Bench-to-bedside review: a possible resolution of the glucose paradox of cerebral ischemia

Blood and Brain Metabolites after Cerebral Ischemia

The study of an organism's response to cerebral ischemia at different levels is essential to understanding the mechanism of the injury and protection. A great interest is devoted to finding the links between quantitative metabolic changes and post-ischemic damage. This work aims to summarize the outcomes of the most studied metabolites in brain tissue-lactate, glutamine, GABA (4-aminobutyric acid), glutamate, and NAA (N-acetyl aspartate)-regarding their biological function in physiological conditions and their role after cerebral ischemia/reperfusion. We focused on ischemic damage and post-ischemic recovery in both experimental-including our results-as well as clinical studies. We discuss the role of blood glucose in view of the diverse impact of hyperglycemia, whether experimentally induced, caused by insulin resistance, or developed as a stress response to the cerebral ischemic event. Additionally, based on our and other studies, we analyze and critically discuss post-ischemic alterations in energy metabolites and the elevation of blood ketone bodies observed in the studies on rodents. To complete the schema, we discuss alterations in blood plasma circulating amino acids after cerebral ischemia. So far, no fundamental brain or blood metabolite(s) has been recognized as a relevant biological marker with the feasibility to determine the post-ischemic outcome or extent of ischemic damage. However, studies from our group on rats subjected to protective ischemic preconditioning showed that these animals did not develop post-ischemic hyperglycemia and manifested a decreased metabolic infringement and faster metabolomic recovery. The metabolomic approach is an additional tool for understanding damaging and/or restorative processes within the affected brain region reflected in the blood to uncover the response of the whole organism via interorgan metabolic communications to the stressful cerebral ischemic challenge.

Ischemic Stroke, Glucocorticoids, and Remote Hippocampal Damage: A Translational Outlook and Implications for Modeling

Progress in treating ischemic stroke (IS) and its delayed consequences has been frustratingly slow due to the insufficient knowledge on the mechanism. One important factor, the hypothalamic-pituitary-adrenocortical (HPA) axis is mostly neglected despite the fact that both clinical data and the results from rodent models of IS show that glucocorticoids, the hormones of this stress axis, are involved in IS-induced brain dysfunction. Though increased cortisol in IS is regarded as a biomarker of higher mortality and worse recovery prognosis, the detailed mechanisms of HPA axis dysfunction involvement in delayed post-stroke cognitive and emotional disorders remain obscure. In this review, we analyze IS-induced HPA axis alterations and supposed association of corticoid-dependent distant hippocampal damage to post-stroke brain disorders. A translationally important growing point in bridging the gap between IS pathogenesis and clinic is to investigate the involvement of the HPA axis disturbances and related hippocampal dysfunction at different stages of SI. Valid models that reproduce the state of the HPA axis in clinical cases of IS are needed, and this should be considered when planning pre-clinical research. In clinical studies of IS, it is useful to reinforce diagnostic and prognostic potential of cortisol and other HPA axis hormones. Finally, it is important to reveal IS patients with permanently disturbed HPA axis. Patients-at-risk with high cortisol prone to delayed remote hippocampal damage should be monitored since hippocampal dysfunction may be the basis for development of post-stroke cognitive and emotional disturbances, as well as epilepsy.

Metabolome of Cerebral Thrombi Reveals an Association between High Glycemia at Stroke Onset and Good Clinical Outcome

Despite the fact that glucose is the main fuel of the brain, hyperglycemia at hospital admission is generally associated with a poor functional outcome in stroke patients. This paradox may be explained by the lack of information about the blood glucose level at stroke onset. Here, we analyzed the metabolome of blood cells entrapped in cerebral thrombi to gain insight into their metabolism at stroke onset. Fourty-one consecutive stroke patients completely recanalized by mechanical thrombectomy within 6 h were included. The metabolome of retrieved thrombi was analyzed by liquid chromatography tandem with mass spectrometry. Discriminant Analysis (sparse Partial Least Squares Discriminant Analysis (sPLS-DA)) was performed to identify classification models and significant associated features of favorable clinical outcome at 3 months (modified Rankin Scale (mRS) < 2). sPLS-DA of the metabolomes of cerebral thrombi discriminated between stroke patients with a favorable or poor clinical outcome (Area Under the Curve (AUC) = 0.992 (0.931–1)). In addition, our results revealed that high sorbitol and glucose levels in the thrombi positively correlated with favorable clinical outcomes. Sorbitol, a short-term glycemic index reflecting a high blood glucose level at stroke onset, was found to be an independent predictor of good outcome (AUC = 0.908 (0.807–0.995)). This study demonstrates that a high blood glucose level at stroke onset is beneficial to the clinical outcome of the patient.

