Assessing Cerebral Hemodynamic Stability After Brain Injury

Optimal Cerebral Perfusion Pressure in Brain Injury: Physiological Relationships and Outcome

BACKGROUND AND OBJECTIVES

The priority for measuring and optimizing physiological metrics in brain injury care remains to be determined. Calculating and targeting optimal cerebral perfusion pressure (CPP opt ) is an emerging treatment paradigm, but its association with other parameters is uncertain. A previous analysis of 22 patients found that brain tissue oxygenation (P bt O 2 ) peaked when CPP values were near CPP opt . This study sought to validate those findings using a distinct, larger cohort. It also studied the relationship between CPP opt and physiological parameters related to intracranial dynamics and with neurological outcome.

METHODS

P bt O 2 , intracranial pressure (ICP), and arterial blood pressure data were collected during a 15-year period from 432 brain injury patients at 4 cooperating trauma centers. CPP opt was retrospectively computed.

RESULTS

The median age was 36 years ( n = 316), the median admission Glasgow coma score was 6 ( n = 323), and 75% of the patients were men ( n = 324). In aggregate data, P bt O 2 peaked at CPP values near CPP opt (+/- 2 mm Hg). Proportion of out-of-range ICP measurements (>22 mm Hg) and positive pressure reactivity index were higher in dying and unfavorable outcome groups, and increased with worsening outcome. Time spent near CPP opt was significantly lower in dying patients but not in patients with unfavorable outcome. Time near CPP opt was, however, correlated with better outcome. Proportion of out-of-range P bt O 2 (<20 mm Hg) was not associated with outcome or mortality.

CONCLUSION

The results verify CPP opt as physiologically significant and that in aggregate data achievement of CPP opt is associated with maximized P bt O 2 . Compliance with the ICP treatment threshold was, though, the only modifiable physiological variable associated with both functional outcome and mortality. Our results support optimization of ICP with highest priority. Further study is required in patients in whom CPP opt is specifically targeted.

Simulating the impact of tumor mechanical forces on glymphatic networks in the brain parenchyma

The brain glymphatic system is currently being explored in the context of many neurological disorders and diseases, including traumatic brain injury, Alzheimer's disease, and ischemic stroke. However, little is known about the impact of brain tumors on glymphatic function. Mechanical forces generated during tumor development and growth may be responsible for compromised glymphatic transport pathways, reducing waste clearance and cerebrospinal fluid (CSF) transport in the brain parenchyma. One such force is solid stress, i.e., growth-induced forces from cell hyperproliferation and excess matrix deposition. Because there are no prior studies assessing the impact of tumor-derived solid stress on glymphatic system structure and performance in the brain parenchyma, this study serves to fill an important gap in the field. We adapted a previously developed Electrical Analog Model using MATLAB Simulink for glymphatic transport coupled with Finite Element Analysis for tumor mechanical stresses and strains in COMSOL. This allowed simulation of the impact of tumor mechanical force generation on fluid transport within brain parenchymal glymphatic units-which include perivascular spaces, astrocytic networks, interstitial spaces, and capillary basement membranes. We conducted a parametric analysis to compare the contributions of tumor size, tumor proximity, and ratio of glymphatic subunits to the stress and strain experienced by the glymphatic unit and corresponding reduction in flow rate of CSF. Mechanical stresses intensify with proximity to the tumor and increasing tumor size, highlighting the vulnerability of nearby glymphatic units to tumor-derived forces. Our stress and strain profiles reveal compressive deformation of these surrounding glymphatics and demonstrate that varying the relative contributions of astrocytes vs. interstitial spaces impact the resulting glymphatic structure significantly under tumor mechanical forces. Increased tumor size and proximity caused increased stress and strain across all glymphatic subunits, as does decreased astrocyte composition. Indeed, our model reveals an inverse correlation between extent of astrocyte contribution to the composition of the glymphatic unit and the resulting mechanical stress. This increased mechanical strain across the glymphatic unit decreases the venous efflux rate of CSF, dependent on the degree of strain and the specific glymphatic subunit of interest. For example, a 20% mechanical strain on capillary basement membranes does not significantly decrease venous efflux (2% decrease in flow rates), while the same magnitude of strain on astrocyte networks and interstitial spaces decreases efflux flow rates by 7% and 22%, respectively. Our simulations reveal that solid stress from growing brain tumors directly reduces glymphatic fluid transport, independently from biochemical effects from cancer cells. Understanding these pathophysiological implications is crucial for developing targeted interventions aimed at restoring effective waste clearance mechanisms in the brain. This study opens potential avenues for future experimental research in brain tumor-related glymphatic dysfunction.

