search for a new outcome parameter for therapeutic trials in MS.