Abstracts

Pre-operative lateralization and localization of brain areas involved in language function is an important part of pre-surgical planning. Currently, determination of language dominance relies on an invasive procedure, the Wada test. Magnetoencephalography (MEG) studies have demonstrated that produces estimates of language laterality that closely match the Wada-derived ones. Here we present the largest epilepsy patient series reported to date that have undergone both a functional neuroimaging and the Wada procedure.
Ninety-nine patients with intractable seizure disorder were evaluated in this study. Event related fields (ERFs) were recorded in response to individual word stimuli in the context of a continuous (old-new) recognition task. Recordings were made in a magnetically shielded room, using a whole-head neuromagnetometer (4D Neuroimaging, Inc. Model WH2500). After ECD analysis, resulting raw scores (number of late activity sources) were used to create a laterality index according to the formula: where R represents the number of acceptable late activity sources observed in the right hemisphere and L the corresponding number on the left. This index was compared with the Wada index.
Independent judgments based on MEG and WADA data are compared in the table below. Concordance was excellent (Fisher[rsquo]s exact test, p [lt] .0001) with complete agreement on 73/84 of the patients There were 11 discordant cases. In the majority of patients (n=7) MEG detected considerable activity in both hemispheres, although the Wada suggested LH dominance. As in 4/7 of these patients the epileptogenic zone was in the RH, it is possible that the degree of neurophysiological activity from that hemisphere was inflated by the presence of focal epileptogenic activity. In 3 additional cases (judged to be bilateral by the Wada, although MEG indicated substantial hemispheric asymmetries), the Wada test had also indicated non-exclusive hemispheric dominance. There was no case in which exclusive language dominance was indicated by both modalities, but in different hemispheres. From a practical standpoint, MEG results would have resulted in additional invasive testing (electrocortical stimulation mapping) in 4/84 patients.
The exceptionally high degree of concordance between the two methods indicates that it may be possible in the near future to substitute the invasive Wada procedure through the completely noninvasive method of MEG. We currently use the procedure routinely as an adjunct to intra- and extraoperative language mapping in planning surgical resection for both adults and children. [table1]
[Supported by: The research presented in this report was partly supported by NIH Grant ROI N537941 to A.C. Papanicolaou.]