Abstract: Nine RCTs, including a total of 968 participants, reported outcomes specified by this review. Only one RCT reported the use of an adequate method of allocation concealment; this trial was a large, single-centre, high quality study and was adequately reported. All included trials reported a reduced recurrence of CSDH with external subdural drains. We found a significant reduction in the risk of recurrence with subdural drains (RR 0.45, 95% CI 0.32 to 0.61, I2 = 38%; 9 studies, 968 participants; moderate-quality evidence). There was no strong evidence of any increase in complications (RR 1.15; 95% CI 0.77 to 1.72, I2 = 0%; 7 studies, 710 participants; low-quality evidence), mortality (RR 0.78, 95% CI 0.45 to 1.33, I2 = 22%; 5 studies, 539 participants; low-quality evidence), or poor functional outcome (which included deaths) (RR 0.68, 95% CI 0.44 to 1.05, I2 = 31%; 5 studies, 490 participants; low-quality evidence). There is some evidence that postoperative drainage is effective in reducing the symptomatic recurrence of CSDH. Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate. Due to the low quality of the evidence for the secondary outcomes, the effect of drainage on the occurrence of surgical complications, mortality and poor functional outcome is uncertain. This uncertainty can be clarified with data from high-quality studies which may be conducted in the future. There is no strong evidence of any increase in complications when drains are used.

Plain-language summary: The review authors conducted extensive searches of the medical literature up to November 2014 for relevant randomized controlled trials (RCTs), which provide the most reliable evidence. They identified nine RCTs, involving 968 participants, that compared the use of external drains in some patients versus no drains in other patients after burr-hole surgery for CSDH. The trials were conducted in India, Turkey, Iran, Germany, the UK and Japan. All participants were adults, mostly over 60 years of age. All the trials used very similar surgical procedures. Six trials followed participants for six months, the remaining trials followed them for three months, one month, or three weeks (one trial each). The authors were able to pool the results of the trials statistically, and this showed that use of drains does reduce the risk of recurrence of CSDH after burr-hole surgery by about 50% compared to the risk in the group of patients who did not have drains (the control group). However, there were no clear differences between the drain and no-drain treatment groups for postoperative complications (i.e. infection, seizures or sudden bleeding), death, or functional outcome (i.e. regaining abilities affected by the CSDH). The results of this review may change in the future when data are available from additional studies. The existing studies have either too few participants or events to give a reliable result, even when the results are pooled. Some of the studies did not describe the randomisation procedures in detail, and are considered to be of lower quality because of this. Further research will also help to establish: - the effects of external drains on postoperative complications, death, and functional outcome; - whether it is better to use one or two burr holes during surgery; - the best way to position the drain’s tube within the brain; - the best duration of drainage.