Regional heterogeneity of post-traumatic brain metabolism as studied by microdialysis, magnetic resonance spectroscopy and positron emission tomography

Decompressive Craniectomy for Traumatic Brain Injury: Postoperative TCD Cerebral Hemodynamic Evaluation

Background: There are no studies describing the cerebral hemodynamic patterns that can occur in traumatic brain injury (TBI) patients following decompressive craniectomy (DC). Such data have potentially clinical importance for guiding the treatment. The objective of this study was to investigate the postoperative cerebral hemodynamic patterns, using transcranial Doppler (TCD) ultrasonography, in patients who underwent DC. The relationship between the cerebral circulatory patterns and the patients' outcome was also analyzed. Methods: Nineteen TBI patients with uncontrolled brain swelling were prospectively studied. Cerebral blood circulation was evaluated by TCD ultrasonography. Patients and their cerebral hemispheres were categorized based on TCD-hemodynamic patterns. The data were correlated with neurological status, midline shift on CT scan, and Glasgow outcome scale scores at 6 months after injury. Results: Different cerebral hemodynamic patterns were observed. One patient (5.3%) presented with cerebral oligoemia, 4 patients (21%) with cerebral hyperemia, and 3 patients (15.8%) with cerebral vasospasm. One patient (5.3%) had hyperemia in one cerebral hemisphere and vasospasm in the other hemisphere. Ten patients (52.6%) had nonspecific circulatory pattern. Abnormal TCD-circulatory patterns were found in 9 patients (47.4%). There was no association between TCD-cerebral hemodynamic findings and outcome. Conclusion: There is a wide heterogeneity of postoperative cerebral hemodynamic findings among TBI patients who underwent DC, including hemodynamic heterogeneity between their cerebral hemispheres. DC was proved to be effective for the treatment of cerebral oligoemia. Our data support the concept of heterogeneous nature of the pathophysiology of the TBI and suggest that DC as the sole treatment modality is insufficient.

Acute glucose and lactate metabolism are associated with cognitive recovery following traumatic brain injury

Traumatic brain injury (TBI) is associated with acute cerebral metabolic crisis (ACMC). ACMC-related atrophy appears to be prominent in frontal and temporal lobes following moderate-to-severe TBI. This atrophy is correlated with poorer cognitive outcomes in TBI. The current study investigated ability of acute glucose and lactate metabolism to predict long-term recovery of frontal-temporal cognitive function in participants with moderate-to-severe TBI. Cerebral metabolic rate of glucose and lactate were measured by the Kety-Schmidt method on days 0-7 post-injury. Indices of frontal-temporal cognitive processing were calculated for six months post-injury; 12 months post-injury; and recovery (the difference between the six- and 12-month scores). Glucose and lactate metabolism were included in separate regression models, as they were highly intercorrelated. Also, glucose and lactate values were centered and averaged and included in a final regression model. Models for the prediction frontal-temporal cognition at six and 12 months post-injury were not significant. However, average glucose and lactate metabolism predicted recovery of frontal-temporal cognition, accounting for 23% and 22% of the variance, respectively. Also, maximum glucose metabolism, but not maximum lactate metabolism, was an inverse predictor in the recovery of frontal-temporal cognition, accounting for 23% of the variance. Finally, the average of glucose and lactate metabolism predicted frontal-temporal cognitive recovery, accounting for 22% of the variance. These data indicate that acute glucose and lactate metabolism both support cognitive recovery from TBI. Also, our data suggest that control of endogenous fuels and/or supplementation with exogenous fuels may have therapeutic potential for cognitive recovery from TBI.

Cellular and molecular mechanisms of injury and spontaneous recovery

Until recently, most have assumed that traumatic brain injury (TBI) was singularly associated with the overt destruction of brain tissue resulting in subsequent morbidity or death. More recently, experimental and clinical studies have shown that the pathobiology of TBI is more complex, involving a host of cellular and subcellular changes that impact on neuronal function and viability while also affecting vascular reactivity and the activation of multiple biological response pathways. Here we review the brain's response to injury, examining both focal and diffuse changes and their implications for post-traumatic brain dysfunction and recovery. TBI-induced neuronal dysfunction and death as well as the diffuse involvement of multiple fiber projections are discussed together with considerations of how local axonal membrane changes or channelopathy translate into local ionic dysregulation and axonal disconnection. Concomitant changes in the cerebral microcirculation are also discussed and their relationship with the parallel changes in the brain's metabolism is considered. These cellular and subcellular events occurring within neurons and their blood supply are correlated with multiple biological response modifiers evoked by generalized post-traumatic inflammation and the parallel activation of oxidative stress processes. The chapter closes with considerations of recovery following focal or diffuse injury. Evidence for dynamic brain reorganization/repair is presented, with considerations of traumatically induced circuit disruption and their progression to either adaptive or in some cases, maladaptive reorganization.

