gms | German Medical Science

Administration of appropriate sedation, and analgesia during awake craniotomy for tumor resection near the speech area is a challenge for the anaesthetist, and is mainly based on the clinical assessment of the patient throughout the procedure. We have therefore investigated if the bispectral index (BIS) - as an objective criterion for depth of hypnosis - could be an appropriate method for targeting analgosedation in this setting.

After approval of the study protocol by the local ethic committee, 18 patients undergoing elective surgery for intracranial tumors gave informed consent and were included in this study. Patients with seizure, EEG modifying medications or additional neurological diseases were excluded. Tumors location was frontal in 8 patients, temporal in 4 patients, occipital in 4 patients, and parieto-frontal in 2 patients. Histological diagnosis included gliomas grade III and IV in 11 cases, brain metastasis in 5 cases, and low-grade gliomas in 2 cases. Pre-operatively we measured the BIS values with an Aspect A-1000 EEG analyser (Aspect, Natick, USA) on the right and left frontal side and calculated the side-to-side BIS difference. In addition, a 12 channel EEG was recorded. Absolute BIS values, side-to-side BIS difference and EEG aberration were correlated to the level of consciousness for each patient according to the sedation agitation scale (SAS).

At the time of examination all patients were completely awake, orientated, and without a neurological deficit (SAS-score = 4). All Bis values recorded from the unaffected hemisphere were over 92 and correlated to a SAS-score of 4. EEG findings were normal in 6 patients, showed a focal aberration in 3 patients, and revealed an abnormal slowing in 6 patients. In 3 patients, distinct muscle artefacts made EEG interpretation unreliable. In 3 of 18 patients we found a qualitative side-to-side BIS difference (36, 25, 18) with a normal BIS value on the unaffected (96, 95, 98) and a lower BIS value on the affected hemisphere (60, 70, 80) simulating an impaired level of consciousness (SAS-score = 3). Location of the tumors was two times frontal (BIS values 60 and 70) and one time central (BIS value 80). Interpretation of the 12-channel EEG revealed an abnormal slowing of the EEG pattern in the left frontal area in two of these three patients, whereas one patient had a normal EEG.

The bispectral index can be used as an additional tool for targeting analgosedation in awake craniotomy when recorded from the unaffected brain hemisphere as this parameter can be altered by frontal tumors associated with a theta EEG activity.