Developmental trajectories of cerebrovascular reactivity in healthy children and young adults assessed with magnetic resonance imaging

Middle cerebral artery blood velocity and end-tidal carbon dioxide responses to moderate intensity cycling in children, adolescents, and adults

This study investigated the middle cerebral artery blood velocity (MCAv) response to constant work-rate moderate-intensity cycling exercise in 21 children (9.3 ± 0.8 yr), 17 adolescents (12.3 ± 0.4 yr), and 20 young adults (23.6 ± 2.4 yr). Participants completed an incremental ramp test to exhaustion on a cycle ergometer to determine maximal oxygen uptake and gas exchange threshold (GET) before completing three 6-min transitions at a moderate intensity (90% GET) on separate visits. On each visit, bilateral MCAv was measured by transcranial Doppler ultrasonography and breath-by-breath end-tidal carbon dioxide ([Formula: see text]) via a metabolic cart. Data were ensemble-averaged for each participant and analyzed using a monoexponential model. Absolute MCAv was significantly higher throughout exercise in children and adolescents compared with adults (P < 0.001). Children had a significantly lower relative increase in MCAv from baseline (∼12%) compared with adolescents (∼20%) and adults (∼18%, P < 0.040). All adolescents and adults had a monoexponential rise in MCAv and [Formula: see text], but this was observed in only eight children. Children and adolescents had a significantly faster MCAv time constant (τ, 12 ± 6 and 14 ± 8 s, respectively) compared with adults (27 ± 9 s, P < 0.001). MCAv τ was positively associated with faster [Formula: see text] τ in adolescents (r = 0.70, P = 0.002) but not in children (r = -0.20, P = 0.640). Time- and amplitude-based response parameters of MCAv kinetics were significantly associated with [Formula: see text] kinetics in adults (r = 0.50-0.74, P ≤ 0.025), but not in children (r = -0.19 to -0.48, P > 0.227). These findings suggest that the transition from childhood to adulthood impacts the MCAv response to exercise and the relationships between [Formula: see text] and MCAv kinetics during exercise.NEW & NOTEWORTHY This is the first study to find that children have smaller increases in Δ%MCAv (∼12%) during moderate-intensity exercise compared with adolescents and adults (∼18%-20%). Furthermore, MCAv kinetics were significantly faster in children and adolescents, compared with adults. MCAv kinetic responses were significantly and positively associated with [Formula: see text] kinetics in adults, but not in children. These novel data also suggest that the regulatory role of [Formula: see text] on MCAv during exercise begins to strengthen during adolescence.

Study on cerebral oxygen saturation in children with sleep-disordered breathing

To explore the association between the severity of sleep-disordered breathing, different types of respiratory events, peripheral oxygen saturation (SpO2), age and sleep stage on cerebral oxygen saturation (rSO2) in children. We enrolled children aged 4-14 years who were treated for snoring or mouth breathing at the Sleep Center of Beijing Children's Hospital, from February 2022 to July 2022. All children completed polysomnography, and SpO2, rSO2, and heart rate (HR) were recorded synchronously. A total of 70 children were included, including 16 (22.9%) with primary snoring, 38 (54.3%) with mild obstructive sleep apnea (OSA), and 16 (22.9%) with moderate-to-severe OSA. There were no significant differences in the mean rSO2 or minimum rSO2 among the primary snoring, mild OSA, and moderate-to-severe OSA groups (all p > 0.05). A total of 1119 respiratory events were included in the analysis. Regardless of the type of respiratory event, rSO2 and HR changes occur prior to fluctuations in SpO2. A mixed-effects model showed that ΔrSO2 was positively correlated with ΔSpO2, duration of respiratory event, mixed and obstructive apnea, central apnea, while negatively correlated with age and rapid eye movement (REM) sleep stage (all p < 0.05). Larger rSO2 fluctuations were impacted by a greater ΔSpO2, longer duration of respiratory events, younger age, apnea-related respiratory events and non-REM sleep stage. Thus, sleep disordered breathing in younger children warrants more attention. More research is needed to determine whether REM sleep has special protective effects on rSO2.

Macrovascular blood flow and microvascular cerebrovascular reactivity are regionally coupled in adolescence

Cerebrovascular imaging assessments are particularly challenging in adolescent cohorts, where not all modalities are appropriate, and rapid brain maturation alters hemodynamics at both macro- and microvascular scales. In a preliminary sample of healthy adolescents (n=12, 8-25 years), we investigated relationships between 4D flow MRI-derived blood velocity and blood flow in bilateral anterior, middle, and posterior cerebral arteries and BOLD cerebrovascular reactivity in associated vascular territories. As hypothesized, higher velocities in large arteries are associated with an earlier response to a vasodilatory stimulus (cerebrovascular reactivity delay) in the downstream territory. Higher blood flow through these arteries is associated with a larger BOLD response to a vasodilatory stimulus (cerebrovascular reactivity amplitude) in the associated territory. These trends are consistent in a case study of adult moyamoya disease. In our small adolescent cohort, macrovascular-microvascular relationships for velocity/delay and flow/CVR change with age, though underlying mechanisms are unclear. Our work emphasizes the need to better characterize this key stage of human brain development, when cerebrovascular hemodynamics are changing, and standard imaging methods offer limited insight into these processes. We provide important normative data for future comparisons in pathology, where combining macro- and microvascular assessments may better help us prevent, stratify, and treat cerebrovascular disease.

Alterations of cerebrovascular reactivity following pediatric mild traumatic brain injury are independent of neurodevelopmental changes

Cerebrovascular dysfunction following mild traumatic brain injury (mTBI) is understudied relative to other microstructural injuries, especially during neurodevelopment. The blood-oxygen level dependent response was used to investigate cerebrovascular reactivity (CVR) in response to hypercapnia following pediatric mTBI (pmTBI; ages 8-18 years), as well as pseudocontinuous arterial spin labeling to measure cerebral blood flow (CBF). Data were collected ∼1-week (N = 107) and 4 months (N = 73) post-injury. Sex- and age-matched healthy controls (HC) underwent identical examinations at comparable time points (N = 110 and N = 91). Subtle clinical and cognitive deficits existed at ∼1 week that resolved for some, but not all domains at 4 months post-injury. At both visits, pmTBI showed an increased maximal fit between end-tidal CO2 regressor and the cerebrovascular response across multiple regions (primarily fronto-temporal), as well as increased latency to maximal fit in independent regions (primarily posterior). Hypoperfusion was also noted within the bilateral cerebellum. A biphasic relationship existed between CVR amplitude and age (i.e., positive until 14.5 years, negative thereafter) in both gray and white matter, but these neurodevelopment effects did not moderate injury effects. CVR metrics were not associated with post-concussive symptoms or cognitive deficits. In conclusion, cerebrovascular dysfunction may persist for up to four months following pmTBI.