Thrombolysis Outcomes in Patients with Diabetes and Previous Stroke: A Meta-Analysis

BACKGROUND

Intravenous tissue-type plasminogen activator (IVtPA) is a proven treatment for acute ischemic stroke; however, diabetes mellitus (DM) and previous cerebral infarction (PCI) were considered relative contraindications for thrombolysis within the 3-4.5 h period.

OBJECTIVE

The study aimed to determine the safety and efficacy of IVtPA among diabetic patients with PCI presenting with acute ischemic stroke.

METHODS

Studies which evaluated the outcome of IVtPA in terms of symptomatic intracerebral hemorrhage (sICH), functional outcome in modified Rankin scale, and death among diabetic patients with PCI presenting with acute ischemic stroke within the 3-4.5 h period were systematically searched until July 2019. Screening and eligibility criteria were applied. Risk of bias was evaluated using the Newcastle-Ottawa Scale. Odds ratios (ORs) with 95% confidence interval (CI) were used to compare measures of treatment effect. Mantel-Haenszel method and random-effects model were also employed.

RESULTS

Four registry-based studies with a total of 44,572 patients were included for quantitative synthesis. Giving IVtPA among DM+/PCI+ patients did not result in significantly increased rate of sICH (OR, 1.09; 95% CI, 0.88, 1.36) compared to No DM+/PCI+ patients. However, there was significantly higher mortality (OR, 1.81; 95% CI, 1.60, 2.06) in the DM+/PCI+ group. Conversely, among those who survived, the DM+/PCI+ patients were more functionally independent at 3 months (OR, 0.76; 95% CI, 0.61, 0.94).

CONCLUSION

Limited evidence suggests that thrombolysis in DM+/PCI+ patients does not result in significantly higher incidence of sICH and may improve functional independence. However, the significantly higher mortality in this group warrants an assessment of the individualized risk-benefit ratio in the use of IVtPA.

Glucocorticoids and preterm hypoxic-ischemic brain injury: the good and the bad

Fetuses at risk of premature delivery are now routinely exposed to maternal treatment with synthetic glucocorticoids. In randomized clinical trials, these substantially reduce acute neonatal systemic morbidity, and mortality, after premature birth and reduce intraventricular hemorrhage. However, the overall neurodevelopmental impact is surprisingly unclear; worryingly, postnatal glucocorticoids are consistently associated with impaired brain development. We review the clinical and experimental evidence on how glucocorticoids may affect the developing brain and highlight the need for systematic research.

Comparison of outcomes following thrombolytic therapy among patients with prior stroke and diabetes in the Virtual International Stroke Trials Archive (VISTA)

OBJECTIVE

The use of alteplase in patients who have had a prior stroke and concomitant diabetes is not approved in Europe. To examine the influence of diabetes and prior stroke on outcomes, we compared data on thrombolysed patients with nonthrombolysed comparators.

RESEARCH DESIGN AND METHODS

We selected patients with ischemic stroke on whom we had data on age, pretreatment baseline National Institutes of Health Stroke Scale (b-NIHSS), and 90-day outcome measures (functional modified Rankin score [mRS]) and neurological measures [NIHSS]) in the Virtual International Stroke Trials Archive. We compared outcomes between thrombolysed patients and nonthrombolysed comparators in those with and without diabetes, those who have had a prior stroke, or both and report findings using the Cochran-Mantel-Haenszel (CMH) test and proportional odds logistic regression analyses. We report an age-adjusted and b-NIHSS-adjusted CMH P value and odds ratio (OR).

