It is shown in the present study that the incidence of conversion to GA and operative time was lower in the MCA group than that in the AAA group. Early functional recovery of patients in MAC group was significantly better than those in AAA group on the 1st, 3rd, 5th, and 7th days after operation. Intraoperative concentrations of PetCO2, PaCO2, and Glu were significantly higher, while SPO2 and PaO2 were significantly lower in MAC group than those in AAA group. Less remifentanil but more dexmedetomidine, antiemetic, and vasoactive drugs were used in the MAC group.
AC is a demanding but safe procedure for brain tumors within or adjacent eloquent areas. According to a European Low-Grade Glioma Network survey , eighteen centers (90%) preferably used either MAC or AAA; only three centers reported AAA and MAC simultaneously [12,13,14]. In fact, there were few studies comparing the advantages or disadvantages of these two anesthetic techniques [13,14,15,16,17], especially when multiple monitoring was in use. A meta-analysis only made comparisons among AC failures, conversion to GA, intraoperative seizures, and neurological dysfunction, but those data was from many published studies which were performed with either MCA or AAA approaches. Three studies are retrospectively comparing the MCA and AAA technique performed by a single center or a single neurosurgeon like our study, in which one study  concluded that the AAA technique may provide better results with respect to agitation and seizure, and another study  suggested a similar perioperative outcome between the two techniques, with shorter operative time for MAC, and the third study demonstrated that MCA with sole dexmedetomidine reduces respiratory and cardiovascular adverse events with a low need for antihypertensive and vasoactive drugs, probably ensuring more rapid surgery and reducing length of hospitalization . Our mean operative time was 3.46 ± 1.26 h in MAC group while 3.90 ± 1.01 h in AAA group; even if we became more and more familiar with the whole procedure in operating room, there has been only a slight reduction in our surgical time in recent years. Consistent with Eseonu’s study , the operative time of MCA group was about 27 min shorter than that of AAA group. The reasons for the shorter operative time may be as follows. First, intubation and extubation of LMA in pre-awake stage prolonged the operation ; however, we did not intubate LMA again after tumor resection consistent with Olsen’s study . Second, we have to waste some time waiting for the patient to be fully conscious before the LMA is removed safely in AAA group, although anesthesiologist can reduce the amount of anesthetic to wake up the patient in advance . In addition, the shorter operative time in MAC group may indicate less cost in America , which was certified by Eseonu  who suggested a better cost effectiveness with MCA technique in the patients undergoing AC. However, we found no difference in hospital stay and costs between the two anesthetic techniques.
In our study, not only the proportion of conversion to GA was significantly lower in MCA group than that in AAA group but also the whole proportion of conversion (0.03%, 7 in 225 patients) is much lower than that reported of 1–2% which is consistent with the outcomes of Stevanovic’s meta-analysis , in which the odds ratio (OR) comparing AAA to MAC was 2.17 and the likelihood ratio test (LR test) showed a significant P-value of 0.03. This may belong to our team of experienced professional anesthetists. The reasons for conversion to GA include LMA leakage, respiratory insufficiency, intraoperative bleeding, pain, brain bulge, seizure, severe restlessness, and acute brain edema . However, intraoperative seizures, which is one of the most common cause of failure in AC , are also one of the causes for conversion to GA in our study. Even though some patients need blood transfusion, intraoperative bleeding can be effectively controlled without the need to switch to GA.
The incidence of airway- and ventilation-related complications in AC has been reported to be 1.8 to 18% in the literature [22,23,24]. In the study with 18% airway- and ventilation-related complications, 4 (9%) patients developed decreases in respiratory rate and oxygen saturation (90%), and all these patients recovered by a brief period of a short period of manual jaw thrust or a short application of oxygen and ventilation by mask . In our study, desaturations or airway obstruction was not observed, possibly because of good oxygenation via the nasal trumpet prior to sedation, real-time monitoring of PetCO2, timely adjustment of drug dosage, less usage of remifentanil, and more dexmedetomidine which is believed to be associated with fewer respiratory adverse events compared with propofol and remifentanil during AC for supratentorial tumor resection . Navdeep’s study  also demonstrated that the episodes of airway obstruction leading to desaturation and hypertension were more in propofol group as compared to dexmedetomidine. Although our study demonstrated that PetCO2, PaCO2, and Glu levels were significantly higher in the MCA group than that in the AAA group, the incidence of brain edema was not observed in the two groups except for patient suffering from generalized seizures.
