TMS and the Neurology of Epilepsy
Basic findings
TMS has been used to study generalised and focal epilepsies for more than a decade. TMS plays an important and varied clinical role; it is currently used to suppress seizures or treat co-morbid conditions such as mood disorder (Hsu et al 2011, Anninos et al. 2011, Bae et al 2007). It is also used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential of novel drugs.
Ziemann et al (1996) showed, via TMS, that antiepileptic drugs that support the action of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) in the neocortex (vigabatrin, baclofen) reduced intracortical excitability but had no effect on motor threshold. Gabapentin showed a similar profile. By contrast, sodium and calcium channel blockers without considerable neurotransmitter properties (carbamazepine, lamotrigine, losigamone) elevated motor threshold but did not change intracortical excitability. The cortical silent period was lengthened by gabapentin and carbamazepine.
Ongoing research
Monitoring The Disease
In most studies of generalised epilepsies, TMS has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. TMS results suggest that patients with generalised epilepsy syndromes have increased cortical excitability. Patients with primary generalised epilepsy (PGE) show reduced motor threshold and ICI (Theodore 2003).
Ongoing work into the application of TMS includes the investigation of the effect of diurnal variability of seizures (Badawy et al 2009) and the distinction between cortical excitability in progressive myoclonic epilepsy and juvenile myoclonic epilepsy (Badawy et al 2010).
Modulating The Disease
Until recently, there has been little investigation of the effects of epilepsy surgery on cortical networks and changes in excitability. Läppchen and colleagues (2011) reported on a TMS study on changes in interhemispheric inhibition following successful surgical removal of an epileptic focus. They investigated motor thresholds (MT) and motor evoked potentials (MEPs) of interhemispheric paired pulse paradigms on both hemispheres and found that resection of the epileptic focus resulted in a significant change in interhemispheric inhibition. Through the use of TMS, Läppchen summarised that an epileptic focus can modulate interhemispheric inhibitory interactions between the motor cortices, and that these results and further investigations via TMS may contribute to a better understanding of widespread functional impairments in focal epilepsy.
Links
References
- Anninos et al., The SQUID as Diagnostic Tool in Medicine and its use with other Experimental Stimulation and Theoretical Methods for Evaluation and Treatment of Various Diseases, 2011.
- Badawy et al., Epilepsia, 2010Theodore, Epilepsy Curr, 2003.
- Badawy et al., Neurology, 2009.
- Bae et al., Epilepsy & Behavior, 2007.
- Hsu et al, Epilepsy Research, 2011.
- Läppchen et al., Journal of Neurology, 2011.
- Ziemann et al., Annals of Neurology, 1996.
