Sharing the same site with neurosurgeons: the bispectral index sensor
Authors: Şahinkaya Halide Hande 
Published: January 2025

To the Editor:
Electroencephalographic monitoring with the bispectral index (BIS) is a method for assessing level of consciousness and depth of anesthesia. Incidence of accidental awareness during anesthesia can be prevented with BIS monitoring. BIS can provide optimal anesthetic consumption, reduced time to extubation, early recovery from anesthesia and discharge from both the operating room and post anesthetic care unit, low incidence of nause and vomiting, cognitive impairment and delirium, as well (1).
The manufacturer’s recommended placement of BIS sensor for monitoring depth of anesthesia usually conflicts with the surgical site. Neurosurgeons and anesthesiologists have to share the same anatomic area, where BIS sensor is placed between the forehead and eyebrows. Wet clothes due to blood and irrigation fluids in the surgical site, mechanical and magnetic interference of surgical equipments like mayfield pins placement, neuro-navigation and electrocautery prevent the proper use of BIS sensor. There is a decrease and interruption in signal quality. Even drapping may be ineffective for protecting the sensor.
Alternative placements of BIS sensor have been applied and the correlation between the frontal placement have been evaluated. Nasal dorsum and under the eye is an appropriate site for monitoring. Nelson et al., determined slightly more variability when compared with the standard positioning, but this variability was not found clinically significant (2). It has been shown that an alternative position across the mandible can be availably used. Shiraishi et al. found a good correlation between frontal and occipital BIS placements. Post auricular sensor placement was also found as a practical alternative (3). Overall, a cross-sectional study was conducted comparing BIS scores derived from frontal and supralabial electrode placement. Supralabial placement was found as an alternative place (4). On the other hand, false elevation of BIS may be possible due to electromyographic (EMG) activity, use of surgical devices, and electrocardiogram (ECG) artifacts (5).
According to the literature, we mostly use nasal dorsum placement when the surgical skin insicion is close to the forehead (Fig. 1). This provides better surgical satistaction by staying away from the surgical field. The reliability of the sensor may decrease when the sensor is far from the standart placement site. So, causion should be taken due to interferences that may affect BIS scores.
In summary, an alternative positioning of BIS can be easily used without a clinically significant variability. Anaesthesiologists should consider different placements when using BIS monitoring during neurosurgical procedures especially close to frontal site.

References
1. Lewis SR, Pritchard MW, Fawcett LJ, Punjasawadwong Y. Bispectral index for improving intraoperative awareness and early postoperative recovery in adults. Cochrane Database Syst Rev 2019;9:CD003843.
2. Nelson P, Nelson JA, Chen AJ, Kofke WA. An alternative position for the BIS-Vista montage in frontal approach neurosurgical cases. J Neurosurg Anesthesiol 2013;25:135-142.
3. Akavipat P, Hungsawanich N, Jansin R. Alternative Placement of Bispectral Index Electrode for Monitoring Depth of Anesthesia during Neurosurgery. Acta Med. Okayama. 2014;68(3):151-155.
4. Dubey JK, Goel N, Chawla R, Gupta M, Bhardwaj M. Supralabial Site: An Alternative Site for Bispectral Index Monitoring: A Cross-sectional Study. J Neuroanaesthesiol Crit Care 2022;9:149–154.
5. Kim YS, Lim BG, Lee IO. False elevation of bispectral index with a mandibular position in a patient undergoing craniotomy. J Neurosurg Anesthesiol 2014;26(3):265-267.