Imaging and decision-making in neurocritical care

Critically ill neurologic patients are common in the hospital practice of neurology and are often in extreme states requiring accurate and specific information. Imaging, especially using advanced imaging techniques, can provide an important means of garnering this information. This article focuses on the clinical utilization of selective imaging methods that are commonly used in critically ill neurologic patients to render diagnoses, to monitor effects of treatment, or have contributed to a better understanding of pathophysiology in the intensive care unit.

Early metabolic crisis-related brain atrophy and cognition in traumatic brain injury

Traumatic brain injury often results in acute metabolic crisis. We recently demonstrated that this is associated with chronic brain atrophy, which is most prominent in the frontal and temporal lobes. Interestingly, the neuropsychological profile of traumatic brain injury is often characterized as 'frontal-temporal' in nature, suggesting a possible link between acute metabolic crisis-related brain atrophy and neurocognitive impairment in this population. While focal lesions and diffuse axonal injury have a well-established role in the neuropsychological deficits observed following traumatic brain injury, no studies to date have examined the possible contribution of acute metabolic crisis-related atrophy in the neuropsychological sequelae of traumatic brain injury. In the current study we employed positron emission tomography, magnetic resonance imaging, and neuropsychological assessments to ascertain the relationship between acute metabolic crisis-related brain atrophy and neurocognitive outcome in a sample of 14 right-handed traumatic brain injury survivors. We found that acute metabolic crisis-related atrophy in the frontal and temporal lobes was associated with poorer attention, executive functioning, and psychomotor abilities at 12 months post-injury. Furthermore, participants with gross frontal and/or temporal lobe atrophy exhibited numerous clinically significant neuropsychological deficits in contrast to participants with other patterns of brain atrophy. Our findings suggest that interventions that reduce acute metabolic crisis may lead to improved functional outcomes for traumatic brain injury survivors.

Posttraumatic refractory intracranial hypertension and brain herniation syndrome: cerebral hemodynamic assessment before decompressive craniectomy

Background. The pathophysiology of traumatic brain swelling remains little understood. An improved understanding of intracranial circulatory process related to brain herniation may have treatment implications. Objective. To investigate the cerebral hemodynamic changes associated with brain herniation syndrome due to traumatic brain swelling. Methods. Nineteen head-injured patients with evidence of refractory intracranial hypertension and transtentorial herniation were prospectively studied. Cerebral hemodynamic assessment by transcranial Doppler (TCD) ultrasonography was performed prior to decompressive craniectomy. Patients and their cerebral hemispheres were classified according to TCD-hemodynamic patterns, and the data correlated with neurological status, midline shift on CT scan, and Glasgow outcome scale scores at 6 months after injury. Results. A wide variety of cerebral hemodynamic findings were observed. Ten patients (52.7%) presented with cerebral oligoemia, 3 patients (15.8%) with cerebral hyperemia, and 6 patients with nonspecific circulatory pattern. Circulatory disturbances were more frequently found in the side of maximal cerebral swelling than in the opposite side. Pulsatility index (PI) values suggested that ICP varied from acceptable to considerably high; patients with increased PI, indicating higher microvascular resistance. No correlation was found between cerebral hemodynamic findings and outcome. Conclusions. There is a marked heterogeneity of cerebral hemodynamic disturbances among patients with brain herniation syndrome.

Analyses of cerebral microdialysis in patients with traumatic brain injury: relations to intracranial pressure, cerebral perfusion pressure and catheter placement

BACKGROUND

Cerebral microdialysis (MD) is used to monitor local brain chemistry of patients with traumatic brain injury (TBI). Despite an extensive literature on cerebral MD in the clinical setting, it remains unclear how individual levels of real-time MD data are to be interpreted. Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are important continuous brain monitors in neurointensive care. They are used as surrogate monitors of cerebral blood flow and have an established relation to outcome. The purpose of this study was to investigate the relations between MD parameters and ICP and/or CPP in patients with TBI.