Intra- and postoperative nausea and vomiting have to be stopped for AC because these nausea and vomiting may contribute to inadvertent brain swelling and enhanced risk of aspiration, discomfort, and distress. The incidence of intra- and postoperative nausea and vomiting is between 0 and 30% [15, 24], while our incidence is 2.17% in MAC group and 0 in AAA group. This low incidence may be due to the use of pre- and postoperative intravenous antiemetics when necessary .
Although the pulse pressure difference in MAC group was greater than that in AAA group, there was no significant difference in systolic blood pressure, diastolic blood pressure, heart rate, and pulse between MAC and AAA group. This result is coincident with Eseonu’s study that intraoperative hypertension occurs equally in 8% of MAC and 9.7% of AAA but different from Dilmen’s study [13, 14] in which Dilmen et al. demonstrated that blood pressures were significantly higher in the MCA group during pinning, and heart rate and blood pressures were significantly higher in the MCA group than that in the AAA group during the skin incision. The incidence of intraoperative hypertension in this study occurred 22.82% in MAC and 17.07% in AAA group. This was coincident with the reported incidence of AC intraoperative hypertension that ranges from 16.7 to 24% [22, 27], but the incidence of intraoperative hypertension is higher than Eseonu’s study .
The principle of modern glioma surgery is to remove the tumor safely and maximally and improve symptom management, quality of life, progression-free survival (PFS), and prognosis in both low- and high-grade glioma. AC, known to preserve the quality of life in patients with low-grade glioma, is also able to significantly increase the extent of resection for lesions located in functional regions [4, 28]. Intraoperative application of electrophysiological monitoring, fluoroscopy, and intraoperative magnetic resonance helps to increase the advantages of AC to resect the tumor safely and maximally [29,30,31] because supertotal resection may provide survival benefits in HGG. In a large study reviewing 1229 patients with GBM over 19 years, prolonged survival was seen in patients that underwent greater than 53% resection of the T2/FLAIR abnormality in addition to GTR of the 259 contrast-enhancing region (20.7 vs. 15.5 months, p < 0.001) and low-grade gliomas . In our study, the extent of tumor resection was compared for the first time between MAC and AAA groups, and it was found that there was no significant difference of the extent of tumor resection between the two groups, but patients in the MAC group seemed to be more likely to undergo total resection. It is suspected that there are more patients converted to GA because of generalized seizures in AAA group which is in fact equal to general anesthesia. The neurosurgeons’ mood may also have influence on the manipulation of tumor removal. It is believed the intervention of AAA techniques during operation may result in bad temper of the surgeons.
In order to follow the principle of modern glioma surgery, new techniques such as neuro-navigation, brain-mapping, and brain-monitoring techniques were applied during our tumor resection. Besides, functional boundaries have to be beard in mind constantly. With the help of these techniques, we have to locate the tumor boundaries and then resect the tumors inside the functional boundaries. Normally after we approach the functional boundary by 1 cm with the help of cortical and subcortical electrical stimulation, communication was carried on continuously by a neurologist and speech pathologists simultaneously with the awake patient. The surgery keeps on and does not stop until the onset of neurologic dysfunction of the patient occurred like aphasia or paralysis. In our study, early functional recovery of patients in MAC group was significantly better than those in AAA group on the 1st, 3rd, 5th, and 7th days after operation. The reasons for this difference may be explained as follows: first, there are more patients converted to GA because of generalized seizures in AAA group which is equivalent to general anesthesia; second, it may be related to the better cooperation of doctors with patients in MAC group, while in AAA group, cooperation is usually hard to proceed between the doctors and the just arousal lethargy patients.
The limitations of study are that the study is a retrospective one. Even though demographic characteristics were consistent in group AAA and MAC, AAA management in glioma awake craniotomy was used in the early stage of our clinical practice since 2009, and we started using MAC since 2013. That is why the sample was different between the two groups. So, the potential statistical bias in this two groups may exist. In addition, our study was conducted in a single institution. Therefore, large randomized controlled trials are necessary to further evaluate the benefits of the two techniques.