The baseline value of the Bispectral Index (BIS) is 96-99 in the awake state. Patients with Alzheimer's disease or vascular dementia may show an increase in slow wave and a decrease in fast wave activity of the electroencephalogram (EEG). BIS is presumed to decrease with EEG slowing. We hypothesized that the baseline "awake" BIS is lower in dementia than in normal elderly patients. We studied 36 patients with Alzheimer's disease or multiinfarct dementia and 36 control patients aged >75 yr. Both groups were assessed with a Mini-Mental State Test. BIS (version 3.4) was recorded from a frontal derivation using an Aspect A-2000 EEG monitor. Off-line data analysis was also performed with the newer version 4.0 of the BIS algorithm. Fourteen of 36 (38%) dementia patients and 4 of 36 (11%) controls had mean baseline BIS 3.4 <93 (P = 0.006). Eighteen of 36 (50%) dementia patients and 8 of 36 (22%) controls had mean BIS 4.0 <93 (P = 0.026). Mean (95% confidence interval) BIS 3.4 was 92.9 (91-95) in the dementia and 96.1 (95-97) in the control group (P = 0.02). Values with BIS 4.0 were, respectively, 89.1 (86-92) and 94.7 (93-96) (P = 0.002). No significant difference was found in age, sex, activity from the electromyogram, and signal quality index. As expected, the difference in Mini-Mental State Test scores was significant (P < 0.0001). A significant proportion of patients with dementia shows a low baseline BIS. The utility of the BIS monitor in detecting dementia warrants further investigation.
Implications: This prospective, controlled, observational study demonstrates that electroencephalogram slowing associated with dementia affects the Bispectral Index of the electroencephalogram. A significant proportion of patients with dementia have a lower than normal "awake" Bispectral Index.