METHODS

Cerebral MD, ICP and CPP were monitored in 90 patients with TBI. Data were extensively analyzed, using over 7,350 samples of complete (hourly) MD data sets (glucose, lactate, pyruvate and glycerol) to seek representations of ICP, CPP and MD that were best correlated. MD catheter positions were located on computed tomography scans as pericontusional or nonpericontusional. MD markers were analyzed for correlations to ICP and CPP using time series regression analysis, mixed effects models and nonlinear (artificial neural networks) computer-based pattern recognition methods.

RESULTS

Despite much data indicating highly perturbed metabolism, MD shows weak correlations to ICP and CPP. In contrast, the autocorrelation of MD is high for all markers, even at up to 30 future hours. Consequently, subject identity alone explains 52% to 75% of MD marker variance. This indicates that the dominant metabolic processes monitored with MD are long-term, spanning days or longer. In comparison, short-term (differenced or Δ) changes of MD vs. CPP are significantly correlated in pericontusional locations, but with less than 1% explained variance. Moreover, CPP and ICP were significantly related to outcome based on Glasgow Outcome Scale scores, while no significant relations were found between outcome and MD.

CONCLUSIONS

The multitude of highly perturbed local chemistry seen with MD in patients with TBI predominately represents long-term metabolic patterns and is weakly correlated to ICP and CPP. This suggests that disturbances other than pressure and/or flow have a dominant influence on MD levels in patients with TBI.

Analysis of the changes in the oxidation of brain tissue cytochrome-c-oxidase in traumatic brain injury patients during hypercapnoea: a broadband NIRS study

Using broadband near-infrared spectroscopy (NIRS) and cerebral microdialysis (MD),we investigated cerebral cellular metabolism and mitochondrial redox states, following hypercapnoea in 6 patients with traumatic brain injury (TBI). In all patients hypercapnoea increased intracranial pressure and cerebral blood flow velocity measured with transcranial Doppler. Despite the likely increase in cerebral oxygen delivery, we did not see an increase in the oxidation status of cytochrome-c-oxidase [oxCCO] in every patient. Analysis of the NIRS data demonstrated two patterns of the changes; Group A (n = 4) showed an increase in [oxCCO] of 0.34(± 0.34)µM and Group B (n = 2) a decrease of 0.40(± 0.41)µM. Although no obvious association was seen between the Δ[oxCCO] and the MD, measured changes in lactate and pyruvate concentrations. Further work using model informed data interpretation may be helpful in understanding the multimodal signals acquired in this heterogeneous patient group.

Hypoxia-induced extracellular acidosis increases p-glycoprotein activity and chemoresistance in tumors in vivo via p38 signaling pathway

Due to inadequate perfusion, tumors develop hypoxia and extracellular acidosis. In vitro, this acidic environment (pH=6.6) has a strong impact on the activity of the p-glycoprotein (pGP) drug transporter responsible for multidrug resistance. This effect is most probably mediated via p38 and/or ERK1/2 signalling pathways. The aim of the study was to analyze whether these findings also play a role for chemosensitivity in solid growing tumors in vivo. Therefore, experimental R3327-AT1 tumors of the rat were exposed to an acidifying treatment leading to forced glycolysis. The intratumoral pO(2)1 was determined polarographically and the extracellular pH was measured with needle electrodes. In addition the cytotoxicity of daunorubicin chemotherapy was assessed. Tumor oxygenation was markedly worsened by the acidosis treatment leading to a mean tumor pO(2) of 1 mmHg. This hypoxia resulted in a pronounced acidification of the tumor tissue from pH 7.04 down to 6.65. Extracellular acidosis in vivo decreased caspase 3-activity after daunorubicin treatment by 30%indicating a reduced chemosensitivity. Inhibition of the p38 signalling pathway restored the reduced chemosensitivity at least partially. However, in contrast to the in vitro findings inhibition of ERK1/2 had practically no impact in vivo.

