Exploratory study of electroencephalogram markers of responsiveness to surgical noxious stimuli under propofol...

While general anaesthesia typically induces unconsciousness, some patients retain the capacity to respond behaviourally to noxious stimulation. Reduced frontal alpha (8-12 Hz) power has been proposed as a marker of arousal, but its clinical reliability remains inconsistent. This exploratory study aimed to identify multichannel EEG spectral and connectivity markers associated with intraoperative behavioural responsiveness around noxious stimulation.

Sixty-four-channel EEG was recorded intraoperatively from seven patients undergoing microlaryngoscopy under propofol anaesthesia with analgesia. Responsiveness was assessed via reactions to verbal commands. Alpha-band spectral features of responders and non-responders were compared pre- and post-noxious stimulation using descriptive statistics. Linear mixed models evaluated the relationship between channel-wise spectral power, and the factors of noxious stimulation and responsiveness. Event-related synchronization/desynchronization and connectivity via weighted Symbolic Mutual Information (wSMI) were analysed across frequency bands and response categories, with correction for multiple comparisons.

Alpha power and global coherence showed minimal changes following noxious stimulation across patients. In contrast, volitional responses to noxious stimulation were associated with increased high-frequency power in sensory-motor and auditory cortices. These responses showed event-related synchronisation in left-central channels, whereas incoherent movements were marked by desynchronisation in the same areas. Theta-band activity further differentiated response types: cognitive responses showed suppression, while incoherent movements showed enhancement, particularly over the same regions. Cognitive responses were associated with increased global whole-brain integration, especially in the theta-band linking motor, auditory, and premotor cortices. Incoherent movements, by contrast, were associated with reductions in global brain connectivity. None of these patients reported postoperative awareness with recall.

These preliminary findings suggest that localised high-frequency power increases and enhanced theta-band connectivity may reflect "connected consciousness," in which patients retain the capacity to process sensory input despite anaesthesia. As this state may precede awareness with recall, particularly during noxious stimulation, its detection remains a key clinical challenge, underscoring the potential of these multichannel EEG features as markers. Despite limitations in sample size and variability in anaesthetic and patient factors, the surgical setting enhances the clinical relevance of these findings. However, further validation is required before clinical application for intraoperative monitoring and optimisation of analgesia prior to noxious stimulation.