RESULTS

Rankin data were available for 5,817 patients: 1,585 thrombolysed patients and 4,232 nonthrombolysed comparators. A total 1,334 (24.1%) patients had diabetes, 1,898 (33.7%) patients have had a prior stroke, and 491 (8%) patients had both. Diabetes and nondiabetes had equal b-NIHSS (median 13; P = 0.3), but patients who have had a prior stroke had higher b-NIHSS than patients who have not had a prior stroke (median 13 vs. 12; P < 0.0001). Functional outcomes were better for thrombolysed patients versus nonthrombolysed comparators among both nondiabetic (P < 0.0001; OR 1.4 [95% CI 1.3-1.6]) and diabetic (P = 0.1; 1.3 [1.05-1.6 ]) subjects. Similarly, outcomes were better for thrombolysed patients versus nonthrombolysed comparators among who have not had a prior stroke (P < 0.0001; 1.4 [1.2-1.6 ]) and those who have (P = 0.02; 1.3 [1.04-1.6 ]). There was no interaction of diabetes and prior stroke with treatment (P = 0.8). Neurological outcomes were consistent with the mRS.

CONCLUSIONS

Outcomes from thrombolysis are better among patients with diabetes and/or those who have had a prior stroke than in control subjects. Withholding thrombolytic treatment from otherwise-eligible patients may not be justified.

Treatment of lactic acidosis: appropriate confusion

BACKGROUND

Lactic acidosis (LA) is common in hospitalized patients and is associated with poor clinical outcomes. There have been major recent advances in our understanding of lactate generation and physiology. However, treatment of LA is an area of controversy and uncertainty, and the use of agents to raise pH is not clearly beneficial.

AIM AND METHODS

We reviewed animal and human studies on the pathogenesis, impact, and treatment of LA, published in the English language and available through the PubMed/MEDLINE database. Our aim was to clarify the physiology of the generation of LA, its impact on outcomes, and the different treatment modalities available. We also examined relevant data regarding LA induced by medications commonly prescribed by hospitalists: biguanides, nucleoside analog reverse-transcriptase inhibitors (NRTIs), linezolid, and lorazepam.

RESULTS/CONCLUSIONS

Lactic acid is a marker of tissue ischemia but it also may accumulate without tissue hypoperfusion. In the latter circumstance, lactic acid accumulation may be an adaptive mechanism-a novel possibility quite in contrast to the traditional view of lactic acid as only a marker of tissue ischemia. Studies on the treatment of LA with sodium bicarbonate or other buffers fail to show consistent clinical benefit. Severe acidemia in the setting of LA is a particularly poorly studied area. In the settings of medication-induced LA, optimal treatment, apart from prompt cessation of the offending agent, is still unclear.

Exacerbation of systemic inflammation and increased cerebral infarct volume with cardiopulmonary bypass after focal cerebral ischemia in the rat

OBJECTIVE

Stroke remains a significant contributor to morbidity and mortality after cardiac surgery. Cardiopulmonary bypass is known to induce a significant inflammatory response, which could adversely influence outcomes. We hypothesized that cardiopulmonary bypass, through an enhanced systemic inflammatory response, might affect outcomes after focal cerebral ischemia.

METHODS

Wistar rats (275-300 g) were anesthetized, surgically prepared for cardiopulmonary bypass and right middle cerebral artery occlusion, and randomly allocated to 2 groups: focal cerebral ischemia alone (n = 9) and focal cerebral ischemia combined with normothermic cardiopulmonary bypass (n = 8). Serum cytokines (tumor necrosis factor alpha and interleukins 1beta, 6, and 10) were measured at baseline, at end of bypass, and at 2, 6, and 24 hours after bypass. On postoperative day 3, animals underwent neurologic testing, after which the brains were prepared for assessment of cerebral infarct volume. Data were compared between groups by Mann-Whitney U test.

RESULTS

Compared with the ischemia-alone group, the ischemia plus bypass group had significantly higher levels of circulating tumor necrosis factor alpha and interleukins 1beta and 10 at the end of bypass and 2 hours after bypass. In addition, the ischemia plus bypass animals had larger total cerebral infarct volumes (286 +/- 125 mm(3)) than did those with ischemia alone (144 +/- 85 mm(3), P = .0124).

CONCLUSIONS

Cardiopulmonary bypass increased cerebral infarct size after focal cerebral ischemia in rats. This worsening of outcome may in part be related to an augmented inflammatory response that accompanies cardiopulmonary bypass.

A review of perioperative glucose control in the neurosurgical population

Significant fluctuations in serum glucose levels accompany the stress response of surgery or acute injury and may be associated with vascular or neurologic morbidity. Maintenance of euglycemia with intensive insulin therapy (IIT) continues to be investigated as a therapeutic intervention to decrease morbidity associated with derangements in glucose metabolism. Hypoglycemia is a common side effect of IIT with potential for significant morbidity, especially in the neurologically injured patient. Differences in cerebral versus systemic glucose metabolism, the time course of cerebral response to injury, and heterogeneity of pathophysiology in neurosurgical patient populations are important to consider in evaluating the risks and benefits of IIT. While extremes of glucose levels are to be avoided, there are little data to support specific use of IIT for maintenance of euglycemia in the perioperative management of neurosurgical patients. Existing data are summarized and reviewed in this context.

