Introduction
Aneurysmal subarachnoid hemorrhage (aSAH) is a medical emergency with high mortality and morbidity [
1,
2]. The contribution of delayed cerebral ischemia (DCI) on outcome is undisputed, although the relief of cerebral vasospasm in the subacute phase after aSAH failed to improve functional outcome [
3]. Despite advances in neurointensive care, the underlying mechanisms of secondary brain injury remain incompletely understood. Animal data support the importance of pathophysiologic mechanisms in the very early phase after SAH with changes including early vasospasm, inflammation, and global cerebral edema (GCE) [
4]. Early brain injury (EBI) is now being recognized as an important cause of mortality and disability after SAH in humans and may be associated with DCI [
5]. So far, pathophysiologic mechanisms related to EBI are under-investigated in humans, and no treatment is available to adequately address these processes. Although difficulties exist in translating findings from the experimental setting to the patients’ bedside, animal data convincingly provide evidence of neuronal damage within minutes after SAH triggered by brain tissue hypoxia, cerebral inflammation, blood-brain barrier (BBB) breakdown, and others [
4]. Monitoring of such events in the very early phase in humans is challenging; however, invasive multimodal neuromonitoring devices allow continuous data acquisition for intracranial pressure (ICP), brain tissue oxygen tension (P
btO
2), cerebral blood flow, and at least hourly information on brain metabolism already within the first 24 hours after aneurysm bleeding [
6]. Using multimodal neuromonitoring data, we previously showed derangement in cerebral metabolism and increased episodes of brain tissue hypoxia in the first days after aSAH in patients with radiologic evidence of GCE compared with those without GCE [
7]. The pro-inflammatory cytokine interleukin-6 (IL-6) in the cerebral microdialysate as a marker for neuroinflammation has been shown to be associated with DCI and unfavorable outcome following aSAH [
8-
10]. Matrix metalloproteinases (MMPs) are involved in vascular remodeling, neuroinflammation, BBB breakdown, and neuronal apoptosis [
11-
13]. In the experimental setting, MMP-9 potentiates EBI and was associated with apoptosis of hippocampal neurons of rats [
11]. In patients with SAH, MMP-9 was associated with disease severity and the development of cerebral vasospasm [
14,
15].
The goal of the current study was to study pathophysiological events involved in the development of EBI in poor-grade aSAH patients by investigating brain hemodynamics—ICP, cerebral perfusion pressure (CPP), and PbtO2—and brain metabolic changes in combination with the local inflammatory response by cerebral microdialysis (CMD)-IL-6 and the function of the BBB by CMD-MMP-9 in the brain extracellular fluid. We intended to focus on the early phase after aSAH and relate these findings to clinical course and outcome.
Discussion
EBI is increasingly recognized to play a key role in pathophysiologic changes contributing to poor functional outcome and mortality after aSAH. Here, we report evidence of brain metabolic derangement, brain tissue hypoxia, neuroinflammation, and BBB disruption in the first 72 hours of neuromonitoring in patients with poor-grade aSAH. Discovering mechanisms of EBI in humans may open the opportunity to target specific treatment endpoints in the early phase after SAH.
Neuroinflammation is increasingly recognized as an innate cerebral response to primary brain injury [
21]. In the present study, we did not find an association between higher CMD-IL-6 levels and systemic inflammation, supporting the idea of compartmentalization of the central nervous system.
Pro-inflammatory cytokines may enhance brain edema through disruption of the BBB and induce neuronal apoptosis and therefore directly contribute to early brain damage [
22,
23]. Cerebral IL-6 has an estimated half-life of several hours and is produced by microglia, astrocytes, and neurons [
24]. In previous studies using cerebral microdialysis, the pro-inflammatory cytokine IL-6 was associated with SAH disease severity, the development of DCI, and poor outcome [
8-
10]. In the present study, we furthermore found an association with admission GCE, metabolic derangement, and a CPP of less than 70 mm Hg. The association between high CMD-LPR and a CPP<70mmHg has been previously reported in SAH patients with admission GCE [
7]. Defining the optimal CPP in the early phase after SAH remains a challenge without having predefined brain physiologic endpoints even after aneurysm securing. Brain multimodal monitoring data may be used to target endpoints on the cellular level. In a series of 30 patients with poor-grade SAH, a CPP of less than 70 mm Hg was associated with metabolic distress and brain tissue hypoxia; however, these data cannot be extrapolated to the first 72 hours after SAH [
25]. A higher CPP was associated with improved brain metabolism reflected by a lower LPR in a retrospective analysis of aSAH patients with admission GCE [
7]. Improving substrate delivery especially in the early phase after SAH may be beneficial in patients with increased need. As shown in patients with traumatic brain injury, CPP augmentation may translate into increased P
btO
2 and a reduction in oxygen extraction fraction [
26]. However, a beneficial effect on brain metabolism was not observed. Defining the optimal CPP in the early phase after SAH and identifying patients who may benefit from early augmentation of CPP remain important issues for future research and should include multimodal neuromonitoring data as treatment endpoints.