Physiologically based modeling reveals different risk of respiratory depression after fentanyl overdose between adults and children

Despite a rapid increase in pediatric mortality rate from prescription and illicit opioids, there is limited research on the dose-dependent impact of opioids on respiratory depression in children, the leading cause of opioid-associated death. In this article, we extend a previously developed translational model to cover pediatric populations by incorporating age-dependent pharmacokinetic, pharmacodynamic, and physiological changes compared to adults. Our model reproduced previous perioperative clinical findings that adults and children have similar risk of respiratory depression at the same plasma fentanyl concentration when specific endpoints (minute ventilation, CO2 tension in the blood) were used. However, our model points to a potential caveat that, in a perioperative setting, routine use of mechanical ventilation and supplemental oxygen maintained the blood and tissue oxygen partial pressures in patients and prevented the use of oxygen-related endpoints to evaluate the consequences of respiratory depression. In a community setting when such oxygenation procedures are not immediately available, our model suggests that the higher oxygen demand and reduced cerebrovascular reactivity could make children more susceptible to severe hypoxemia and brain hypoxia, even with the same plasma fentanyl concentration as adults. Our work indicates that when developing intervention strategies to protect children from opioid overdose in a community setting, these pediatric-specific factors may need to be considered.

The influence of maturation and sex on intracranial blood velocities during exercise in children

Cerebral blood velocity (CBv) increases in response to moderate exercise in humans, but the magnitude of change is smaller in children compared with postpubertal adolescents and adults. Whether sex differences exist in the anterior or posterior CBv response to exercise across pubertal development remains to be determined. We assessed middle cerebral artery (MCAv) and posterior cerebral artery (PCAv) blood velocity via transcranial Doppler in 38 prepubertal (18 males) and 48 postpubertal (23 males) with cerebrovascular and cardiorespiratory measures compared at baseline and ventilatory threshold. At baseline, MCAv was higher in both sexes pre- versus postpuberty. Females demonstrated a greater MCAv (P < 0.001) than their male counterparts (prepubertal females; 78 ± 11 cm·s-1 vs. prepubertal males; 72 ± 8 cm·s-1, and postpubertal females; 68 ± 10 cm·s-1 vs. postpubertal males; 62 ± 7 cm·s-1). During exercise, MCAv remained higher in postpubertal females versus males (81 ± 15 cm·s-1 vs. 73 ± 11 cm·s-1), but there were no differences in prepuberty. The relative increase in PCAv was greater in post- versus prepubertal females (51 ± 9 cm·s-1 vs. 45 ± 11 cm·s-1; P = 0.032) but was similar in males and females. Our findings suggest that biological sex alters anterior cerebral blood velocities at rest in both pre- and postpubertal youth, but the response to submaximal exercise is only influenced by sex postpuberty.NEW & NOTEWORTHY Cerebral blood velocity (CBv) in the anterior circulation was higher in females compared with males irrespective of maturational stage, but not in the posterior circulation. In response to exercise, females demonstrated a greater CBv compared with males, especially post-peak height velocity (post-PHV) where the CBv response to exercise was more pronounced. Our findings suggest that both CBv at rest and in response to acute submaximal exercise are altered by biological sex in a maturity-dependent manner.

Measuring Quantitative Cerebral Blood Flow in Healthy Children: A Systematic Review of Neuroimaging Techniques

Cerebral blood flow (CBF) is an important hemodynamic parameter to evaluate brain health. It can be obtained quantitatively using medical imaging modalities such as magnetic resonance imaging and positron emission tomography (PET). Although CBF in adults has been widely studied and linked with cerebrovascular and neurodegenerative diseases, CBF data in healthy children are sparse due to the challenges in pediatric neuroimaging. An understanding of the factors affecting pediatric CBF and its normal range is crucial to determine the optimal CBF measuring techniques in pediatric neuroradiology. This review focuses on pediatric CBF studies using neuroimaging techniques in 32 articles including 2668 normal subjects ranging from birth to 18 years old. A systematic literature search was conducted in PubMed, Embase, and Scopus and reported following the preferred reporting items for systematic reviews and meta-analyses (PRISMA). We identified factors (such as age, gender, mood, sedation, and fitness) that have significant effects on pediatric CBF quantification. We also investigated factors influencing the CBF measurements in infants. Based on this review, we recommend best practices to improve CBF measurements in pediatric neuroimaging. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 2.

Cerebrovascular reactivity increases across development in multiple networks as revealed by a breath-holding task: A longitudinal fMRI study

Functional magnetic resonance imaging (fMRI) has been widely used to understand the neurodevelopmental changes that occur in cognition and behavior across childhood. The blood-oxygen-level-dependent (BOLD) signal obtained from fMRI is understood to be comprised of both neuronal and vascular information. However, it is unclear whether the vascular response is altered across age in studies investigating development in children. Since the breath-hold (BH) task is commonly used to understand cerebrovascular reactivity (CVR) in fMRI studies, it can be used to account for developmental differences in vascular response. This study examines how the cerebrovascular response changes over age in a longitudinal children's BH data set from the Nathan Kline Institute (NKI) Rockland Sample (aged 6-18 years old at enrollment). A general linear model approach was applied to derive CVR from BH data. To model both the longitudinal and cross-sectional effects of age on BH response, we used mixed-effects modeling with the following terms: linear, quadratic, logarithmic, and quadratic-logarithmic, to find the best-fitting model. We observed increased BH BOLD signals in multiple networks across age, in which linear and logarithmic mixed-effects models provided the best fit with the lowest Akaike information criterion scores. This shows that the cerebrovascular response increases across development in a brain network-specific manner. Therefore, fMRI studies investigating the developmental period should account for cerebrovascular changes that occur with age.

Multifaceted neural and vascular pathologies after pediatric mild traumatic brain injury

Dynamic changes in neurodevelopment and cognitive functioning occur during adolescence, including a switch from reactive to more proactive forms of cognitive control, including response inhibition. Pediatric mild traumatic brain injury (pmTBI) affects these cognitions immediately post-injury, but the role of vascular versus neural injury in cognitive dysfunction remains debated. This study consecutively recruited 214 sub-acute pmTBI (8-18 years) and age/sex-matched healthy controls (HC; N = 186), with high retention rates (>80%) at four months post-injury. Multimodal imaging (functional MRI during response inhibition, cerebral blood flow and cerebrovascular reactivity) assessed for pathologies within the neurovascular unit. Patients exhibited increased errors of commission and hypoactivation of motor circuitry during processing of probes. Evidence of increased/delayed cerebrovascular reactivity within motor circuitry during hypercapnia was present along with normal perfusion. Neither age-at-injury nor post-concussive symptom load were strongly associated with imaging abnormalities. Collectively, mild cognitive impairments and clinical symptoms may continue up to four months post-injury. Prolonged dysfunction within the neurovascular unit was observed during proactive response inhibition, with preliminary evidence that neural and pure vascular trauma are statistically independent. These findings suggest pmTBI is characterized by multifaceted pathologies during the sub-acute injury stage that persist several months post-injury.

