Background
The reliability of near-infrared spectroscopy (NIRS) for measuring cerebral oxygenation (ScO2) is controversial due to the possible contamination from extracranial tissues. We compared ScO2 measured with the NIRS optode on the forehead, the skull and the dura mater in anaesthetised patients undergoing craniotomy. We hypothesised that ScO2 measured directly on the skull and the dura mater would differ from ScO2 measured on the skin.

Methods
This prospective observational study included 17 adult patients scheduled for elective craniotomy. After induction of general anaesthesia, ScO2 was measured on the forehead skin, as well as on the skull and on the dura mater in the surgical field. The primary comparison was the difference in ScO2 measured on the dura mater and on ScO2 measured on the skin; secondary comparisons were the differences in ScO2 on the skull and ScO2 on the skin and the dura mater, respectively. Data were described with median (5%–95% range) and analysed with the Wilcoxon signed-rank test.

Results
ScO2 values on the dura mater were obtained in 11 patients, and median ScO2 (48%, 29%–95%) did not differ significantly from ScO2 on the skin (73%, 49%–92%; p = .052), median difference −25% (−35.6% to −1.2%). ScO2 on the skull (N = 16) was lower than that on the skin (63% [43%–79%] vs. 75% [61%–94%]; p = .0002), median difference −10% (−20.8 to −3.0).

Conclusion
In adults undergoing craniotomy, NIRS-based ScO2 measured on the dura mater did not reach statistically significantly lower values than ScO2 measured on the skin, whereas values on the skull were lower than on the skin, indicating a contribution from scalp tissue to the signal.