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New developments in clinical magnetoencephalography
Jeffrey LewineUniversity of Utah, Salt Lake City, Utah 84108, USA
The availability of large array and whole-head biomagnetometer systems has allowed for a renaissance in clinical MEG. In the past six years, my colleagues and I, first at the Albuquerque VAMC and now at the University of Utah in Salt Lake City, have studied more than 1000 patients with a variety of neuorologic and psychiatric diseases. As a result of our work and that of many other biomagnetism investigators throughout the world, clinical application of MEG is now routine at several hospitals.
Preoperative localization of sensorimotor cortex in neurosurgical patients has emerged as the most well established clinical use of MEG, and comparative studies with other noninvasive imaging modalities prove MEG to be the most reliable and accurate noninvasive method available. New paradigms for assessing lateralization of language function are now in routine use at several medical centers.
The utility of MEG in the evaluation of epilepsy has long been proclaimed, and outcome data are finally in hand to substantiate the claim. For example, my colleagues and I have found that, when interictal MEG findings agree with ictal scalp EEG and MRI, surgical outcomes are excellent in more than 70% of cases. This is true for both temporal and extratemporal lobe epilepsy. In contrast, if interictal MEG shows bilateral or multifocal epileptiform activity, focal neurosurgery fails to provide good outcomes, even in cases where MRI and ictal EEG indicate only a single region of abnormality. Overall, interictal MEG provides an excellent indication of the epileptogenic zone. The main limitation of MEG in this regard is not the source model (provided that a multiple-dipole spatio-temporal approach is employed), but rather, the fact that nearly 1/3 of patients with epilepsy fail to demonstrate significant interictal activity on a 1 - 2 hour MEG examination. On the other hand, activation procedures, using low dose methohexital, appear quite promising, especially in temporal lobe epilepsy.
An exciting new clinical use of MEG is in the autism spectrum disorders where MEG often reveals significant epileptiform activity during stage III sleep. In many cases this activity arises from the opercular surface of the superior temporal gyrus, deep into the Sylvian plane. The orientation of dendrites of the relevant cells is parallel to the scalp surface, which is favorable for MEG and relatively unfavorable for EEG. In many cases, spikes which are clear in the MEG are not readily identified on simultaneous EEG. Recent data indicate that MEG-guided epilepsy surgery in this population can lead to significant improvements in language and eye contact along with a reduction in autistic behaviors. These applications of MEG, together with recent developments in mild head trauma and schizophrenia paint a bright picture for the future of clinical MEG.