Cerebral blood flow and cerebrovascular reactivity are modified by maturational stage and exercise training status during youth

NEW FINDINGS

What is the central question of this study? Gonadal hormones modulate cerebrovascular function while insulin-like growth factor 1 (IGF-1) facilitates exercise-mediated cerebral angiogenesis; puberty is a critical period of neurodevelopment alongside elevated gonadal hormone and IGF-1 activity: but whether exercise training across puberty enhances cerebrovascular function is unkown. What is the main finding and its importance? Cerebral blood flow is elevated in endurance trained adolescent males when compared to untrained counterparts. However, cerebrovascular reactivity to hypercapnia is faster in trained vs. untrained children, but not adolescents. Exercise-induced improvements in cerebrovascular function are attainable as early as the first decade of life.

ABSTRACT

Global cerebral blood flow (gCBF) and cerebrovascular reactivity to hypercapnia (CV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) are modulated by gonadal hormone activity, while insulin-like growth factor 1 facilitates exercise-mediated cerebral angiogenesis in adults. Whether critical periods of heightened hormonal and neural development during puberty represent an opportunity to further enhance gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ is currently unknown. Therefore, we used duplex ultrasound to assess gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ in n = 128 adolescents characterised as endurance-exercise trained (males: n = 30, females: n = 36) or untrained (males: n = 29, females: n = 33). Participants were further categorised as pre- (males: n = 35, females: n = 33) or post- (males: n = 24, females: n = 36) peak height velocity (PHV) to determine pubertal or 'maturity' status. Three-factor ANOVA was used to identify main and interaction effects of maturity status, biological sex and training status on gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Data are reported as group means (SD). Pre-PHV youth demonstrated elevated gCBF and slowerCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response times than post-PHV counterparts (both: P ≤ 0.001). gCBF was only elevated in post-PHV trained males when compared to untrained counterparts (634 (43) vs. 578 (46) ml min-1 ; P = 0.007). However,CV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time was faster in pre- (72 (20) vs. 95 (29) s; P ≤ 0.001), but not post-PHV (P = 0.721) trained youth when compared to untrained counterparts. Cardiorespiratory fitness was associated with gCBF in post-PHV youth (r2  = 0.19; P ≤ 0.001) andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time in pre-PHV youth (r2  = 0.13; P = 0.014). Higher cardiorespiratory fitness during adolescence can elevate gCBF while exercise training during childhood primes the development of cerebrovascular function, highlighting the importance of exercise training during the early stages of life in shaping the cerebrovascular phenotype.

Impaired cerebrovascular reactivity in pediatric sickle cell disease using diffuse correlation spectroscopy

Cerebrovascular reactivity (CVR), defined as the ability of cerebral vasculature to dilate in response to a vasodilatory stimulus, is an integral mechanism in brain homeostasis that is thought to be impaired in sickle cell disease (SCD). This study used diffuse correlation spectroscopy and a simple breath-hold stimulus to quantify CVR non-invasively in a cohort of 12 children with SCD and 14 controls. Median [interquartile range] CVR was significantly decreased in SCD compared to controls (2.03 [1.31, 2.44] versus 3.49 [3.00, 4.11] %/mmHg, p = 0.028). These results suggest DCS may provide a feasible means to routinely monitor CVR impairments in pediatric SCD.

Comparing end-tidal CO2, respiration volume per time (RVT), and average gray matter signal for mapping cerebrovascular reactivity amplitude and delay with breath-hold task BOLD fMRI

Cerebrovascular reactivity (CVR), defined as the cerebral blood flow response to a vasoactive stimulus, is an imaging biomarker with demonstrated utility in a range of diseases and in typical development and aging processes. A robust and widely implemented method to map CVR involves using a breath-hold task during a BOLD fMRI scan. Recording end-tidal CO₂ (P_ETCO₂) changes during the breath-hold task is recommended to be used as a reference signal for modeling CVR amplitude in standard units (%BOLD/mmHg) and CVR delay in seconds. However, obtaining reliable P_ETCO₂ recordings requires equipment and task compliance that may not be achievable in all settings. To address this challenge, we investigated two alternative reference signals to map CVR amplitude and delay in a lagged general linear model (lagged-GLM) framework: respiration volume per time (RVT) and average gray matter BOLD response (GM-BOLD). In 8 healthy adults with multiple scan sessions, we compare spatial agreement of CVR maps from RVT and GM-BOLD to those generated with P_ETCO₂. We define a threshold to determine whether a P_ETCO₂ recording has "sufficient" quality for CVR mapping and perform these comparisons in 16 datasets with sufficient P_ETCO₂ and 6 datasets with insufficient P_ETCO₂. When P_ETCO₂ quality is sufficient, both RVT and GM-BOLD produce CVR amplitude maps that are nearly identical to those from P_ETCO₂ (after accounting for differences in scale), with the caveat they are not in standard units to facilitate between-group comparisons. CVR delays are comparable to P_ETCO₂ with an RVT regressor but may be underestimated with the average GM-BOLD regressor. Importantly, when P_ETCO₂ quality is insufficient, RVT and GM-BOLD CVR recover reasonable CVR amplitude and delay maps, provided the participant attempted the breath-hold task. Therefore, our framework offers a solution for achieving high quality CVR maps in both retrospective and prospective studies where sufficient P_ETCO₂ recordings are not available and especially in populations where obtaining reliable measurements is a known challenge (e.g., children). Our results have the potential to improve the accessibility of CVR mapping and to increase the prevalence of this promising metric of vascular health.

Assessment of cerebrovascular function in patients with sickle cell disease using transfer function analysis

In patients with sickle cell disease (SCD), the delivery of oxygen to the brain is compromised by anemia, abnormal rheology, and steno-occlusive vascular disease. Successful compensation depends on an increase in oxygen supply such as that provided by an increase in cerebral blood flow (CBF). We used magnetic resonance imaging to provide a high-resolution assessment of the ability of SCD patients to respond to a vasoactive stimulus in middle, anterior, and posterior cerebral artery territories for both white and gray matter. Cerebrovascular reactivity (CVR) was measured as the blood oxygen level dependent signal (a surrogate for CBF) response to an increase in the end tidal partial pressure of CO2 (PET CO2 ). The dynamic aspect of the response was measured as the time constant of the first order response kinetics (tau). To confirm and support these findings we used an alternative examination of the response, transfer function analysis (TFA), to measure the responsiveness (gain), the speed of response (phase), and the consistency of the response over time (coherence). We tested 34 patients with SCD and compared the results to those of 24 healthy controls participants. The results from a three-way ANOVA showed that patients with SCD have reduced CVR (p < 0.001) and lower coherence (p < 0.001) in gray matter and white matter and reduced gain in gray matter only (p < 0.001). In terms of the speed of the response to CO2 , tau (p < 0.001) and TFA phase (p < 0.001) were increased in SCD patients compared to healthy control subjects. These findings show that the cerebrovascular responsiveness to CO2 in patients with SCD is both decreased and slowed compared to healthy controls.

