Chronic subdural hematoma (cSDH) is one of the most common neurosurgical pathologies. CSDH can lead to substantial morbidity and mortality, which makes optimal treatment paramount (1). The gold standard of treatment remains surgical drainage of the hematoma through burr-hole trepanation and insertion of a drain (2). A randomized controlled trial (RCT) showed that the insertion of a subdural drain (SDD) after burr-hole drainage of cSDH significantly reduces the rate of recurrence and improves outcome (3). However, the insertion of SDD carries a risk of parenchymal injury due to their proximity to the cortex or bridging veins (4–7). Therefore, subperiosteal drains (SPD), which are inserted between the calvarium and the periosteum have been recommended by some surgeons (2, 6). Several studies, amongst others a recently published RCT, were carried out comparing these two different drain types, with regard to outcome and recurrence, with somewhat controversial results (4, 5, 7–14).

The aim of this systematic review and meta-analysis is to compare SPD and SDD with regard to rate of recurrence, morbidity, mortality, and clinical outcome.

After screening 1,109 articles, 10 were included in our qualitative systematic review as well as quantitative meta-analysis (Figure 2). One study was an RCT (7), three studies were non-randomized prospective cohort studies (4, 10, 12), and six were retrospective cohort studies or sub-group analysis of other studies (Table 1) (5, 8, 9, 11, 13, 14). For our analysis 1,553 patients from the SPD group and 1,782 patients from the SDD group were included.

The aim of this meta-analysis was, to evaluate whether there is a difference in recurrence rate of cSDH following insertion of SPD or SDD. This meta-analysis shows that SPD has a significantly lower risk for recurrence, drain misplacement and intraparenchymal injury, as well as morbidity, when compared to SDD. Concerning mortality and clinical outcome, no significant difference was seen between the drain groups.

Although cSDH remains one of the most common neurosurgical disease entities, there is a paucity of studies on outcome differences in cSDH after insertion of SPD and SDD. Recurrence in cSDH occurs in ~10% of surgically drained patients and causes higher morbidity in the affected patients (20, 21). It was shown, that the insertion of a drain after burr-hole drainage of cSDH reduces recurrence rates and improves outcome significantly (3). Technical nuances, amongst others, the insertion of a drain, or the localization of the drain (above or under the bone), seem important to achieve better surgical outcome. For this reason, in recent years, some authors compared the outcome of SPD insertion with SDD insertion after burr-hole drainage of cSDH. Unfortunately, to date, only one RCT exists on the matter, while all other series are of retrospective nature, post-hoc analysis of prospective cohorts, or consist of rather small cohorts.

To date, the only RCT published comparing SPD and SDD showed no significant difference in recurrence rate between the two groups (11.9 and 12.3%, respectively) (7). Similarly, Zhang et al. (11 and 13%) (9), Chih et al. (7 and 10%) (4), Sjavik et al. (11 and 16%) (13) and Oral et al. (6 and 8%) (5) did not show a significant difference in recurrence rates, while the study by Kaliaperumal et al. showed no recurrences at all (12). Häni et al. (24 and 22%), Glancz et al. (9 and 8%) (11) and Ishfaq (13 and 10%) (10) showed lower recurrence rates in the SDD group compared to the SPD group; however, statistical significance was not observed in any of these studies.

Sjavik et al. (13) compared in their study three different drain techniques: (1) SDD with irrigation, (2) passive SDD, (3) active SPD. For our analyses, both SDD types were included in the SDD group. In Sjavik et al.'s analysis passive SDD showed a higher recurrence rate compared to active SPD. However, the drains with subdural irrigation showed no significant difference in recurrence rate; hence, their results could be due to the active negative pressure within the drain and is most probably less affected by the drain's location. Data on the influence of inserting an active suction drain after surgical drainage of cSDH is sparse, while to our knowledge trials comparing the outcome of active vs. passive drainage do not exist.

Parenchymal hemorrhage due to drain misplacement is a feared complication when inserting an SDD after burr hole drainage of cSDH, potentially increasing perioperative morbidity and mortality (2, 7, 22). All studies reporting misplacement rates showed lower rates in the SPD group (4, 7, 8, 10, 12, 14). The overall misplacement rates in the included studies was 1.2% for SPD and 7.8% for SDD, while only the study by Häni et al. showed a higher misplacement rate in the SPD group. In their post-hoc analysis, Häni et al. distinguished between two groups, defined by the year of treatment within their study, namely: “SDD recommended” (n = 214, with a possibility to switch to SPD) and SDD treated (n = 135). All misplacements (n = 6) occurred in the “SDD recommended” group, however, four of these patients ultimately were treated with an SPD since the placement of an SDD was difficult and caused brain injury. These patients were however allocated to the SPD group leading to the above-described overall misplacement rate of 1.2% in the SPD group.

Based on the included studies mortality rate in the SPD group was 5% and in the SDD group 4.6%. Interestingly, although drain misplacement, parenchymal injury, and overall morbidity rate were significantly higher in the SDD group, mortality rate was comparable in both drain groups. This might be because the studies included were not sufficiently powered to show such an association. Further, most intraparenchymal injuries might lead to transient or permanent morbidity (e.g., hemiparesis or aphasia), however, they are not necessarily fatal.

