In about 20% of epileptic patients the results of medication are unsatisfactory. Many such persons may benefit from removal of the tissue producing the epileptic activity. In presurgical evaluation of patients with intractable seizures originating in the cortex, MEG complements, and may even replace invasive techniques such as depth or subdural electrode recordings, which are used at present to locate the seizure-provoking area. These invasive methods carry some risk, their use requires considerable expertise and, above all, they cause discomfort and inconvenience to the patient. MEG could aid in the selection of suitable candidates for intracranial implantation and in focusing the surgical procedure to the right location. Furthermore, simultaneous MEG measurements of epileptic activity and sensory evoked responses may show an epileptic trigger in an essential brain area, thus preventing surgery.
The advantages of MEG were well displayed in our study of a 36-year old woman who had complex partial seizures of temporal lobe origin and up to 7 short losses of consciousness per day for 23 years. Computed tomography showed a large cyst in the right parietal region. In videotelemetric EEG, most epileptic discharges were seen in the right temporal region but some peaked at varying locations in the frontal midline. MEG showed frequent discharges with maximum amplitudes over the right temporal area. The estimated sources had two clearly different orientations but were restricted to a 2 × 2 cm2 zone in the right temporal lobe, 3 - 4 cm below the anatomical abnormality and 4 cm posterior to the sources of evoked fields in the primary auditory cortex. Currents with varying orientations at this location explained the EEG spikes, whose maxima were several centimeters from their source. Surgery was guided by the MEG localization, and an enlarged cortical gyrus was found. In direct intracranial recordings, this area displayed maximal epileptic activity and was resected. The patient has remained seizure-free since then.
Fig 13. (Left) Magnetic field patterns during one epileptic
spike over the right (above) and left (below) hemispheres; the
isocontours are separated by 400 fT. Yellow indicates magnetic field out of
and blue into the head. (Right) Dipole strengths as a function of time.
Field patterns during three single spikes were analyzed and the results
superimposed. The acticity in the right hemisphere precedes that in the left
one by about 20 ms.
Until now, MEG studies have aimed at predicting the area involved in producing the epileptic activity and in defining the regional extent of the abnormal areas. However, the good spatial accuracy and the high temporal resolution of whole-head MEG may also allow follow-up studies of the spread of epileptic activity and elucidation of mechanisms producing epileptic foci. This was demonstrated by our study of a 18-year old female epileptic patient whose complex partial epilepsy, which had started at age 8, had initially been well controlled by medication. During daytime seizures, she had tingling and clumsiness in her left hand. In seizures during sleep, upper limbs were drawn to extension and she had convulsions in them. Despite medication, her seizures increased. MEG revealed epileptic activity in the right parietal region, posterior to the source areas of the somatosensory evoked fields. Corresponding spikes were seen 20 ms later in the homologous cortical area of the opposite hemisphere. This "mirror focus" had probably developed during the disease.