Dynamic onset response of the internal carotid artery to hypercapnia is blunted in children compared with adults

Intracranial blood velocity reactivity to a steady‐state hypercapnic stimulus has been shown to be similar in children and adults, but the onset response to hypercapnia is slower in the child. Given the vasodilatory effect of hypercapnia on the cerebrovasculature, assessment of vessel diameter, and blood flow are vital to fully elucidate whether the temporal hypercapnic response differs in children versus adults. Assessment of internal carotid artery (ICA) vessel diameter (ICAd), blood velocity (ICAv), volumetric blood flow (Q_ICA), and shear rate (ICA_SR) in response to a 4 min hypercapnic challenge was completed in children (n = 14, 8 girls; 9.8 ± 0.7 years) and adults (n = 17, 7 females; 24.7 ± 1.8 years). The dynamic onset responses of partial pressure of end‐tidal CO₂ (P_ETCO₂), Q_ICA, ICAv, and ICA_SR to hypercapnia were modeled, and mean response time (MRT) was computed. Following 4 min of hypercapnia, ICA reactivity and ICAd were comparable between the groups. Despite a similar MRT in P_ETCO₂ in children and adults, children had slower Q_ICA (children 108 ± 60 s vs. adults 66 ± 37 s; p = 0.023), ICAv (children 120 ± 52 s vs. adults 52 ± 31 s; p = 0.001), and ICA_SR (children 90 ± 27 s vs. adults 47 ± 36 s; p = 0.001) MRTs compared with adults. This is the first study to show slower hypercapnic hyperemic kinetic responses of the ICA in children. The mechanisms determining these differences and the need to consider the duration of hypercapnic exposure when assessing CVR in children should be considered in future studies.

Lag-Optimized Blood Oxygenation Level Dependent Cerebrovascular Reactivity Estimates Derived From Breathing Task Data Have a Stronger Relationship With Baseline Cerebral Blood Flow

Cerebrovascular reactivity (CVR), an important indicator of cerebrovascular health, is commonly studied with the Blood Oxygenation Level Dependent functional MRI (BOLD-fMRI) response to a vasoactive stimulus. Theoretical and empirical evidence suggests that baseline cerebral blood flow (CBF) modulates BOLD signal amplitude and may influence BOLD-CVR estimates. We address how acquisition and modeling choices affect the relationship between baseline cerebral blood flow (bCBF) and BOLD-CVR: whether BOLD-CVR is modeled with the inclusion of a breathing task, and whether BOLD-CVR amplitudes are optimized for hemodynamic lag effects. We assessed between-subject correlations of average GM values and within-subject spatial correlations across cortical regions. Our results suggest that a breathing task addition to a resting-state acquisition, alongside lag-optimization within BOLD-CVR modeling, can improve BOLD-CVR correlations with bCBF, both between- and within-subjects, likely because these CVR estimates are more physiologically accurate. We report positive correlations between bCBF and BOLD-CVR, both between- and within-subjects. The physiological explanation of this positive correlation is unclear; research with larger samples and tightly controlled vasoactive stimuli is needed. Insights into what drives variability in BOLD-CVR measurements and related measurements of cerebrovascular function are particularly relevant when interpreting results in populations with altered vascular and/or metabolic baselines or impaired cerebrovascular reserve.

Pediatric Traumatic Brain Injury: An Update on Preclinical Models, Clinical Biomarkers, and the Implications of Cerebrovascular Dysfunction

Traumatic brain injury (TBI) is a leading cause of pediatric morbidity and mortality. Recent studies suggest that children and adolescents have worse post-TBI outcomes and take longer to recover than adults. However, the pathophysiology and progression of TBI in the pediatric population are studied to a far lesser extent compared to the adult population. Common causes of TBI in children are falls, sports/recreation-related injuries, non-accidental trauma, and motor vehicle-related injuries. A fundamental understanding of TBI pathophysiology is crucial in preventing long-term brain injury sequelae. Animal models of TBI have played an essential role in addressing the knowledge gaps relating to pTBI pathophysiology. Moreover, a better understanding of clinical biomarkers is crucial to diagnose pTBI and accurately predict long-term outcomes. This review examines the current preclinical models of pTBI, the implications of pTBI on the brain’s vasculature, and clinical pTBI biomarkers. Finally, we conclude the review by speculating on the emerging role of the gut-brain axis in pTBI pathophysiology.

Kids With Altitude: Acute Mountain Sickness and Changes in Body Mass and Total Body Water in Children Travelling to 3800 m

INTRODUCTION

We explored the incidence of acute mountain sickness (AMS) and extravascular lung water (ELW) in children in relation to changes in body composition and peripheral blood oxygenation (S_pO₂) during 1 week of acclimatization to 3800 m.

METHODS

In a prospective cohort study, 10 children (7 female, ages 7-14 y) and 10 sex-matched adults (ages 23-44 y) traveled via automobile from sea level to 3000 m for 2 nights, followed by 4 nights at 3800 m. Each morning, body mass and body water (bioelectrical impedance), S_pO₂ (pulse oximetry), AMS (Lake Louise Questionnaire), and ELW (transthoracic echocardiography) were measured.

RESULTS

No differences were found between children and adults in S_pO₂ or ELW. At 3800 m 7 of 10 children were AMS+ vs 4 of 10 adults. Among those AMS+ at 3800 m, the severity was greater in children compared to adults (5±1 vs 3 ± 0; P=0.005). Loss of body mass occurred more quickly in children (day 5 vs day 7) and to a greater extent (-7±3% vs -2±2%; P<0.001); these changes were mediated via a larger relative loss in total body water in children than in adults (-6±5% vs -2±2%; P=0.027).

CONCLUSIONS

Children demonstrated a higher incidence of AMS than adults, with greater severity among those AMS+. The loss of body water and body mass at high altitude was also greater in children, albeit unrelated to AMS severity. In addition to awareness of AMS, strategies to maintain body weight and hydration in children traveling to high altitudes should be considered.

Cerebrovascular reactivity changes in acute concussion: a controlled cohort study

Background

Evidence suggests that cerebrovascular reactivity (CVR) increases within the first week after the incidence of concussion, indicating a disruption of normal autoregulation. We sought to extend these findings by investigating the effects of acute concussion on the speed of CVR response and by visualizing global and regional impairments in individual patients with acute concussion.

Methods

Twelve patients aged 18-40 years who experienced concussion less than a week before this prospective study were included. Twelve age and sex-matched healthy subjects constituted the control group. In all subjects, CVR was assessed using blood oxygenation level-dependent (BOLD) echo-planar imaging with a 3.0T MRI scanner, in combination with changes in end-tidal partial pressure of CO₂ (P_ETCO₂). In each subject, we calculated the CVR amplitude and CVR response time in the gray and white matter using a step and ramp P_ETCO₂ challenge. In addition, a separate group of 39 healthy controls who underwent the same evaluation was used to create atlases with voxel-wise mean and standard deviation of CVR amplitude and CVR response time. This allowed us to convert each metric of the 12 patients with concussion and the 12 healthy controls into z-score maps. These maps were then used to generate and compare z-scores for each of the two groups. Group differences were calculated using an unpaired t-test.

