A case of subarachnoid hemorrhage with different BIS values observed between the right and left forehead

We experienced a case of subarachnoid hemorrhage with different BIS values observed between the right and left forehead. A 71-year-old woman was scheduled for endovascular coiling. She had been already intubated and showed little response to painful stimulation without sedation (Japan coma scale; III-200). Anesthesia was induced and maintained with propofol. BIS monitoring was started from the right forehead before anesthetic induction (r-BIS), and additional monitoring from the left side was started after induction (1-BIS). When the operation was started, 1-BIS was 40 and r-BIS was 30. During the operation, 1-BIS was always higher than r-BIS; 1-BIS was 49 +/- 10 and r-BIS was 37 +/- 7 (mean +/- SD). The difference between r-BIS and 1-BIS was more than 10 for 68% of operation time. Since abnormal EEG pattern was observed, we concluded that abnormal EEG influenced BIS value, and 1-BIS after anesthesia was higher than that of her actual conscious level. EEG of a patient with acute stroke sometimes shows an abnormal pattern. Anesthesiologists should keep in mind the possibility that an abnormal EEG might influence BIS value.