New insights into the mechanism of neurally mediated syncope.

OBJECTIVE To determine the role of the cerebral cortex in neurally mediated syncope, the electroencephalograms (EEG) of patients recorded during head up tilt table test were analysed. DESIGN Retrospective study. SETTING University hospital. PATIENTS 18 patients with syncope or near syncope underwent head up tilt table test with simultaneous ECG and EEG monitoring. METHODS Standard 70 degrees tilt table test was done with simultaneous ECG and EEG monitoring. EEG waveforms were analysed by both visual inspection and spectral analysis. RESULTS 6 of 18 patients (33%) had a positive tilt table test. Before syncope slow waves increased in patients with a positive test. In addition, five of six tilt positive patients (83%) had slow wave activity that lateralised to the left side of the brain (mean (SD) 822 (724) v 172 (215) micro V(2), p < 0.05), while none of the tilt negative patients exhibited lateralisation (24 (15) v 26 (19) micro V(2), NS). Spectral analysis showed that the lateralisation occurred in the delta frequency. The lateralisation preceded the event by 5-56 seconds (18 (21) seconds). CONCLUSIONS EEG activity lateralises to the left hemisphere of the brain before syncope. The lateralisation precedes syncope and is associated with the onset of bradycardia, hypotension, and clinical symptoms. These findings suggest that the central nervous system may have a role in neurally mediated syncope.