Results

All studies were well tolerated without any serious adverse events. Anatomical MRI was normal in all study subjects. No differences in CO₂ stimulus and O₂ targeting were observed between the two participant groups during BOLD MRI. With regard to the gray matter, the CVR magnitude step (P=0.117) and ramp + 10 (P=0.085) were not significantly different between patients with concussion and healthy controls. However, the tau value was significantly lower in patients with concussion than in the healthy controls (P=0.04). With regard to the white matter, the CVR magnitude step (P=0.003) and ramp + 10 (P=0.031) were significantly higher and the tau value (P=0.024) was significantly shorter in patients with concussion than in healthy controls. After z-score transformation, the z tau value was significantly lower in patients with concussion than in healthy controls (Grey matter P=0.021, White matter P=0.003). Comparison of the three parameters, z ramp + 10, z step, and z tau, between the two groups showed that z step (Grey matter P=0.035, White matter P=0.005) was the most sensitive parameter and that z ramp + 10 (Grey matter P=0.073, White matter P=0.126) was the least sensitive parameter.

Conclusions

Concussion is associated with patient-specific abnormalities in BOLD cerebrovascular responsiveness that occur in the setting of normal global CVR. This study demonstrates that the measurement of CVR using BOLD MRI and precise CO₂ control is a safe, reliable, reproducible, and clinically useful method for evaluating the state of patients with concussion. It has the potential to be an important tool for assessing the severity and duration of symptoms after concussion.

Differences in cerebrovascular regulation and ventilatory responses during ramp incremental cycling in children, adolescents, and adults

Regulation of cerebral blood flow during exercise in youth is poorly understood. This study investigated the cerebrovascular and ventilatory responses to a ramp incremental cycle test to exhaustion in 14 children (means ± SD age: 9.4 ± 0.9 yr), 14 adolescents (12.4 ± 0.4 yr), and 19 adults (23.4 ± 2.5 yr). Middle cerebral artery blood velocity (MCAv), partial pressure of end-tidal CO₂ ([Formula: see text]), and ventilatory parameters were analyzed at baseline, gas exchange threshold (GET), respiratory compensation point (RCP), and exhaustion. The increase in minute ventilation relative to CO₂ production during exercise was also calculated (V̇e/V̇co₂ slope). Relative change from baseline (Δ%) in MCAv was lower in children, compared with adolescents and adults at GET [15 ± 10% vs. 26 ± 14%, and 24 ± 10%, respectively, P ≤ 0.03, effect size (d) = 0.9] and RCP (13 ± 11% vs. 24 ± 16% and 27 ± 15%, respectively, P ≤ 0.05, d ≥ 0.8). Δ%MCAv was similar in adults and adolescents at all intensities and similar in all groups at exhaustion. The magnitude of the V̇_E/V̇co₂ slope was negatively associated with Δ%MCAv at GET and RCP across all participants (P ≤ 0.01, r = -0.37 to -0.48). Δ%[Formula: see text] was smaller in children and adolescents compared with adults at GET and RCP (P ≤ 0.05, d ≥ 0.6). In children, Δ%[Formula: see text] and Δ%MCAv were not associated from baseline-GET (r¯ = 0.14) and were moderately associated from RCP-exhaustion (r¯ = 0.49). These relationships strengthened with increasing age and were stronger in adolescents (baseline-GET: r¯ = 0.47, RCP-exhaustion: r¯ = 0.62) and adults (baseline-GET: r¯ = 0.66, RCP-exhaustion: r¯ = 0.78). These findings provide the first evidence on the development of the regulatory role of [Formula: see text] on MCAv during exercise in children, adolescents, and adults.NEW & NOTEWORTHY This is the first study to observe similar increases in cerebral blood flow during incremental exercise in adolescents and adults. Increases in cerebral blood flow during exercise were smaller in children compared with adolescents and adults and were associated with a greater V̇_E/V̇co₂ slope. This study also provides the first evidence on the progressive development of the regulatory role of end-tidal CO₂ on cerebral blood flow during exercise during the transition from childhood to adulthood.

Cerebrovascular Reactivity Measurement Using Magnetic Resonance Imaging: A Systematic Review

Cerebrovascular reactivity (CVR) magnetic resonance imaging (MRI) probes cerebral haemodynamic changes in response to a vasodilatory stimulus. CVR closely relates to the health of the vasculature and is therefore a key parameter for studying cerebrovascular diseases such as stroke, small vessel disease and dementias. MRI allows in vivo measurement of CVR but several different methods have been presented in the literature, differing in pulse sequence, hardware requirements, stimulus and image processing technique. We systematically reviewed publications measuring CVR using MRI up to June 2020, identifying 235 relevant papers. We summarised the acquisition methods, experimental parameters, hardware and CVR quantification approaches used, clinical populations investigated, and corresponding summary CVR measures. CVR was investigated in many pathologies such as steno-occlusive diseases, dementia and small vessel disease and is generally lower in patients than in healthy controls. Blood oxygen level dependent (BOLD) acquisitions with fixed inspired CO₂ gas or end-tidal CO₂ forcing stimulus are the most commonly used methods. General linear modelling of the MRI signal with end-tidal CO₂ as the regressor is the most frequently used method to compute CVR. Our survey of CVR measurement approaches and applications will help researchers to identify good practice and provide objective information to inform the development of future consensus recommendations.

MRI study of cerebral blood flow, vascular reactivity, and vascular coupling in systemic hypertension

Chronic hypertension alters cerebrovascular function, which can lead to neurovascular pathologies and increased susceptibility to neurological disorders. The purpose of this study was to utilize in vivo MRI methods with corroborating immunohistology to evaluate neurovascular dysfunction due to progressive chronic hypertension. The spontaneously hypertensive rat (SHR) model at different stages of hypertension was studied to evaluate: i) basal cerebral blood flow (CBF), ii) cerebrovascular reactivity (CVR) assessed by CBF and blood-oxygenation level dependent (BOLD) signal changes to hypercapnia, iii) neurovascular coupling from CBF and BOLD changes to forepaw stimulation, and iv) damage of neurovascular unit (NVU) components (microvascular, astrocyte and neuron densities). Comparisons were made with age-matched normotensive Wistar Kyoto (WKY) rats. In 10-week SHR (mild hypertension), basal CBF was higher (p < 0.05), CVR trended higher, and neurovascular coupling response was higher (p < 0.05), compared to normotensive rats. In 40-week SHR (severe hypertension), basal CBF, CVR, and neurovascular coupling response were reversed to similar or below normotensive rats, and were significantly different from 10-week SHR (p < 0.05). Immunohistological analysis found significantly reduced microvascular density, increased astrocytes, and reduced neuronal density in SHR at 40 weeks (p < 0.05) but not at 10 weeks (p > 0.05) in comparison to age-matched controls. In conclusion, we observed a bi-phasic basal CBF, CVR and neurovascular coupling response from early to late hypertension using in vivo MRI, with significant changes prior to changes in the NVU components from histology. MRI provides clinically relevant data that might be useful to characterize neurovascular pathogenesis on the brain in hypertension.