Regulation of cerebral vasculature in normal and ischemic brain

We outline the mechanisms currently thought to be responsible for controlling cerebral blood flow (CBF) in the physiologic state and during ischemia, focusing on the arterial pial and penetrating microcirculation. Initially, we categorize the cerebral circulation and then review the vascular anatomy. We draw attention to a number of unique features of the cerebral vasculature, which are relevant to the microcirculatory response during ischemia: arterial histology, species differences, collateral flow, the venous drainage, the blood-brain barrier, astrocytes and vascular nerves. The physiology of the arterial microcirculation is then assessed. Lastly, we review the changes during ischemia which impact on the microcirculation. Further understanding of the normal cerebrovascular anatomy and physiology as well as the pathophysiology of ischemia will allow the rational development of a pharmacologic therapy for human stroke and brain injury.

Why Does Acute Hyperglycemia Worsen the Outcome of Transient Focal Cerebral Ischemia?

Background and Purpose— Hyperglycemia adversely affects the outcome of stroke. Global ischemia data support that the harmful effect of hyperglycemia is mediated by glucose-induced elevated plasma glucocorticoids. Here we sought to evaluate the negative effects of hyperglycemia on transient focal ischemia in the rat, and to test whether these could be prevented by inhibition of either corticosteroid production or neutrophil infiltration.

Methods— Sprague-Dawley rats (n=217) were used. Ischemia was induced by 1 hour middle cerebral artery occlusion (n=196). Acute hyperglycemia was induced by IP injection of dextrose 30 minutes before ischemia. Neutrophil infiltration was blocked by neutropenia with vinblastine. Corticosterone synthesis was inhibited by chemical adrenalectomy with metyrapone. We measured MRI lesion and tissue infarct volumes, evaluated the neurological function, brain myeloperoxidase and matrix metalloproteinase-9 activities, and protein O-glycosylation.

Results— Hyperglycemia significantly enhanced MRI diffusion-weighted imaging alterations, increased cortical, but not subcortical, infarct volume, worsened neurological score, and enhanced brain myeloperoxidase and matrix metalloproteinase-9 activities. Metyrapone did not prevent hyperglycemic brain damage despite successful reduction of plasma corticosterone. Yet, metyrapone tended to reduce cortical infarction and apparent diffusion coefficient lesion volume, indicating some negative contribution of corticosterone. Blocking neutrophil infiltration was also ineffective to prevent the harmful effect of hyperglycemia. A new finding was that O-linked glycosylation of cerebral proteins was increased under hyperglycemia.

Conclusions— In transient middle cerebral artery occlusion, the hyperglycemia-exacerbated brain damage cannot be fully explained by the negative effects of plasma corticosteroids or neutrophil infiltration. The contribution of other intrinsic effects of high glucose, such as brain protein O-glycosylation, deserves further investigation.

Single-dose ebselen does not afford sustained neuroprotection to rats subjected to severe focal cerebral ischemia

Oxygen free radicals have been involved in the pathophysiology of cerebral ischemia, especially after spontaneous or thrombolytic reperfusion. In this study with rats, we have combined a severe focal ischemic insult (2 h) and a prolonged reperfusion time (7 days) to assess the possible sustained neuroprotective effect of ebselen (10 or 100 mg/kg), a small, lipophilic organoselenium compound which mimics glutathione peroxidase. Parietal cortical perfusion was measured by laser-Doppler flowmetry, and focal cerebral ischemia was carried out by the intraluminal thread method. We have measured plasma selenium levels, brain reduced glutathione levels, as a marker of oxidative stress, and infarct volume associated with cerebral ischemia. Focal ischemia did not alter reduced glutathione levels, while 60 min reperfusion following ischemia induced a significant (P < 0.05) decrease in reduced glutathione levels of the ipsilateral hemisphere. Pretreatment with ebselen, which induced significant (P < 0.05) increase in plasma selenium levels, did not significantly alter the decrease in reduced glutathione levels. The ischemic insult induced 30% mortality on average, with deaths always occurring within 12-48 h. Surviving rats suffered up to 25% body weight loss 1 week after the ischemic insult. Infarct volumes were 26.8 +/- 4.7% of the hemisphere in placebo-treated rats, 26.6 +/- 3.6% in 10 mg/kg ebselen-treated rats, and 25.6 +/- 6.4% in 100 mg/kg ebselen-treated rats (not significantly different). Single-dose administration of ebselen does not reduce the size of brain infarct resulting from severe focal cerebral ischemia in rats. In contrast to previous studies with relatively earlier endpoints, we have delayed the measurement of infarct volume to 1 week after the ischemic insult.

