Traumatic brain injury (TBI) is an important cause of disability and mortality in the western world. While the initial injury sustained results in damage, it is the subsequent secondary cascade that is thought to be the significant determinant of subsequent outcomes. The changes associated with the secondary injury do not become irreversible until some time after the start of the cascade. This may present a window of opportunity for therapeutic interventions aiming to improve outcomes subsequent to TBI. A prominent contributor to the secondary injury is a multifaceted inflammatory reaction. The complement system plays a notable role in this inflammatory reaction; however, it has often been overlooked in the context of TBI secondary injury. The complement system has homeostatic functions in the uninjured central nervous system (CNS), playing a part in neurodevelopment as well as having protective functions in the fully developed CNS, including protection from infection and inflammation. In the context of CNS injury, it can have a number of deleterious effects, evidence for which primarily comes not only from animal models but also, to a lesser extent, from human post-mortem studies. In stark contrast to this, complement may also promote neurogenesis and plasticity subsequent to CNS injury. This review aims to explore the role of the complement system in TBI secondary injury, by examining evidence from both clinical and animal studies. We examine whether specific complement activation pathways play more prominent roles in TBI than others. We also explore the potential role of complement in post-TBI neuroprotection and CNS repair/regeneration. Finally, we highlight the therapeutic potential of targeting the complement system in the context of TBI and point out certain areas on which future research is needed.

Traumatic brain injury (TBI) is a major cause of mortality and morbidity worldwide. There are currently no early biomarkers for prognosis in routine clinical use. Interleukin-6 (IL-6) is a potential biomarker in the context of the established role of neuroinflammation in TBI recovery. Therefore, a systematic review of the literature was performed to assess and summarise the evidence for IL-6 secretion representing a useful biomarker for clinical outcomes. A multi-database literature search between January 1946 and July 2021 was performed. Studies were included if they reported adult TBI patients with IL-6 concentration in serum, cerebrospinal fluid (CSF) and/or brain parenchyma analysed with respect to functional outcome and/or mortality. A synthesis without meta-analysis is reported. Fifteen studies were included, reporting 699 patients. Most patients were male (71.7%), and the pooled mean age was 40.8 years; 78.1% sustained severe TBI. Eleven studies reported IL-6 levels in serum, six in CSF and one in the parenchyma. Five studies on serum demonstrated higher IL-6 concentrations were associated with poorer outcomes, and five showed no signification association. In CSF studies, one found higher IL-6 levels were associated with poorer outcomes, one found them to predict better outcomes and three found no association. Greater parenchymal IL-6 was associated with better outcomes. Despite some inconsistency in findings, it appears that exaggerated IL-6 secretion predicts poor outcomes after TBI. Future efforts require standardisation of IL-6 measurement practices as well as assessment of the importance of IL-6 concentration dynamics with respect to clinical outcomes, ideally within large prospective studies. Prospero registration number: CRD42021271200

Objectives: Acute subdural haematoma (ASDH) is a devastating pathology commonly found on CT brain scans of patients with traumatic brain injury. The role of surgical intervention in the elderly has been increasingly questioned due to its associated morbidity and mortality. Therefore, a systematic review and meta-analysis of the literature to quantify the mortality and functional outcomes associated with surgical management of ASDH in the elderly was performed. Design/setting: A multidatabase literature search between January 1990 and May 2020, and meta-analysis of proportions was performed to quantify mortality and unfavourable outcome (Glasgow Outcome scale 1–3; death/ severe disability) rates. Participants: Studies reporting patients aged 60 years or older. Interventions: Craniotomy, decompressive craniectomy, conservative management. Outcome measures: Mortality and functional outcomes (discharge, long-term follow-up (LTFU)). Results: 2572 articles were screened, yielding 21 studies for final inclusion and 15 for meta-analysis. Pooled estimates of mortality were 39.83% (95% CI 32.73% to 47.14%; 10 studies, 308/739 patients, I ² =73%) at discharge and 49.30% (95% CI 42.01% to 56.61%; 10 studies, 277/555 patients, I ² =63%) at LTFU. Mean duration of follow-up was 7.1 months (range 2–12 months). Pooled estimate of percentage of poor outcomes was 81.18% (95% CI 75.61% to 86.21%; 6 studies, 363/451 patients, I ² =45%) at discharge, and 79.25% (95% CI 72.42% to 85.37%; 8 studies, 402/511 patients, I ² =66%) at LTFU. Mean duration of follow-up was 6.4 months (range 2–12 months). Potential risk factors for poor outcome included age, baseline functional status, preoperative neurological status and imaging parameters. Conclusions: Outcomes following surgical evacuation of ASDH in patients aged 60 years and above are poor. This constitutes the best level of evidence in the current literature that surgical intervention for ASDH in the elderly carries significant risks, which must be weighed against benefits. PROSPERO registration number: CRD42020189508.