Persistent alterations in cerebrovascular reactivity in response to hypercapnia following pediatric mild traumatic brain injury

Much attention has been paid to the effects of mild traumatic brain injury (mTBI) on cerebrovascular reactivity in adult populations, yet it remains understudied in pediatric injury. In this study, 30 adolescents (12-18 years old) with pediatric mTBI (pmTBI) and 35 age- and sex-matched healthy controls (HC) underwent clinical and neuroimaging assessments during sub-acute (6.9 ± 2.2 days) and early chronic (120.4 ± 11.7 days) phases of injury. Relative to controls, pmTBI reported greater initial post-concussion symptoms, headache, pain, and anxiety, resolving by four months post-injury. Patients reported increased sleep issues and exhibited deficits in processing speed and attention across both visits. In grey-white matter interface areas throughout the brain, pmTBI displayed increased maximal fit/amplitude of a time-shifted end-tidal CO2 regressor to blood oxygen-level dependent response relative to HC, as well as increased latency to maximal fit. The alterations persisted through the early chronic phase of injury, with maximal fit being associated with complaints of ongoing sleep disturbances during post hoc analyses but not cognitive measures of processing speed or attention. Collectively, these findings suggest that deficits in the speed and degree of cerebrovascular reactivity may persist longer than current conceptualizations about clinical recovery within 30 days.

The influence of age and sex on cerebrovascular reactivity and ventilatory response to hypercapnia in children and adults

NEW FINDINGS

What is the central question of this study? In this study, we investigated intracranial cerebrovascular and ventilatory reactivity to 6% CO₂ in children and adults and explored dynamic ventilatory and cerebrovascular onset responses. What is the main finding and its importance? We showed that cerebrovascular reactivity was similar in children and adults, but the intracranial blood velocity onset response was markedly attenuated in children. Sex differences were apparent, with greater increases in intracranial blood velocity in females and lower ventilatory reactivity in adult females. Our study confirms the importance of investigating dynamic onset responses when assessing the influence of development on cerebrovascular regulation.

ABSTRACT

The purpose of this study was to compare the integrated intracranial cerebrovascular reactivity (CVR) and hypercapnic ventilatory response between children and adults and to explore the dynamic response of the middle cerebral artery mean velocity (MCA_V ). Children (n = 20; 9.9 ± 0.7 years of age) and adults (n = 21; 24.4 ± 2.0 years of age) completed assessment of CVR over 240 s using a fixed fraction of inspired CO₂ (0.06). Baseline MCA_V was higher in the adult females compared with the males (P ≤ 0.05). The MCA_V was greater in female children compared with male children (P ≤ 0.05) and in female adults compared with male adults (P ≤ 0.05) with hypercapnia. Relative CVR was similar in children and adults (3.71 ± 1.06 versus 4.12 ± 1.32% mmHg^-1 ; P = 0.098), with absolute CVR being higher in adult females than males (3.27 ± 0.86 versus 2.53 ± 0.70 cm s^-1  mmHg^-1 ; P ≤ 0.001). Likewise, the hypercapnic ventilatory response did not differ between the children and adults (1.89 ± 1.00 versus 1.77 ± 1.34 l min^-1  mmHg^-1 ; P = 0.597), but was lower in adult females than males (1.815 ± 0.37 versus 2.33 ± 1.66 l min^-1  mmHg^-1 ; P ≤ 0.05). The heart rate response to hypercapnia was greater in children than in adults (P = 0.001). A monoexponential regression model was used to characterize the dynamic onset, consisting of a delay term, amplitude and time constant (τ). The results revealed that MCA_V τ was faster in adults than in children (34 ± 18 versus 74 ± 28 s; P = 0.001). Our study provides new insight into the impact of age and sex on CVR and the dynamic response of the MCA_V to hypercapnia.

Fetal acute cerebral vasoreactivity to maternal hyperoxia in low-risk pregnancies: a cross-sectional study

OBJECTIVE

To establish whether fetal cerebral vasoreactivity (CVR_O2 ), following maternal hyperoxia, is predicted by fetal cerebral and uteroplacental Doppler pulsatility indices (PI) at baseline, fetal pulmonary vasoreactivity to oxygen (PVR_O2 ), gestational age (GA), or sex.

METHODS

Pulsatility index of middle (MCA), anterior (ACA), posterior cerebral (PCA), umbilical (UA), uterine (UtA), and branch of the pulmonary arteries (PA) were obtained, by ultrasound, before (baseline), during (hyperoxia) and after 15 minutes of maternal administration of 8 L/min of 100% oxygen, through a non-rebreathing face mask, in normal singleton pregnancies within 20 to 38 weeks' gestation. CVR_O2 was defined as changes greater than zero in z score of PI of the cerebral arteries from baseline to hyperoxia. Logistic modeling was applied to identify CVR_O2 predictors.

RESULTS

A total of 97 pregnancies were eligible. In the overall population, median z scores of PI of MCA, ACA, and PCA did not differ between study phases. Based on the logistic model, baseline z scores for cerebral PI and GA were the best predictors of CVR_O2 .

CONCLUSIONS

In low-risk pregnancies, fetal CVR_O2 to hyperoxia does not occur uniformly but depends on cerebral PI and GA at baseline. These findings may provide useful reference points when oxygen is administered in high-risk pregnancies.

Cerebral oxygenation in children with sleep-disordered breathing

Sleep-disordered breathing (SDB) is associated with neurocognitive and behavioral dysfunction, and structural brain abnormalities. Near infrared spectroscopy allows a continuous and non-invasive monitoring of brain tissue oxygenation, giving insight in some pathophysiological mechanisms potentially associated with SDB-related neurocognitive dysfunction. The present review summarizes the finding of studies describing brain tissue oxygenation in adults and children with SDB. Contrary to adults, mean nocturnal tissue oxygenation index (TOI) during sleep does not seem to be different in children with SDB as compared to healthy controls. During respiratory events such as apnoeas and hypopnoeas, the decrease in TOI precedes the peripheral, systemic desaturation. The decrease in TOI has been shown to be greater during apnoeas as compared to hypopnoeas, during rapid-eye movement sleep as compared to other sleep stages, in younger children as compared to their older counterparts, and in those with a high apnoea-hypopnoea index as compared with a low apnoea-hypopnoea index. Studies analyzing the association between repetitive changes in TOI and neurocognitive and behavioral dysfunction may help to decipher the pathophysiology of neurocognitive dysfunction associated with SDB in children.

