Chapter 62 Transcranial magnetic stimulation in epilepsy and Parkinson's disease: drug induced changes in motor excitability

Abstract

Transcranial magnetic stimulation (TMS) of the human cerebral cortex study neurologic disorders likely to affect the excitability of the central motor pathways. In epilepsy, single-pulse (s) and repetitive (r) TMS have been employed to assess their potential for the induction of seizures, localization of the epileptic focus, identification of the speech dominant hemisphere, and antiepileptic properties of drugs. In Parkinson's disease (PD), TMS was used to detect the changes of cortical excitability; a defect of the intra cortical inhibition was found across the studies. Epilepsy and PD are two chronic disorders, requiring a long lasting medical treatment. TMS excitability parameters were found to be sensitive to antiepileptic drugs (AEDs) and antiparkinsonian drugs (APDs). This has led to the development of a new application of TMS— that is, the possibility to evaluate noninvasively the effects of neuroactive drugs on motorcortex excitability in humans. The rationale is to better understand the principal modes of drug action at the level of the human cerebral cortex and to assess whether TMS may be of clinical value for monitoring medical treatment in neurologic disorders. This chapter describes the general methodology of such studies and experience with AEDs and APDs effects on motor excitability. Results suggest that l -DOPA and pergolide differentially affect the cortical inhibitory circuits at 12 months, with a progressive deterioration of the restored intracortical inhibition, with pergolide. Motor threshold (MT) is elevated in epilepsy and this change may be attributed to a drug effect. AEDs with mode of action on voltage-gated sodium-channels increase MT, whereas AEDs, supporting the action of gamma-aminobutyric acid (GABA), have no significant effect on MT.