We included 27 studies (n = 3287) that met our selection criteria in our review (Additional file; Fig.1)[14–20, 22–41]. Year of publication ranged from 1996 to 2020 (Additional file; Table2). We did not identify additional studies through bibliographical review.
The median duration of follow up was 6 months (IQR 3–12, Table1), the median sample size of the multivariable analysis was 104 (IQR 80–154), and 76% of patients received aneurysm treatment. One study did not report on how many patients were provided aneurysm treatment and in one study aneurysm treatment was not provided at all. Most studies had a single center (n = 17, 63%) and retrospective (n = 24, 89%) design.
The studies investigated 82 early predictors of functional outcome with multivariable regression analysis. Taking into account predictor definition, reporting quality, and, if present, categorization we were able to conduct a systematic review of sixteen and meta-analysis of nine early predictors. We meta-analyzed age per decade increase, sex, clinical grade, pupillary light reflex, clinical improvement before aneurysm treatment, modified Fisher grade, and presence of hydrocephalus, intraventricular hemorrhage (IVH), and intracerebral hematoma (ICH) on admission imaging (Additional file; Table3). Aneurysm size, aneurysm location, Glasgow Coma Scale (GCS), Fisher grade, other concomitant bleeding, brain infarction on admission imaging, and leukocytosis were suitable for systematic review (Additional file; Table4).
We included fifteen studies in the systematic review of the early predictor age. Seven studies were eligible for meta-analysis (n = 865). The likelihood of favorable functional outcome decreased with older age (per decade, pooled aOR 0.7, 95% CI 0.5-1.0, Fig.1)[24–27, 31, 38, 39]. We observed moderate funnel plot asymmetry, and after adjusting for publication bias the effect of age was no longer significant (p = 0.10, Additional file; Fig.2A-B). In the eight studies not eligible for meta-analysis older age was often associated with worse functional outcome[14, 15, 19, 29, 35–37, 41].
We included six studies in the systematic review investigating the effect of sex on functional outcome. Five studies (n = 427) were eligible for meta-analysis [15, 23, 26, 32, 35]. We did not observe an association between sex and the likelihood of favorable functional outcome (pooled aOR 0.5, 95% CI 0.1–1.4, Fig.2). One study was not eligible for meta-analysis and found no association between age and functional outcome[41].
We included thirteen studies in the systematic review of clinical grade on the likelihood of favorable functional outcome. Ten studies (n = 1471) were eligible for meta-analysis. The pooled aOR of WFNS grade IV versus V and H-H grade IV versus V was 2.9 (95% CI 1.9–4.3, Fig.3)[17, 23–25, 27, 30, 34, 36, 37, 40]. The effect estimate for clinical grade was similar when including only studies investigating WFNS grade and not H-H grade[17, 24, 34, 36, 40]. In three studies not included in the meta-analysis higher clinical grade was associated with poorer outcome[14, 16, 18].
We included three studies (n = 560, (11%)) investigating the effect of clinical improvement before aneurysm treatment on the likelihood of favorable functional outcome[17, 30, 36]. The pooled aOR was 3.3 (95% CI 2.0-5.3, Fig.4). Further, we reviewed GCS on admission as an early predictor. Three studies included in the systematic review reported an increased likelihood of favorable functional outcome with increasing GCS[15, 20, 31, 41].
We included three studies (n = 641) in the systematic review and meta-analysis of the effect of intact pupillary light reflex on admission[20, 26, 31]. The pooled aOR was 2.9 (95% CI 1.6–5.1, Fig.5).
We included seven studies in the systematic review of the effect of presence of ICH on admission imaging on the likelihood of favorable functional outcome. Three studies (n = 355) were eligible for meta-analysis[23, 26, 34]. The pooled aOR was 0.4 (95% CI 0.2–0.8, Fig.6). The remaining four studies did not report a significant effect of ICH on functional outcome[25, 28, 32, 41].
We included three studies (n = 726) in the meta-analysis of the effect of modified Fisher grade per grade on the likelihood of favorable functional outcome[18, 20, 31]. We found a pooled aOR of 0.4 (95% CI 0.3–0.5, Fig.7). We included six studies in the systematic review investigating the effect of Fisher grade on functional outcome[19, 23, 25, 36, 37, 40, 41]. Three studies reported a significant association of higher Fisher grade with functional outcome.
We included five studies in the systematic review of the effect of presence of hydrocephalus before aneurysm treatment on functional outcome. Three studies (n = 321) were eligible for meta-analysis[23, 27, 39]. The pooled aOR was 1.0 (95% CI 0.3–2.7, Fig.8). Two studies were not eligible for meta-analysis. Neither found a significant association with functional outcome[29, 40].
We included seven studies in the systematic review of the effect of presence of IVH on admission imaging on the likelihood of favorable functional outcome. Three studies were eligible for meta-analysis (n = 272)[16, 26, 41]. The pooled aOR was 1.8 (95% CI 0.3–12.8, Fig.9). Four studies were not eligible for meta-analysis and analyzed in with systematic review. Two found an association of IVH with functional outcome[30, 41] [32, 40].
Additionally, we conducted a systematic review of aneurysm size, aneurysm location, presence of brain infarction on admission imaging, leukocytosis, and other concomitant bleeding in relation to function outcome (Additional file; Table5).
We performed subgroup analyses for length of follow-up, for favorable outcome definition, and for studies including only patients that received aneurysm treatment for the predictors age, sex, and clinical grade, which showed similar results as the main analysis. The overall risk of bias as assessed with the QUIPS ROB tool for prognostic studies was high (Fig.10, and Additional file; Fig.3).