Topographic electroencephalographic study of transient ischemic attacks.

Twenty-five patients with TIAs in the carotid artery distribution were studied by means of computed mapping of EEG (CME) and conventional EEG. In addition, CT scan and cerebral angiography, and in 10 patients rCBF measurements were performed. The CME provided topographic maps of the average power spectra for each of 6 frequency bands from 2.0 to 29.5 Hz which were displayed two-dimensionally in a color-coded isopower format. EEG abnormalities were analyzed and the results of the two different methods of EEG interpretation were compared. Sixty-eight percent of the patients showed unilateral abnormalities on CME appropriately lateralized to the clinical symptoms even after these symptoms had cleared completely. Furthermore, 88% of those who were examined within 2 weeks of last TIA showed corresponding CME abnormalities. Only 3 out of 10 TIA patients who had rCBF studies showed reduction of blood flow on the appropriate hemisphere, whereas 7 of the 10 patients had corresponding CME abnormalities. Comparing the results of the CME and of conventional EEG reading revealed the CME to be slightly more sensitive in detecting asymmetrical voltage depression of background activity than the conventional reading of the EEG, while the latter detected low amplitude sporadic activities which were missed by CME. Both methods were equally sensitive in detecting slow wave foci and non-transient symmetrical changes. The two most significant points of this report are the following: first, 68% of the TIA patients studied had residual unilateral abnormalities in CME in their symptom-free period. In the subset of patients subjected to rCBF studies only 30% showed residual flow aberrations, whereas 70% of the same subset demonstrated unilateral abnormalities in CME. Second, though conventional EEG reading by an experienced electroencephalographer can nearly match the performance of CME the CME format makes subtle but useful EEG findings readily available to the uninitiated. The CME also quantifies the data making objective comparisons more amenable to software manipulations for further studies.