Adult epilepsy

Both electroencephalography (EEG) and magnetoencephalography (MEG) measure the same underlying brain activities. The greatest advantage of MEG is that source estimation techniques are easier to apply for MEG than for EEG. Magnetic source imaging (MSI) of interictal spikes as part of presurgical evaluations is one of the most successful clinical applications of MEG. MSI provides additional information to conventional presurgical evaluations by other noninvasive modalities in some patients with intractable epilepsy, especially those with neocortical/extratemporal lobe epilepsy or epilepsy with normal magnetic resonance imaging (MRI). MSI is recommended for the following situations: (1) no clear hypothesis regarding ictal onset, (2) insular onset suspected, (3) interhemispheric onset (especially frontal) suspected, (4) mesial temporal onset suspected without clear evidence of hippocampal sclerosis on MRI, (5) intrasylvian onset suspected, (6) multiple or very large epileptogenic lesion on MRI, and (7) planned intracranial EEG (especially previous craniotomy including revision epilepsy surgery). MSI can provide guidance for additional electrode coverage for intracranial EEG and the extent of the resection area when planning surgery. A single tight cluster of MEG spike dipoles is well correlated with ictal onset zone. Complete resection of the MEG focus often results in seizure freedom after surgery.