Association of RAP Compensatory Reserve Index with Continuous Multimodal Monitoring Cerebral Physiology, Neuroimaging, and Patient Outcome in Adult Acute Traumatic Neural Injury: A Scoping Review

Acute traumatic neural injury, known as traumatic brain injury (TBI), stands as a significant contributor to global mortality and disability. Ideally, continuously monitoring cerebral compliance/cerebral compensatory reserve would enable timely interventions and avert further substantial deterioration in TBI cases. RAP, defined as the moving Pearson's correlation between intracranial pressure (ICP) pulse amplitude waveform and ICP, has been proposed as a continuously updating index in this context. However, the literature remains scattered and difficult to navigate. Thus, the goal of this scoping review was to comprehensively characterize the literature regarding RAP and its association with (1) other multimodal cerebral physiological monitoring, (2) neuroimaging features, and (3) long-term patient outcomes. We subsequently conducted a systematic scoping review of the human literature to highlight the association of RAP with continuous multimodal monitoring of cerebral physiology, neuroimaging, and patient outcomes in the context of adult TBI patients. Our review encompassed 21 studies focusing on these topics. The primary findings involve meticulous analysis of studies, categorizing findings into three states of RAP to clearly understand its relation to cerebral physiology and clinical outcomes. State 1 signifies a healthy condition with a small positive value near zero (RAP <0.5). Conversely, state 2, a predominant characterization of TBI patients, indicates compromised compensatory reserve, featuring a large positive RAP value (RAP > 0.4). State 3 emerges in worsened conditions, showcasing further compromised compensatory reserve, exhausted cerebrovascular reactivity, and disturbed cerebral autoregulation. A substantial number of patients with fatal outcomes was found in state 3, marked by a notable occurrence of decreasing and, in some instances, negative RAP. The significance of this review lies in establishing a platform for future research directions to enhance the precision and clinical implications of RAP in TBI care, ultimately aiming to prevent the transition from state 2 to state 3 and mitigate fatal outcomes.

Continuous monitoring methods of cerebral compliance and compensatory reserve: a scoping review of human literature

Objective.Continuous monitoring of cerebrospinal compliance (CC)/cerebrospinal compensatory reserve (CCR) is crucial for timely interventions and preventing more substantial deterioration in the context of acute neural injury, as it enables the early detection of abnormalities in intracranial pressure (ICP). However, to date, the literature on continuous CC/CCR monitoring is scattered and occasionally challenging to consolidate.Approach.We subsequently conducted a systematic scoping review of the human literature to highlight the available continuous CC/CCR monitoring methods.Main results.This systematic review incorporated a total number of 76 studies, covering diverse patient types and focusing on three primary continuous CC or CCR monitoring metrics and methods-Moving Pearson's correlation between ICP pulse amplitude waveform and ICP, referred to as RAP, the Spiegelberg Compliance Monitor, changes in cerebral blood flow velocity with respect to the alternation of ICP measured through transcranial doppler (TCD), changes in centroid metric, high frequency centroid (HFC) or higher harmonics centroid (HHC), and the P2/P1 ratio which are the distinct peaks of ICP pulse wave. The majority of the studies in this review encompassed RAP metric analysis (n= 43), followed by Spiegelberg Compliance Monitor (n= 11), TCD studies (n= 9), studies on the HFC/HHC (n= 5), and studies on the P2/P1 ratio studies (n= 6). These studies predominantly involved acute traumatic neural injury (i.e. Traumatic Brain Injury) patients and those with hydrocephalus. RAP is the most extensively studied of the five focused methods and exhibits diverse applications. However, most papers lack clarification on its clinical applicability, a circumstance that is similarly observed for the other methods.Significance.Future directions involve exploring RAP patterns and identifying characteristics and artifacts, investigating neuroimaging correlations with continuous CC/CCR and integrating machine learning, holding promise for simplifying CC/CCR determination. These approaches should aim to enhance the precision and accuracy of the metric, making it applicable in clinical practice.