Our analysis showed no significant difference between the groups concerning infection rate, while SPD showed significant lower rates of overall morbidity. The infection rates in both groups, based on the included studies, were 1.7% for SPD and 1.9% for SDD. The only study showing significantly lower rates of infections in the SPD group (2 vs. 9%) was the RCT by Soleman et al. (7). A possible explanation is that other studies (5, 11), which also showed absolutely lower numbers of infection in the SPD group, were underpowered for such an analysis and therefore statistical significance was not reached (5, 11). Infection rate in cSDH are reported to occur in up to 20% of the cases, while empyema rates are much lower (around 2%) (6). In theory, SPD might lower the risk of subdural empyema or other types of “deep” infections, since no foreign material is placed within the subdural cavity or in proximity to the cortical surface.

Santarius et al. showed that the insertion of a drain improves outcome and lowers mortality in cSDH when compared to no drain insertion (3). Based on our pooled analysis the type of drain did not seem to influence clinical outcome. From the included studies, only Kaliaperumal et al. showed significantly better outcome in the SPD group compared to the SDD group (12). All other studies describing outcome showed comparable clinical outcomes in both groups. Interestingly, although recurrence rates, overall morbidity, and intraparenchymal injury rates were significantly lower in the SPD group, clinical outcome was comparable between the groups. One possible explanation might be that the included studies were not powered to detect the true outcome within the groups. Further, morbidity due to misplacement of the drain might be only transient showing an improvement with time. Last, in most studies clinical outcome was assessed in a retrospective manner, with many dropouts or missing information, potentially leading to biased results. Studies focusing and analyzing the outcome after insertion of an SPD compared to SDD in a prospective manner are still needed.

Recently published analysis by Pranata et al., Xie et al., and Ding et al. confirmed our results (23–25). Xie et al. (24), Ding et al. (25), and Pranata et al. (23) also observed significantly lower recurrence rates for SPD as opposed to SDD (Xie et al. and Ding et al.: 12 and 12.7%, Pranata et al.: 12 and 13.2%).

Significantly lower misplacement rates were reported in favor of SPD by Xie et al. (1.% and 2.6%) (24), Ding et al. (0.6 and 2.3%) (25) and Pranata et al. (2.2 and 4.7%) (23). Mortality rates were similar for both groups in all three analysis. Xie et al. (24) reported a mortality rate of 3.7 and 3.8%, in favor of SPD. Ding et al. (25) observed a lower mortality rate for SDD (4.8 and 4.5%) while Pranata et al. (23) published much higher absolute values of 15.7 and 9.4% in favor of SDD.

Comparable outcome rates for both groups were published by Xie et al. (87.4 and 82.1%) (24) Ding et al. (68.1 and 67.5%) (25) and Pranata et al. (percentage not available) (23).

Our assessment and classification of the included studies differed from the meta-analyses by Pranata et al. and Ding et al. while concurred with Pranata et al., Xie et al., Ding et al. (23–25). We considered the study by Soleman et al. (7) the only RCT amongst the included studies, while Pranata et al. and Ding et al. (23, 25) considered Häni et al. (8) and Kaliaperumal et al., respectively, only Kaliaperumal et al. (12), as additional RCTs. We did not consider the study by Kaliaperumal et al. an RCT since the drain type was not randomized (the patients were assigned alternately to SPD and SDD). Similarly, the study by Häni and colleagues was not considered by us as an RCT, since it was not primarily designed to compare the recurrence rate after insertion of SPD or SDD, but rather a post-hoc analysis of their RCT initially designed to assess the need for follow up CT after evacuation of cSDH (8). In addition, as opposed to Pranata et al., Xie et al., and Ding et al. (23–25), we combined the two different subdural drain types described by Sjavik et al. into one SDD group (13) and we included the “as treated” rather than the “intention to treat” results from the study by Häni et al. (8) in our analysis. Ding et al. was the only study to report subgroup analysis in their analysis (25). Due these reasons, some of our results differed from the previous reports published.

Despite conducting a current systematic review and meta-analysis of the existing literature, our study consists of some limitations. First, we only searched two databases (Pubmed and Embase) and only included English literature, which carries a risk of omitting important data published elsewhere. Second, in our review and analysis we included RCTs, as well as non-randomized prospective cohort studies and retrospective cohort studies. Therefore, the data included was somewhat heterogeneous, potentially influencing the validity of our results. However, to date only one RCT and very few well-designed prospective trials have been published on the matter. Third, even though we assessed for publication bias we cannot exclude a general publication bias, due to unpublished negative studies, which are not included in our meta-analysis. Last, most data included in this meta-analysis derived from retrospective cohorts, inherent to all limitations of such studies, potentially influencing the validity of the results as well.

Based on the existing evidence, the insertion of an SPD after burr-hole drainage of cSDH seems superior to SDD, in terms of recurrence, overall morbidity, drain misplacement, and intraparenchymal injury rates, while showing comparable infection rates, mortality, and clinical outcome. Therefore, the insertion of SPD after drainage of cSDH should be encouraged. Further prospective studies on the clinical outcome and mortality after insertion of SPD or SDD should be focus of future research.

All datasets generated for this study are included in the article/supplementary material. The R code is available upon request.

LG carried out the database search, review of the literature, data extraction, and analysis as well as drafting of the manuscript. NH was the second researcher who independently reviewed the literature and performed quality and bias assessments. JS the senior author, overviewed the project and took final decisions about inclusion of papers if LG and NH could not reach a conclusion. He critically reviewed the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.