This result has also to be seen within its limits. Only a large-scale multi-centre randomised controlled trial, with a standardised perioperative protocol would enable a definitive distinction of these two procedures. Furthermore, the AAA technique [33], with its low failure rate of 1 out of 50 patients seems to have potential for implementation in AC, but further clinical data are required to confirm the feasibility of this technique to larger populations. Dexmedetomidine has been successfully used for MAC as well as the SAS technique in our included studies. Further investigations are required to show a potential significant superiority of dexmedetomidine to especially propofol.LimitationsFirst, we accessed only two databases and restricted our search to English language, which might not have identified all clinical studies meeting our inclusion criteria. Second, we focused our search on the years 2007?015, to analyse the recent RG7666 site development of anaesthesia techniques for AC. One justification for our chosen time-span is the continuous development of the anaesthetics and our aim to provide a SR for the actually usually used anaesthetics. Another one is that the information quality of clinical articles is significantly depending on the reporting quality. As the most of the identified studies were of observational nature, we have decided to include only studies since 2007, when the latest STROBE statement for improving reporting quality of observational trials was published [78]. Of note, some of our included studies were already published in 2007 before the latest STROBE statement release. Due to our specific search strategy we identified only forty-seven studies, which were mostly observational, retrospective and heterogeneous. All studies, including the two small RCTs with 26 [32] and 30 patients [56], and one pseudo-RCT with 29 patients [36] had low methodological quality with a moderate to high risk of bias. Furthermore, the primary endpoint of the pseudo-RCT, which used an alternating assignation method, was the difference between listening to major key or minor key music during AC [36], which was not our focus in this SR. Eleven studies were performed during a large time-scale of more than six years [24,31,35,37,38,42,43,47,55,57,59], and even 18 years [34]. It is likely, that the findings were strongly affected by a learning curve of the whole team involved in conduction of AC. This implies also the anaesthetic techniques, which were subjects to change. Of note, outcome assessment differed significantly between the studies. Our inclusion of small studies 20 patients [19,23,28,39,54,60], bears the risk of overestimation of beneficial outcomes, due to random chance [79]. Furthermore, the estimated treatment-effect tends to be larger in non-randomised studies [80]. Our pre-described outcome variables were not reported in each of the identified studies and hindered therefore a metaanalysis of more than five outcome variables. Inclusion of observational studies into our metaanalysis was justified by the absence of better evidence for the different anaesthetic ACPLOS ONE | DOI:10.1371/journal.pone.EXEL-2880 molecular weight 0156448 May 26,38 /Anaesthesia Management for Awake Craniotomytechniques, presently. Concurrently, a sensitivity analysis could only be performed by inclusion of these observational studies, despite the present high-risk of bias in them. Furthermore, we have excluded studies, which were performed outside the operating room or with the use of.This result has also to be seen within its limits. Only a large-scale multi-centre randomised controlled trial, with a standardised perioperative protocol would enable a definitive distinction of these two procedures. Furthermore, the AAA technique [33], with its low failure rate of 1 out of 50 patients seems to have potential for implementation in AC, but further clinical data are required to confirm the feasibility of this technique to larger populations. Dexmedetomidine has been successfully used for MAC as well as the SAS technique in our included studies. Further investigations are required to show a potential significant superiority of dexmedetomidine to especially propofol.LimitationsFirst, we accessed only two databases and restricted our search to English language, which might not have identified all clinical studies meeting our inclusion criteria. Second, we focused our search on the years 2007?015, to analyse the recent development of anaesthesia techniques for AC. One justification for our chosen time-span is the continuous development of the anaesthetics and our aim to provide a SR for the actually usually used anaesthetics. Another one is that the information quality of clinical articles is significantly depending on the reporting quality. As the most of the identified studies were of observational nature, we have decided to include only studies since 2007, when the latest STROBE statement for improving reporting quality of observational trials was published [78]. Of note, some of our included studies were already published in 2007 before the latest STROBE statement release. Due to our specific search strategy we identified only forty-seven studies, which were mostly observational, retrospective and heterogeneous. All studies, including the two small RCTs with 26 [32] and 30 patients [56], and one pseudo-RCT with 29 patients [36] had low methodological quality with a moderate to high risk of bias. Furthermore, the primary endpoint of the pseudo-RCT, which used an alternating assignation method, was the difference between listening to major key or minor key music during AC [36], which was not our focus in this SR. Eleven studies were performed during a large time-scale of more than six years [24,31,35,37,38,42,43,47,55,57,59], and even 18 years [34]. It is likely, that the findings were strongly affected by a learning curve of the whole team involved in conduction of AC. This implies also the anaesthetic techniques, which were subjects to change. Of note, outcome assessment differed significantly between the studies. Our inclusion of small studies 20 patients [19,23,28,39,54,60], bears the risk of overestimation of beneficial outcomes, due to random chance [79]. Furthermore, the estimated treatment-effect tends to be larger in non-randomised studies [80]. Our pre-described outcome variables were not reported in each of the identified studies and hindered therefore a metaanalysis of more than five outcome variables. Inclusion of observational studies into our metaanalysis was justified by the absence of better evidence for the different anaesthetic ACPLOS ONE | DOI:10.1371/journal.pone.0156448 May 26,38 /Anaesthesia Management for Awake Craniotomytechniques, presently. Concurrently, a sensitivity analysis could only be performed by inclusion of these observational studies, despite the present high-risk of bias in them. Furthermore, we have excluded studies, which were performed outside the operating room or with the use of.