Early metabolic characteristics of lesion and nonlesion tissue after head injury

We defined lesion and structurally normal regions using magnetic resonance imaging at follow-up in patients recovering from head injury. Early metabolic characteristics in these regions of interest (ROIs) were compared with physiology in healthy volunteers. Fourteen patients with severe head injury had positron emission tomography within 72 h, and magnetic resonance imaging at 3 to 18 months after injury. Cerebral blood flow (CBF), oxygen utilization (CMRO(2)), and oxygen extraction fraction (OEF) were all lower in lesion ROIs, compared with nonlesion and control ROIs (P<0.001); however, there was substantial overlap in physiology. Control ROIs showed close coupling among CBF, blood volume (CBV), and CMRO(2), whereas relationships within lesion and nonlesion ROIs were abnormal. The relationship between CBF and CMRO(2) generally remained coupled but the slope was reduced; that for CBF and OEF was variable; whereas that between CBF and CBV was highly variable. There was considerable heterogeneity between and within patients. Although irreversibly damaged tissue is characterized by marked derangements in physiology, a more detailed analysis shows acute changes in physiology and physiologic relationships within regions of the brain that appear structurally normal at follow-up. Such pathophysiological derangements may result in selective neuronal loss and impact on functional outcome.

Traumatic brain injury research priorities: the Conemaugh International Brain Injury Symposium

In 2005, an international symposium was convened with over 100 neuroscientists from 13 countries and major research centers to review current research in traumatic brain injury (TBI) and develop a consensus document on research issues and priorities. Four levels of TBI research were the focus of the discussion: basic science, acute care, post-acute neurorehabilitation, and improving quality of life (QOL). Each working group or committee was charged with reviewing current research, discussion and prioritizing future research directions, identifying critical issues that impede research in brain injury, and establishing a research agenda that will drive research over the next five years, leading to significantly improved outcomes and QOL for individuals suffering brain injuries. This symposium was organized at the request of the Congressional Brain Injury Task Force, to follow up on the National Institutes of Health Consensus Conference on TBI as mandated by the TBI ACT of 1996. The goal was to review what progress had been made since the National Institutes of Health (NIH) Consensus Conference, and also to follow up on the 1990's Decade of the Brain Project. The major purpose of the symposium was to provide recommendations to the U.S. Congress on a priority basis for research, treatment, and training in TBI over the next five years.

Continuous monitoring of cerebral metabolism in traumatic brain injury: a focus on cerebral microdialysis

PURPOSE OF REVIEW

This review highlights recent advances in cerebral microdialysis as a tool for neurochemical monitoring of patients with traumatic brain injury.

RECENT FINDINGS

Progress in microdialysis research has come from validation studies of microdialysis biomarkers and clinical outcome in large cohorts of traumatic brain injury patients and by combining microdialysis with other methods, such as positron emission tomography, magnetic resonance spectroscopy, brain tissue oximetry and electrophysiology. The combination of rapid-sampling microdialysis and electrocorticography has revealed complex, transient fluctuations of microdialysis glucose and lactate and depolarization-like events that may affect the secondary injury process. The use of microdialysis to monitor global cerebral metabolic events (related to intracranial hypertension or reduced cerebral perfusion pressure for example) as opposed to focal events in peri-lesional brain tissue need to be clearly distinguished and the microdialysis catheter location verified by neuroimaging to ensure proper data interpretation. Differences in microdialysis biomarker levels between grey and white matter following traumatic brain injury need clarification.

SUMMARY

Microdialysis is established as a neurochemical research tool in neurointensive care, particularly in combination with other monitoring methods, and contributes to a growing knowledge of secondary injury mechanisms in traumatic brain injury. The value of microdialysis as a tool in routine neurointensive care decision-making remains unclear.

Brain ischaemia after traumatic brain injury: lessons from 15O2 positron emission tomography

PURPOSE OF REVIEW

To describe the role of O2 positron emission tomography in studies aimed at understanding ischaemia in head injury. It has been difficult to use cerebral blood flow levels to provide a secure definition of cerebral ischaemia in head injury, since primary changes in cerebral metabolism may be responsible for coupled reductions in cerebral blood flow. Further, regional heterogeneity of pathophysiology can confound global measures of adequacy of cerebral oxygen delivery. There is a need for a technique that can provide a comprehensive and quantitative description of cerebral physiology in this setting.

RECENT FINDINGS

O2 positron emission tomography can image cerebral blood flow, cerebral blood volume, cerebral metabolic rate for oxygen and oxygen extraction fraction, and thus allows a robust and specific definition of true ischaemia. When used in combination with other monitoring tools and imaging modalities, positron emission tomography has also been used to validate and refine bedside monitors of cerebrovascular physiology, study the impact of therapeutic interventions and provide clues to novel pathophysiology.

SUMMARY

There is a clear role for O2 positron emission tomography in elucidating pathophysiology in head injury. The technique may provide most information when combined with other imaging and monitoring tools.