Longitudinal Neuroimaging in Pediatric Traumatic Brain Injury: Current State and Consideration of Factors That Influence Recovery

Traumatic brain injury (TBI) is a leading cause of death and disability for children and adolescents in the U.S. and other developed and developing countries. Injury to the immature brain varies greatly from that of the mature, adult brain due to numerous developmental, pre-injury, and injury-related factors that work together to influence the trajectory of recovery during the course of typical brain development. Substantial damage to brain structure often underlies subsequent functional limitations that persist for years following pediatric TBI. Advances in neuroimaging have established an important role in the acute management of pediatric TBI, and magnetic resonance imaging (MRI) techniques have a particular relevance for the sequential assessment of long-term consequences from injuries sustained to the developing brain. The present paper will discuss the various factors that influence recovery and review the findings from the present neuroimaging literature to assess altered development and long-term outcome following pediatric TBI. Four MR-based neuroimaging modalities have been used to examine recovery from pediatric TBI longitudinally: (1) T1-weighted structural MRI is sensitive to morphological changes in gray matter volume and cortical thickness, (2) diffusion-weighted MRI is sensitive to changes in the microstructural integrity of white matter, (3) MR spectroscopy provides a sensitive assessment of metabolic and neurochemical alterations in the brain, and (4) functional MRI provides insight into the functional changes that occur as a result of structural damage and typical developmental processes. As reviewed in this paper, 13 cohorts have contributed to only 20 studies published to date using neuroimaging to examine longitudinal changes after TBI in pediatric patients. The results of these studies demonstrate considerable heterogeneity in post-injury outcome; however, the existing literature consistently shows that alterations in brain structure, function, and metabolism can persist for an extended period of time post-injury. With larger sample sizes and multi-site cooperation, future studies will be able to further examine potential moderators of outcome, such as the developmental, pre-injury, and injury-related factors discussed in the present review.

Educational fMRI: From the Lab to the Classroom

Functional MRI (fMRI) findings hold many potential applications for education, and yet, the translation of fMRI findings to education has not flowed. Here, we address the types of fMRI that could better support applications of neuroscience to the classroom. This 'educational fMRI' comprises eight main challenges: (1) collecting artifact-free fMRI data in school-aged participants and in vulnerable young populations, (2) investigating heterogenous cohorts with wide variability in learning abilities and disabilities, (3) studying the brain under natural and ecological conditions, given that many practical topics of interest for education can be addressed only in ecological contexts, (4) depicting complex age-dependent associations of brain and behaviour with multi-modal imaging, (5) assessing changes in brain function related to developmental trajectories and instructional intervention with longitudinal designs, (6) providing system-level mechanistic explanations of brain function, so that useful individualized predictions about learning can be generated, (7) reporting negative findings, so that resources are not wasted on developing ineffective interventions, and (8) sharing data and creating large-scale longitudinal data repositories to ensure transparency and reproducibility of fMRI findings for education. These issues are of paramount importance to the development of optimal fMRI practices for educational applications.

Cerebral Oxygenation During Respiratory Events in Children with Sleep-Disordered Breathing and Associated Disorders

OBJECTIVES

To evaluate changes in cerebral oxygenation by means of near-infrared spectroscopy during respiratory events in children with sleep-disordered breathing (SDB) and associated disorders.

STUDY DESIGN

Sixty-five children suspected of having SDB underwent a respiratory polygraphy with simultaneous recording of cerebral oxygenation indices. Respiratory events were analyzed by type of event, duration, variations of pulse oximetry (oxygen saturation [SpO₂]), cerebral tissue oxygenation index (TOI), and heart rate. Data were categorized according to the severity of SDB and age.

RESULTS

There were 540 obstructive and mixed apneas, 172 central apneas, and 393 obstructive hypopneas analyzed. The mean decreases in SpO₂ and TOI were 4.1 ± 3.1% and 3.4 ± 2.8%, respectively. The mean TOI decrease was significantly smaller for obstructive hypopnea compared with apneas. The TOI decrease was significantly less in children with mild SDB as compared with those with moderate-to-severe SDB and in children >7 years as compared with those <7 years old. TOI decreases correlated significantly with SpO₂ decreases, duration of event, and age, regardless of the type of event. In a multivariable regression model, predictive factors of TOI decreases were the type of respiratory event, SpO₂ decrease, apnea-hypopnea index, and age.

CONCLUSIONS

In children with SDB and associated disorders, cerebral oxygenation variations depend on the type of respiratory event, severity of SDB, and age.

An Investigation of Dynamic Cerebral Autoregulation in Adolescent Concussion

PURPOSE

Although cerebrovascular impairments are believed to contribute to concussion symptoms, little information exists regarding brain vasomotor control in adolescent concussion, particularly autoregulatory control that forms a fundamental response mechanism during changes in blood pressure. This research tested the hypothesis that adolescent concussion is marked by impaired dynamic cerebral autoregulation.

METHODS

Nineteen concussed adolescents (15 ± 2 yr, 13 females) and 18 healthy controls (15 ± 2 yr, 9 females) completed two sit-to-stand trials. Brachial artery blood pressure and cerebral blood flow velocity in the right middle cerebral artery were measured continuously. Dynamic rate of regulation was calculated as the rate of change in cerebrovascular resistance relative to the change in arterial blood pressure. The concussed adolescents were followed through their rehabilitation for up to 12 wk.

RESULTS

At the first visit, the concussed adolescents demonstrated reduced rate of regulation compared with the healthy controls (0.12 ± 0.04 vs 0.19 ± 0.06 s, P ≤ 0.001). At the concussed adolescents final visit, after symptom resolution, the rate of regulation improved to levels that were not different from the healthy controls (n = 9; 0.15 ± 0.08 vs 0.19 ± 0.06 s, P= 0.06). Two distinct groups were observed at the final visit with some individuals experiencing recovery of dynamic cerebral autoregulation and others showing no marked change from the initial visit.

CONCLUSION

Adolescents demonstrate an impairment in dynamic cerebral autoregulation after concussion that improves along with clinical symptoms in some individuals and remains impaired in others despite symptom resolution.

Reduced cerebrovascular reactivity among adolescents with bipolar disorder

OBJECTIVES

Cardiovascular disease (CVD) is excessive and premature among individuals with bipolar disorder (BD). Cerebrovascular reactivity (CVR), reflecting vasodilatory capacity of cerebral blood vessels in response to vasoactive substances, is a marker of cerebrovascular health. Despite informative findings in other diseases, CVR has not previously been examined in BD.

METHODS

Twenty-five adolescents with BD and 25 age and sex-matched psychiatrically healthy controls (HCs) completed six 15-second breath-holds (BHs) during functional magnetic resonance imaging (fMRI) at 3-Tesla. CVR was determined by comparing blood-oxygenation-level dependent (BOLD) signal changes. Voxel-wise contrasts were analyzed. Body mass index (BMI) was examined as a potential confound.

RESULTS

CVR in the posterior cingulate gyrus and periventricular white matter was lower in BD vs HC. After controlling for differences in BMI, additional between-group CVR differences were observed in the temporal poles, supramarginal gyrus, and lingual gyrus. There were no regions in which CVR was significantly greater in BD vs HC. CVR was not associated with mood symptoms.

CONCLUSIONS

This preliminary study provides evidence of cerebrovascular dysfunction in BD, including regions known to be susceptible to cerebrovascular dysfunction and/or disease. These findings warrant additional research on the causes and consequences of cerebrovascular dysfunction in early-onset BD.

