12 patients with focal epilepsy were examined by magnetoencephalography (MEG). Source localisations of interictal epileptiform activity (spikes) yielded clear results. Slow wave dipole density in the frequency range from 2 to 6 Hz, using time selections from an automatic principal component analysis (PCA), was calculated.
Results of spike and slow wave dipole density localisations were superimposed on MR-images of each patient. Slow wave dipole densities were increased close to spike localisations. Distances between spike center of mass and slow wave maxima were calculated, average mean distance was 2.0 cm.
Independant of the localisation in either TLE or ETLE a concordance of slow wave and spike localisations were found. Slow wave localisations were found in patients with lesions in MRI and patients with no abnormalities on the MRI.
In comparison to healthy subjects, slow wave dipole density in patients with epilepsy was clearly increased.
The localisation of slow wave dipole density yielded additional important information and may contribute to defining the irritative zone.