Median nerve somatosensory evoked potential monitoring during carotid endarterectomy: does reference choice matter?

Reconstructing subcortical and cortical somatosensory activity via the RAMUS inverse source analysis technique using median nerve SEP data

This study concerns reconstructing brain activity at various depths based on non-invasive EEG (electroencephalography) scalp measurements. We aimed at demonstrating the potential of the RAMUS (randomized multiresolution scanning) technique in localizing weakly distinguishable far-field sources in combination with coinciding cortical activity. As we have shown earlier theoretically and through simulations, RAMUS is a novel mathematical method that by employing the multigrid concept, allows marginalizing noise and depth bias effects and thus enables the recovery of both cortical and subcortical brain activity. To show this capability with experimental data, we examined the 14-30 ms post-stimulus somatosensory evoked potential (SEP) responses of human median nerve stimulation in three healthy adult subjects. We aim at reconstructing the different response components by evaluating a RAMUS-based estimate for the primary current density in the nervous tissue. We present source reconstructions obtained with RAMUS and compare them with the literature knowledge of the SEP components and the outcome of the unit-noise gain beamformer (UGNB) and standardized low-resolution brain electromagnetic tomography (sLORETA). We also analyzed the effect of the iterative alternating sequential technique, the optimization technique of RAMUS, compared to the classical minimum norm estimation (MNE) technique. Matching with our previous numerical studies, the current results suggest that RAMUS could have the potential to enhance the detection of simultaneous deep and cortical components and the distinction between the evoked sulcal and gyral activity.

The Efficacy of Near-Infrared Spectroscopy Monitoring in Carotid Endarterectomy: A Prospective, Single-Center, Observational Study

There has been no gold standard for intraoperative monitoring in carotid endarterectomy (CEA) till now. The purpose of the current study was to investigate the value of near-infrared spectroscopy (NIRS) monitoring in CEA and explore the thresholds for intraoperative cerebral hypoperfusion. Eighty-four consecutive patients who underwent CEA surgery in Xuan Wu Hospital of Capital Medical University from August 2015 to June 2016 were enrolled in this study. All patients were intraoperatively monitored by transcranial Doppler ultrasonography (TCD) and NIRS. Regional oxygen saturation (rSO₂) monitored by NIRS and blood flow velocity of the middle cerebral artery (V-MCA) monitored by TCD were continuously recorded. Correlation analysis was conducted for NIRS and TCD monitoring values. Intraoperative shunting was performed in five patients according to the TCD monitoring results and surgeon preference. During clamping of the carotid artery, the Pearson correlation index between rSO₂ and V-MCA was 0.581 (P<0.001). A cut-off of 12.3% decrease of rSO₂ was identified as the optimal threshold for intraoperative hypoperfusion indicated by TCD monitoring, when the sensitivity and specificity were 74.6% and 91.7%, respectively, with a 0.609 Kappa value. Physical examination immediately after operation showed no ischemic injury occurred, and no death and stroke occurred during the postoperative hospitalization. Our study demonstrated that NIRS could serve as a favorable monitoring tool during CEA. A 12.3% decrease of rSO₂ could be adopted as a reliable threshold for intraoperative cerebral hypoperfusion.