Simulating the Impact of Tumor Mechanical Forces on Glymphatic Networks in the Brain Parenchyma

Background

The brain glymphatic system is currently being explored in the context of many neurological disorders and diseases, including traumatic brain injury, Alzheimer's disease, and ischemic stroke. However, little is known about the impact of brain tumors on glymphatic function. Mechanical forces generated during tumor development and growth may be responsible for compromised glymphatic transport pathways, reducing waste clearance and cerebrospinal fluid (CSF) transport in the brain parenchyma. One such force is solid stress, i.e., growth-induced forces from cell hyperproliferation and excess matrix deposition. Because there are no prior studies assessing the impact of tumor-derived solid stress on glymphatic system structure and performance in the brain parenchyma, this study serves to fill an important gap in the field.

Methods

We adapted a previously developed Electrical Analog Model using MATLAB Simulink for glymphatic transport coupled with Finite Element Analysis for tumor mechanical stresses and strains in COMSOL. This allowed simulation of the impact of tumor mechanical force generation on fluid transport within brain parenchymal glymphatic units - which include paravascular spaces, astrocytic networks, interstitial spaces, and capillary basement membranes. We conducted a parametric analysis to compare the contributions of tumor size, tumor proximity, and ratio of glymphatic subunits to the stress and strain experienced by the glymphatic unit and corresponding reduction in flow rate of CSF.

Results

Mechanical stresses intensify with proximity to the tumor and increasing tumor size, highlighting the vulnerability of nearby glymphatic units to tumor-derived forces. Our stress and strain profiles reveal compressive deformation of these surrounding glymphatics and demonstrate that varying the relative contributions of astrocytes vs. interstitial spaces impact the resulting glymphatic structure significantly under tumor mechanical forces. Increased tumor size and proximity caused increased stress and strain across all glymphatic subunits, as does decreased astrocyte composition. Indeed, our model reveals an inverse correlation between extent of astrocyte contribution to the composition of the glymphatic unit and the resulting mechanical stress. This increased mechanical strain across the glymphatic unit decreases the venous efflux rate of CSF, dependent on the degree of strain and the specific glymphatic subunit of interest. For example, a 20% mechanical strain on capillary basement membranes does not significantly decrease venous efflux (2% decrease in flow rates), while the same magnitude of strain on astrocyte networks and interstitial spaces decreases efflux flow rates by 7% and 22%, respectively.

Conclusion

Our simulations reveal that solid stress from brain tumors directly reduces glymphatic fluid transport, independently from biochemical effects from cancer cells. Understanding these pathophysiological implications is crucial for developing targeted interventions aimed at restoring effective waste clearance mechanisms in the brain.This study opens potential avenues for future experimental research in brain tumor-related glymphatic dysfunction.

Intracranial compliance and volumetry in patients with traumatic brain injury

BACKGROUND

Cerebral edema (CE) and intracranial hypertension (IHT) are complications of numerous neurological pathologies. However, the study of CE and noninvasive methods to predict IHT remains rudimentary. This study aims to identify in traumatic brain injury (TBI) patients the relationship between the volume of the lateral ventricles and the parameters of the noninvasive intracranial pressure waveform (nICPW).