Another potential treatment target in the early phase after aSAH is to suppress neuroinflammation by the application of systemic anti-inflammatory drugs. Potential benefits in patients with SAH have been postulated [
27-
29] and are furthermore supported by the improvement of cerebral edema and decreasing neuronal cell apoptosis in experimental SAH models [
30]. With the limitation of associated hemodynamic side effects [
31] when applied as a rapid infusion, a continuous low-dose infusion may be considered [
32].
We found an early upregulation of CMD-MMP-9 in our study population, and higher levels were associated with disease severity, loss of consciousness at ictus, and early brain tissue hypoxia. Loss of consciousness at ictus is highly correlated with poor clinical grade and the development of early or delayed brain edema [
18]. MMP-9 contributes to endothelial basal membrane damage, neuroinflammation, and apoptosis and therefore plays a pivotal role in EBI [
11-
13]. Serum-MMP-9 levels were elevated in patients who developed cerebral vasospasm, although both an initial upregulation and a sustained prolonged increase have been described [
15,
33]. This again supports the importance of local measurements in the brain as serum markers may reflect a dilution of the innate cerebral response or exaggerated systemic levels originating from multiple organ systems [
14,
21]. Antagonizing MMP-9 diminished cortical apoptosis, was associated with improved outcome after experimental SAH [
34,
35], and was recently postulated as potential therapy in ischemic stroke [
36].
Bedside analysis of standard metabolic parameters in the cerebral microdialysate revealed a high LPR and an increased release of the excitatory amino-acid glutamate into the extracellular compartment. LPR expresses the redox state of the cell, which is determined by oxygen availability and oxidative metabolism. Glutamate levels were highest at the start of monitoring and gradually returned to near normal baseline values, which has been nicely documented in experimental SAH models [
37]. This parallel increased level of LPR is indicative for tissue ischemia and therefore strongly suggestive of global cerebral ischemia in our poor-grade population. However, our monitoring devices were implanted when cerebral recirculation already occurred. Based on pyruvate levels in the normal range in combination with a high LPR, the metabolic profile may also suggest post-ischemic mitochondrial dysfunction, especially in the absence of brain tissue hypoxia 48 hours after ictus. Mitochondrial dysfunction may be diagnosed bedside by using standard metabolic data derived from CMD and was recently investigated in 55 patients with poor-grade SAH [
38]. The authors describe a more-than-sevenfold-higher incidence of episodes of mitochondrial dysfunction compared with episodes of cerebral ischemia as cause for disturbed cerebral energy metabolism in patients with SAH [
38]. Although no specific treatment to improve mitochondrial dysfunction is currently available, further research is warranted as mitochondrial dysfunction may increase tissue sensitivity to secondary adverse events such as vasospasm and decreased cerebral blood flow.
We observed improvement in brain metabolism and P
btO
2 over the monitoring time most likely secondary to the parallel increase in CPP. Brain extracellular glucose concentrations significantly decreased to a critical level in a substantial amount of patients, whereas systemic glucose levels remained constant and this is suggestive of increased cerebral glucose consumption. Achieving normal cerebral glucose levels should be recommended as neuroglucopenia is associated with metabolic distress and poor outcome after SAH [
39].
Quantifying brain metabolism and neuroinflammation may be of importance as both were associated with poor functional outcome. All statistical models were corrected for important covariates, including probe location, as in half of our patients the microdialysis catheter was within 1 cm from the lesion. ‘Perilesional’ probe positioning implies that the microdialysate was collected adjacent to radiological damaged brain tissue, where cell necrosis, blood compounds, autophagy, and apoptosis may alter brain metabolism and ameliorate cytokine release into the extracellular compartment.
Our study was designed as a pilot study and included only a small number of patients and this is a potentially limiting factor. Moreover, early pathophysiologic changes described in the present study may be relevant for patients with poor-grade aSAH and not be generalizable to all clinical grades. We were not able to define specific treatment targets based on the following limitations: (1) a localized metabolic information using cerebral microdialysis technique (2) the small sample size and (3) local treatment strategies which may differ from other institutional protocols and substantially influence longitudinal brain physiologic data. Importantly, patient- and disease-specific data were prospectively documented, and statistical models were corrected for important covariates.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
RH was involved in the idea, study design, interpretation of data, statistical analysis, and writing of the manuscript and final revision of the manuscript. AS made substantial contributions to the design and data acquisition, analysis, and interpretation and performed laboratory analysis of brain-derived biomarkers. RB and BP made substantial contributions to the idea, study design, data analysis and interpretation, and final revision of the manuscript. ES made substantial contributions to the idea, study design, data interpretation, and final revision of the manuscript. AD, APA, FS, MF, and CH contributed to the design, data acquisition, and interdisciplinary data interpretation and performed laboratory analysis of brain-derived biomarkers. WOH made substantial contributions to the design, data acquisition (multimodal neuromonitoring high-frequency data), and data interpretation. PL substantially contributed to the study design, statistical analysis, and data interpretation. PR performed radiographic analysis as independent radiologist and substantially contributed to the data interpretation. CT substantially contributed to the design and data interpretation and performed placement of multimodal neuromonitoring devices. All authors critically reviewed, drafted, and approved the final version of the manuscript and agree to be accountable for all aspects concerning the work.