Glucose transport to the brain: a systems model

Glucose transport to the brain involves sophisticated interactions of solutes, transporters, enzymes, and cell signaling processes, within an intricate spatial architecture. The dynamics of the transport are influenced by the adaptive nature of the blood-brain barrier (BBB), the semi-impermeable membranes of brain capillaries. As both the gate and the gatekeeper between blood-borne nutrients and brain tissue, the BBB helps govern brain homeostasis. Glucose in the blood must cross the BBB's luminal and abluminal membranes to reach neural tissue. A robust representation of the glucose transport mechanism can highlight a target for brain therapeutic intervention, help characterize mechanisms behind several disease phenotypes, or suggest a new delivery route for drugs. The challenge for researchers is understanding the relationships between influential physiological variables in vivo, and using that knowledge to predict how alterations or interventions affect glucose transport. This paper reviews factors influencing glucose transport and approaches to representing blood-to-brain glucose transport including in vitro, in vivo, and kinetic models. Applications for different models are highlighted, while their limitations in answering arising questions about the human in vivo BBB lead to a discussion of an alternate approach. A developing complex systems simulation is introduced, initiating a single platform to represent the dynamics of glucose transport across the adapting human blood-brain barrier.

Hyperglycemia and Brain Tissue pH after Traumatic Brain Injury

OBJECTIVE

Hyperglycemia occurring after head injury is associated with poor neurological outcome. We tested the hypothesis that blood glucose levels are associated with brain tissue pH (pH(b)) and that the correction of hyperglycemia would result in an improvement in pH(b).

METHODS

This is a retrospective analysis of a prospectively collected database. Thirty-four patients in a tertiary care neuroscience critical care unit with major traumatic brain injury underwent pH(b) monitoring.

RESULTS

A total of 428 glucose measurements were recorded during pH(b) monitoring. Mean glucose level was 7.1 mmol/L (range, 2.8-21.7 mmol/L) and median (interquartile range) pH(b) was 7.11 mmol/L (7.00-7.19 mmol/L). To account for the correlated, unbalanced nature of the data, a linear generalized estimating equation model was created. This model predicted that for each 1 mmol/L increase in blood glucose, pH(b) changed by -0.011 mmol/L (95% confidence interval, -0.016 to -0.005 mmol/L; P < 0.001). This relationship remained significant in a multivariable model that included cerebral perfusion pressure, brain tissue oxygen and carbon dioxide tension, and brain temperature. Twenty-one episodes of significant hyperglycemia (>or=11.1 mmol/L) treated with intravenous insulin were identified. Insulin therapy significantly reduced blood glucose concentration from a median (interquartile range) of 11.9 mmol/L (range, 11.4-13.6 mmol/L) to 8.8 mmol/L (range, 7.3-9.6 mmol/L; P < 0.001). Baseline pH(b) was not significantly different from pH(b) associated with the subsequent glucose reading of less than 11.1 mmol/L (P = 0.29), but there was a suggestion of improvement if the change in blood glucose was large.

CONCLUSION

Blood glucose is associated with brain tissue acidosis in patients with major head injury. Prospective studies are required to confirm these results and to determine whether treatment of hyperglycemia improves outcome.

Lactate: A key metabolite in the intercellular metabolic interplay

Most physicians involved in intensive care consider lactate solely as a deleterious metabolite, responsible for high morbidity and bad prognosis in severe patients. For the physiologist, however, lactate is a key metabolite, alternatively produced or consumed. Many studies in the literature have infused animals or humans with exogenous lactate, demonstrating its safety and usefulness, but the bad reputation of lactate is still widespread. The metabolic meaning of glucose–lactate cycling exceeds its initial role described by Cori and Cori. According to recent works concerning lactate, it can be predicted that a new role as a therapeutic agent will arise for this metabolite.