Can Jaw Replace the Forehead for Anaesthesia Monitoring? Research Indicates It's Not That Simple

When the Forehead Is Off-Limits: Seeking Alternatives

In many surgical procedures-especially neurosurgery and head-and-neck operations-access to the patient’s forehead for depth-of-anaesthesia monitoring can be limited. Traditionally, processed electroencephalography (EEG) electrodes are placed on the forehead to gather the Patient State Index (PSI), a key indicator of how deeply a patient is anaesthetised. But what happens when this isn’t possible? Could placing sensors on the mandible (the chin/jaw area) serve as a reliable alternative?

A recent prospective observational study published in the Indian Journal of Anaesthesia set out to answer this very question.

The Study at a Glance

Researchers enrolled adults scheduled for elective, non-neurosurgical, and non-head-and-neck surgeries under general anaesthesia. PSI values were recorded simultaneously from traditional frontal (forehead) placements and from the mandibular region. The main aim? To assess whether PSI values from the jaw could be treated as a direct substitute for those from the forehead.

In total, 36 patients (median age 62, predominantly women) completed the study, yielding over 83,000 paired observations across different phases of anaesthesia—induction, surgery, and emergence.

Crunching the Numbers: Moderate Association, But Wide Differences

The study found that while there was a moderate correlation between mandibular (MAND) and frontal (HEAD) PSI values overall (median r = 0.70), the actual agreement was limited, with wide variability. Only about 29% of paired readings were within ±10 PSI units of each other, and 69% within ±20. Interestingly, the level of agreement varied depending on the phase of anaesthesia:

Induction: The closest match (median r = 0.85), with 64% of readings within ±10 PSI.

Surgery: The weakest link (median r = 0.06), with only 24% within ±10 PSI.

Emergence: Some improvement, but still limited (median r = 0.77; 29% within ±10 PSI).

EMG Activity: The Hidden Culprit

A key finding was that electromyographic (EMG) activity—the electrical signal from muscle movements—was often much higher in the mandible than the forehead. This EMG difference explained a significant portion of the divergence in PSI readings, especially during phases when neuromuscular blockers wore off or patients became aroused.

Clinical Takeaway: Use With Caution

While alternative montages like the nasal/infraorbital have shown promise, this study suggests that the mandibular PSI should only be considered as an adjunct when forehead access is impossible. Its values should not be used interchangeably with forehead readings, particularly during surgery and emergence, and always interpreted with close attention to EMG artefacts and overall trends rather than absolute numbers.

Limitations and Future Directions

The research was conducted at a single centre with a modest sample size, and signal failure of the mandibular montage occurred in a meaningful minority of patients. The findings highlight the need for device-specific validation studies and suggest future research should focus on standardised low-EMG conditions.

Key Takeaways

• Mandibular PSI does not reliably match frontal PSI values and is not a direct substitute.

• Agreement between the two locations varies significantly by anaesthetic phase.

• Increased EMG activity in the mandible often skews PSI readings.

• Mandibular PSI should only be used as an adjunct and interpreted with caution.

• More research is needed for alternative placements when the forehead is inaccessible.

Citation: Shiraishi T, Yaguchi Y. Agreement between mandibular and frontal patient state index values during general anaesthesia: A prospective observational study. Indian J Anaesth 2026;70:485-9. https://journals.lww.com/ijaweb