Multimodality monitoring and telemonitoring in neurocritical care: from microdialysis to robotic telepresence

PURPOSE OF REVIEW

This review will highlight the state-of-the-art in brain monitoring in neurointensive care and define methods of integrating this technology into patient care using telemedicine methods.

RECENT FINDINGS

Several new methods of brain monitoring have been established over the last several years including continuous EEG monitoring, brain tissue oxygenation, jugular venous oxygenation, and cerebral microdialysis. Observational research using these monitors has documented that the brain metabolism, blood flow and function are dynamic after a primary insult. The dynamic nature of the brain can predispose the brain to secondary insults that can occur in the setting of intensive care. Several variables of brain metabolism and function can be monitored and directly impact treatment decisions as well as provide diagnostic and prognostic information. General treatment guidelines for brain injury and brain hemorrhage were developed, in part, prior to implementation of use of these monitors, and there is a trend away from adoption of a one-size-fits-all approach and a trend towards monitor-guided therapy. Dealing with the data provided by multimodality monitoring can be overwhelming. Efficient use of such information requires methods to integrate diverse sets of information, and methods to access the online monitoring information remotely and at any time, day or night. Such remote access integration methods will be reviewed.

SUMMARY

Multimodality and telemedicine techniques have advanced the state of knowledge about brain function in critically ill patients, and are presently being implemented to direct therapy. Increasing complexity of care will become commonplace, but will be facilitated by computer-enhanced tools that permit the intensivist to integrate this information into an improved treatment regimen.

Metabolic crisis without brain ischemia is common after traumatic brain injury: a combined microdialysis and positron emission tomography study

Brain trauma is accompanied by regional alterations of brain metabolism, reduction in metabolic rates and possible energy crisis. We hypothesize that microdialysis markers of energy crisis are present during the critical period of intensive care despite the absence of brain ischemia. In all, 19 brain injury patients (mean GCS 6) underwent combined positron emission tomography (PET) for metabolism of glucose (CMRglu) and oxygen (CMRO(2)) and cerebral microdialysis (MD) at a mean time of 36 h after injury. Microdialysis values were compared with the regional mean PET values adjacent to the probe. Longitudinal MD data revealed a 25% incidence rate of metabolic crisis (elevated lactate/pyruvate ratio (LPR) > 40) but only a 2.4% incidence rate of ischemia. Positron emission tomography imaging revealed a 1% incidence of ischemia across all voxels as measured by oxygen extraction fraction (OEF) and cerebral venous oxygen content (CvO(2)). In the region of the MD probe, PET imaging revealed ischemia in a single patient despite increased LPR in other patients. Lactate/pyruvate ratio correlated negatively with CMRO(2) (P < 0.001), but not with OEF or CvO(2). Traumatic brain injury leads to a state of persistent metabolic crisis as reflected by abnormal cerebral microdialysis LPR that is not related to ischemia.

Update of neuropathology and neurological recovery after traumatic brain injury

This review focuses on the potential for traumatic brain injury to evoke both focal and diffuse changes within the brain parenchyma, while considering the cellular constituents involved and the subcellular perturbations that contribute to their dysfunction. New insight is provided on the pathobiology of traumatically induced cell body injury and diffuse axonal damage. The consequences of axonal damage in terms of subsequent deafferentation and any potential retrograde cell death and atrophy are addressed. The regional and global metabolic sequelae are also considered. This detailed presentation of the neuropathological consequences of traumatic brain injury is used to set the stage for better appreciating the neurological recovery occurring after traumatic injury. Although the pathological and clinical effects of focal and diffuse damage are usually intermingled, the different clinical manifestations of recovery patterns associated with focal versus diffuse injuries are presented. The recognizable patterns of recovery, involving unconsciousness, posttraumatic confusion/amnesia, and postconfusional restoration, that typically occur across the full spectrum of diffuse injury are described, recognizing that the patient's long-term recovery may involve more idiosyncratic combinations of dysfunction. The review highlights the relationship of focal lesions to localizing syndromes that may be embedded in the evolving natural history of diffuse pathology. It is noted that injuries with primarily focal pathology do not necessarily follow a comparable pattern of recovery with distinct phases. Potential linkages of these recovery patterns to the known neuropathological sequelae of injury and various reparative mechanisms are considered and it is proposed that potential biological markers and newer imaging technologies will better define these linkages.