Ritva Paetau

Auditory evoked magnetic fields to tones and pseudowords in healthy children and adults.

Neuromagnetic responses to tones and pseudowords were measured with a 24-channel magnetometer in nine adults and in 23 children, the latter aged 0.3–15 years. Both stimulus types elicited substantially similar responses in all subjects. At 0.9-s interstimulus interval (ISI), the adult response was a stable P1 m-N1m-P2m-N2m sequence peaking at 50, 100, 200, and 250 ms, respectively. A biphasic Plm-N1,2m response with peaks at 100 and 260 ms occurred in children up to 12 years of age. At longer ISIs (1.2–2.4 s), an adult-type N1m response appeared in most children. N1 m amplitude suppression at short ISIs was stronger in children than in adults and may reflect a longer refractory period of the N1 generator neurons during early childhood than later in life. Peak latencies of PI m, N1 m, and N2m decreased with age, most rapidly <7 years of age. All deflections originated in nearby cortical areas within the posterior sylvian fissure, and may serve as functional landmarks for that anatomic area.

Magnetoencephalography in neurosurgery

OBJECTIVE:To present applications of magnetoencephalography (MEG) in studies of neurosurgical patients. METHODS:MEG maps magnetic fields generated by electric currents in the brain, and allows the localization of brain areas producing evoked sensory responses and spontaneous electromagnetic activity. The identified sources can be integrated with other imaging modalities, e.g., with magnetic resonance imaging scans of individual patients with brain tumors or intractable epilepsy, or with other types of brain imaging data. RESULTS:MEG measurements using modern whole-scalp instruments assist in tailoring individual therapies for neurosurgical patients by producing maps of functionally irretrievable cortical areas and by identifying cortical sources of interictal and ictal epileptiform activity. The excellent time resolution of MEG enables tracking of complex spaciotemporal source patterns, helping, for example, with the separation of the epileptic pacemaker from propagated activity. The combination of noninvasive mapping of subcortical pathways by magnetic resonance imaging diffusion tensor imaging with MEG source localization will, in the near future, provide even more accurate navigational tools for preoperative planning. Other possible future applications of MEG include the noninvasive estimation of language lateralization and the follow-up of brain plasticity elicited by central or peripheral neural lesions or during the treatment of chronic pain. CONCLUSION:MEG is a mature technique suitable for producing preoperative “road maps” of eloquent cortical areas and for localizing epileptiform activity.

Magnetoencephalography in presurgical evaluation of children with the Landau-Kleffner syndrome

Summary: Purpose: Our aim was (a) to localize the primary epileptogenic cortex for possible multiple subpial transsection in four children with the Landau‐Kleffner syndrome (LKS), and (b) to evaluate the impact of magnetoencephalography (MEG) in the localizing process.

Long‐term outcome of 32 children with encephalopathy with status epilepticus during sleep, or ESES syndrome

Purpose:  To prospectively evaluate the efficacy of drug treatment and long‐term cognitive outcome in children with encephalopathy with status epilepticus during sleep (ESESS).

Magnetoencephalographic Evaluation of Children and Adolescents with Intractable Epilepsy

Summary: Magnetoencephalographic (MEG) discharges were recorded with multichannel superconducting quantum interference device (SQUID) gradiometers in 13 young candidates for epilepsy surgery. The sources of epileptic activity were related to generators of somatosensory and auditory evoked cortical responses and projected on magnetic resonance imaging (MRI) scans. Seven subjects had restricted or regional MEG foci, located in the frontoopercular (1), sensorimotor (3), perisylvian (1), mesiotemporal (1), or temporooccipital cortex (1). The MEG foci in the 3 patients who underwent operation agreed with the intracranial findings. Findings in the other patients emphasize the need to collect further data to define the ultimate role of MEG in preoperative evaluation of epilepsy.

Apparent asynchrony between interictal electric and magnetic spikes

WE recorded simultaneous multichannel electroencephalogram (EEG) and magneto encephalogram (MEG) in four children with partial epilepsy. Sources of averaged spikes were modelled with current dipoles. Of 10 spike averages obtained, three peaked simultaneously in MEG and EEG, and in seven averages, the MEG peak preceded the main EEG peak by 9–40 ms. A small positive early EEG signal coincided with the MEG peak in six asynchronous spikes. The simultaneous MEG and EEG spikes originated within 5–23 mm, while sources of asynchronous peakswere 12–67 mm apart. We conclude that non-identical neurone currents underlie the MEG and EEG signals, and emphasize the importance of modelling early phases of EEG spikes when localizing interictal epileptic zones.

Landau-Kleffner syndrome: epileptic activity in the auditory cortex

The Landau-Kleffner syndrome (LKS) is characterized by electroencephalographic spike discharges and verbal auditory agnosia in previously healthy children. We recorded magnetoencephalographic (MEG) spikes in a patient with LKS, and compared their sources with anatomical information from magnetic resonance imaging. All spikes originated close to the left auditory cortex. The evoked responses were contaminated by spikes in the left auditory area and suppressed in the right--the latter responses recovered when the spikes disappeared. We suggest that unilateral discharges at or near the auditory cortex disrupt auditory discrimination in the affected hemisphere, and lead to suppression of auditory information from the opposite hemisphere, thereby accounting for the two main criteria of LKS.

Cerebral magnetic fields to lingual stimulation

We recorded cerebral magnetic fields to electric stimulation of the tongue in 7 healthy adults. The two main deflections of the response peaked around 55 msec (P55m) and 140 msec (N140m). During both of them the magnetic field pattern, determined with a 7- or 24-channel SQUID magnetometer, suggested a dipolar current source. The topography of P55m can be explained by a tangential dipole at the first somatosensory cortex (SI) in the posterior wall of the central sulcus. The equivalent source of N140m is, on average, about 1 cm lateral to the source of P55m. The reported method allows non-invasive determination of the cortical tongue representation area.

Proton Spectroscopic Imaging Shows Abnormalities in Glial and Neuronal Cell Pools in Frontal Lobe Epilepsy

Summary:  Purpose: Proton magnetic resonance spectroscopic imaging (1H MRSI) can lateralize the epileptogenic frontal lobe by detecting metabolic ratio abnormalities in frontal lobe epilepsy (FLE). We used 1H MRS to lateralize and localize the epileptogenic focus, and we also sought to characterize further the metabolic abnormality in FLE.

The effect of surgery in encephalopathy with electrical status epilepticus during sleep

Purpose:  We analyzed clinical and electroencephalography (EEG) outcomes of 13 patients with pharmacoresistant encephalopathy with electrical status epilepticus during sleep (ESES) following epilepsy surgery.