METHODS

This is an analytical, descriptive, and cross-sectional study with nonsurgical TBI patients. The monitoring of nICPW was performed with a mechanical strain gauge, and the volumetry of the lateral ventricles was calculated using the free 3D Slicer software, both during the acute phase of the injury. The linear model of fixed and random mixed effects with Gamma was used to calculate the influence of nICPW parameters (P2/P1 and time-to-peak [TTP]) values on volumetry.

RESULTS

Considering only the fixed effects of the sample, there was P = 0.727 (95% CI [-0.653; 0.364]) for the relationship between P2/P1 and volumetry and 0.727 (95% CI [-1.657; 1.305]) for TTP and volumetry. Considering the fixed and random effects, there was P = 8.5e-10 (95% CI [-0.759; 0.355]) for the relationship between P2/P1 and volumetry and 8.5e-10 (95% CI [-2.001; 0.274]) for TTP and volumetry.

CONCLUSION

The present study with TBI patients found association between nICPW parameters and the volume of the lateral ventricles in the 1st days after injury.

Intracranial Pressure and Intracranial Elastance Monitoring in Neurocritical Care

Patients with acute brain injuries tend to be physiologically unstable and at risk of rapid and potentially life-threatening decompensation due to shifts in intracranial compartment volumes and consequent intracranial hypertension. Invasive intracranial pressure (ICP) monitoring therefore remains a cornerstone of modern neurocritical care, despite the attendant risks of infection and damage to brain tissue arising from the surgical placement of a catheter or pressure transducer into the cerebrospinal fluid or brain tissue compartments. In addition to ICP monitoring, tracking of the intracranial capacity to buffer shifts in compartment volumes would help in the assessment of patient state, inform clinical decision making, and guide therapeutic interventions. We review the anatomy, physiology, and current technology relevant to clinical management of patients with acute brain injury and outline unmet clinical needs to advance patient monitoring in neurocritical care.

Impaired cerebral compensatory reserve is associated with admission imaging characteristics of diffuse insult in traumatic brain injury

Background

Continuous assessment of cerebral compensatory reserve is possible using the moving correlation between pulse amplitude of intra-cranial pressure (AMP) and intra-cranial pressure (ICP), called RAP. Little is known about the behavior and associations of this index in adult traumatic brain injury (TBI). The goal of this study is to evaluate the association between admission cerebral imaging findings and RAP over the course of the acute intensive care unit stay.

Methods

We retrospectively reviewed 358 adult TBI patients admitted to the Addenbrooke’s Hospital, University of Cambridge, from March 2005 to December 2016. Only non-craniectomy patients were studied. Using archived high frequency physiologic signals, RAP was derived and analyzed over the first 48 h and first 10 days of recording in each patient, using grand mean, percentage of time above various thresholds, and integrated area under the curve (AUC) of RAP over time. Associations between these values and admission computed tomography (CT) injury characteristics were evaluated.

Results

The integrated AUC, based on various thresholds of RAP, was statistically associated with admission CT markers of diffuse TBI and cerebral edema. Admission CT findings of cortical gyral effacement, lateral ventricle compression, diffuse cortical subarachnoid hemorrhage (SAH), thickness of cortical SAH, presence of bilateral contusions, and subcortical diffuse axonal injury (DAI) were all associated with AUC of RAP over time. Joncheere-Terpstra testing indicated a statistically significant increase in mean RAP AUC across ordinal categories of the abovementioned associated CT findings.

Conclusions

RAP is associated with cerebral CT injury patterns of diffuse injury and edema, providing some confirmation of its potential measurement of cerebral compensatory reserve in TBI.

Electronic supplementary material

The online version of this article (10.1007/s00701-018-3681-y) contains supplementary material, which is available to authorized users.