Near-infrared spectroscopy applied over the mastoid process as a proxy of posterior cerebral perfusion: A new application worthy of investigation?

Dear Editor, Intraoperative cerebral oxygenation monitoring during cardiovascular surgery is essential to prevent perioperative complications. Intraoperative cerebral desaturations have been associated with postoperative delirium, cognitive dysfunction, and increased stroke risk, especially among elderly patients.1–3 Near-infrared spectroscopy (NIRS) is widely used for intraoperative cerebral oxygenation monitoring, typically applied bilaterally to the frontal region. However, this technique provides limited information, as it only reflects frontal lobe oxygenation. NIRS measures regional capillary blood saturation at 2–3 cm depth and can detect real-time ischemic events (defined as ≥20%–25% regional capillary blood saturation decrease from baseline). Given that the cochlear system within the mastoid process is supplied by posterior branches of the Circle of Willis,4 we hypothesized that NIRS sensors placed over the mastoid process could provide indirect data on occipital perfusion. To test this, we conducted a prospective observational pilot study enrolling 30 consecutive patients undergoing elective carotid endarterectomy (CEA) under regional anesthesia (ethical committee approval number: 46/23). Neurological examination and NIRS monitoring guided intraoperative decisions, such as the need for shunt insertion. Data were collected at baseline (T0), after carotid clamping (T1), and after unclamping (T2). Transcranial Doppler was not available. Values are reported as median (interquartile range). Four patients required shunting due to loss of consciousness at T1; two had a prior cerebral ischemic event. Although their exclusion from the analysis represents a limitation, we considered that their autoregulation and/or circulatory compensation failure would add some bias to the results. The other 26 patients remained neurologically intact; none had a cerebral ischemic event history. All patients were right-handed and on home therapy for hypertension. Intraoperative mean arterial pressure was targeted within ±10% of baseline. Due to our institutional protocol for carotid endarterectomy, patients did not receive invasive blood pressure monitoring. We adopted the completely noninvasive hemodynamic monitoring Clear-sight (Edwards Lifescience, Irwin, California), which provides continuous blood pressure monitoring by a finger-cuff around the middle phalanx of the second or third finger of the contralateral to surgery hand. In the left CEA group (n = 14), left occipital saturation increased at T2 (from 59 [50–73]% to 69 [61–79]%; P = 0.040), i.e., after the unclamping. Right occipital saturation showed no significant changes. In the right CEA group (n = 16), left occipital saturation exhibited a significant increase at T1 and T2: from 64 [49–72]% to 69 [62–76]% and 68[58–81]%, respectively (P < 0.001), while again right occipital saturation remained stable (±1%). In both groups, the left and right frontal saturations changed predictably following the impairment in cerebral blood flow related to the clamping (see Figure 1). We are aware of the small sample size, which prompts caution in the interpretation of the results. Anyway, we consider these preliminary findings suggest that during carotid clamping, frontal circulation impairment may be associated with increased left occipital oxygenation, independent of the operated side. After reperfusion, the increase persisted only in left-sided CEA. This asymmetric pattern may reflect collateral circulation, regional vascular anatomy, or hemispheric dominance effects. If occipital oxygenation monitoring will be confirmed by further studies, at the internal carotid clamping in case of discordance between anterior and posterior NIRS data, we could have more information about potential ischemic events in the posterior brain regions and try to limit them by shunt insertion. Then, further studies are warranted to validate its reliability for posterior circulation monitoring during cardiovascular surgery.5Figure 1.: The main results and trends of cerebral oxygenation.Conflicts of interest The authors declare that they have no conflicts of interest with regard to the content of this report.