Concordance among indices of intrinsic brain function: Insights from inter-individual variation and temporal dynamics

Various resting-state fMRI (R-fMRI) measures have been developed to characterize intrinsic brain activity. While each of these measures has gained a growing presence in the literature, questions remain regarding the common and unique aspects these indices capture. The present work provided a comprehensive examination of inter-individual variation and intra-individual temporal variation for commonly used measures, including fractional amplitude of low frequency fluctuations, regional homogeneity, voxel-mirrored homotopic connectivity, network centrality and global signal correlation. Regardless of whether examining intra-individual or inter-individual variation, we found that these definitionally distinct R-fMRI indices tend to exhibit a relatively high degree of covariation, which doesn't exist in phase randomized surrogate data. As a measure of intrinsic brain function, concordance for R-fMRI indices was negatively correlated with age across individuals (i.e., concordance among functional indices decreased with age). To understand the functional significance of concordance, we noted that higher concordance was generally associated with higher strengths of R-fMRI indices, regardless of whether looking through the lens of inter-individual (i.e., high vs. low concordance participants) or intra-individual (i.e., high vs. low concordance states identified via temporal dynamic analyses) differences. We also noted a linear increase in functional concordance together with the R-fMRI indices through the scan, which may suggest a decrease in arousal. The current study demonstrated an enriched picture regarding the relationship among the R-fMRI indices, as well as provided new insights in examining dynamic states within and between individuals.

Assessment of cerebral blood flow with magnetic resonance imaging in children with sickle cell disease: A quantitative comparison with transcranial Doppler ultrasonography

INTRODUCTION

Transcranial Doppler ultrasonography (TCD) is a clinical tool for stratifying ischemic stroke risk by identifying abnormal elevations in blood flow velocity (BFV) in the middle cerebral artery (MCA). However, TCD is not effective at screening for subtle neurologic injury such as silent cerebral infarcts. To better understand this disparity, we compared TCD measures of BFV with tissue-level cerebral blood flow (CBF) using arterial spin-labeling MRI in children with and without sickle cell disease, and correlated these measurements against clinical hematologic measures of disease severity.

METHODS

TCD and MRI assessment were performed in 13 pediatric sickle cell disease patients and eight age-matched controls. Using MRI measures of MCA diameter and territory weight, TCD measures of BFV in the MCA [cm/s] were converted into units of CBF [ml min^-1100 g^-1] for comparison.

RESULTS

There was no significant association between TCD measures of BFV in the MCA and corresponding MRI measures of CBF in patients (r = .28, p = .39) or controls (r = .10, p = .81). After conversion from BFV into units of CBF, a strong association was observed between TCD and MRI measures (r = .67, p = .017 in patients, r = .86, p = .006 in controls). While BFV in the MCA showed a lack of correlation with arterial oxygen content, an inverse association was observed for CBF measurements.

CONCLUSIONS

This study demonstrates that BFV in the MCA cannot be used as a surrogate marker for tissue-level CBF in children with sickle cell disease. Therefore, TCD alone may not be sufficient for understanding and predicting subtle pathophysiology in this population, highlighting the potential clinical value of tissue-level CBF.

Cerebrovascular and ventilatory responses to acute normobaric hypoxia in girls and women

Physiological responses to hypoxia in children are incompletely understood. We aimed to characterize cerebrovascular and ventilatory responses to normobaric hypoxia in girls and women. Ten healthy girls (9.9 ± 1.7 years; mean ± SD; Tanner stage 1 and 2) and their mothers (43.9 ± 3.5 years) participated. Internal carotid (ICA) and vertebral artery (VA) velocity, diameter and flow (Duplex ultrasound) was recorded pre- and post-1 h of hypoxic exposure (FIO2 = 0.126;~4000 m) in a normobaric chamber. Ventilation (V˙E) and respiratory drive (VT/TI) expressed as delta change from baseline (∆%), and end-tidal carbon-dioxide (PETCO2) were collected at baseline (BL) and 5, 30 and 60 min of hypoxia (5/30/60 HYP). Heart rate (HR) and oxygen saturation (SpO2) were also collected at these time-points. SpO2 declined similarly in girls (BL-97%; 60HYP-80%, P < 0.05) and women (BL-97%; 60HYP-83%, P < 0.05). Global cerebral blood flow (gCBF) increased in both girls (BL-687; 60HYP-912 mL·min-1, P < 0.05) and women (BL-472; 60HYP-651 mL·min-1, P < 0.01), though the ratio of ICA:VA (%) contribution to gCBF differed significantly (girls, 75:25%; women, 61:39%). The relative increase in V˙E peaked at 30HYP in both girls (27%, P < 0.05) and women (19%, P < 0.05), as did ∆%VT/TI (girls, 41%; women, 27%, P's < 0.05). Tidal volume (VT) increased in both girls and women at 5HYP, remaining elevated above baseline in girls at 30 and 60 HYP, but declined back toward baseline in women. Girls elicit similar increases in gCBF and ventilatory parameters in response to acute hypoxia as women, though the pattern and contributions mediating these responses appear developmentally divergent.

Anterior cerebral blood velocity and end-tidal CO2 responses to exercise differ in children and adults

Little is known about the response of the cerebrovasculature to acute exercise in children and how these responses might differ with adults. Therefore, we compared changes in middle cerebral artery blood velocity (MCAV_mean), end-tidal Pco₂ ([Formula: see text]), blood pressure, and minute ventilation (V̇e) in response to incremental exercise between children and adults. Thirteen children [age: 9 ± 1 (SD) yr] and thirteen sex-matched adults (age: 25 ± 4 yr) completed a maximal exercise test, during which MCAV_mean, [Formula: see text], and V̇e were measured continuously. These variables were measured at rest, at exercise intensities specific to individual ventilatory thresholds, and at maximum. Although MCAV_mean was higher at rest in children compared with adults, there were smaller increases in children (1-12%) compared with adults (12-25%) at all exercise intensities. There were alterations in [Formula: see text] with exercise intensity in an age-dependent manner [F(2.5,54.5) = 7.983, P < 0.001; η² = 0.266], remaining stable in children with increasing exercise intensity (37-39 mmHg; P > 0.05) until hyperventilation-induced reductions following the respiratory compensation point. In adults, [Formula: see text] increased with exercise intensity (36-45 mmHg, P < 0.05) until the ventilatory threshold. From the ventilatory threshold to maximum, adults showed a greater hyperventilation-induced hypocapnia than children. These findings show that the relative increase in MCAV_mean during exercise was attenuated in children compared with adults. There was also a weaker relationship between MCAV_mean and [Formula: see text] during exercise in children, suggesting that cerebral perfusion may be regulated by different mechanisms during exercise in the child.NEW & NOTEWORTHY These findings provide the first direct evidence that exercise increases cerebral blood flow in children to a lesser extent than in adults. Changes in end-tidal CO₂ parallel changes in cerebral perfusion in adults but not in children, suggesting age-dependent regulatory mechanisms of cerebral blood flow during exercise.

Vascular impairment as a pathological mechanism underlying long-lasting cognitive dysfunction after pediatric traumatic brain injury

Traumatic brain injury (TBI) is the leading cause of death and disability in children. Indeed, the acute mechanical injury often evolves to a chronic brain disorder with long-term cognitive, emotional and social dysfunction even in the case of mild TBI. Contrary to the commonly held idea that children show better recovery from injuries than adults, pediatric TBI patients actually have worse outcome than adults for the same injury severity. Acute trauma to the young brain likely interferes with the fine-tuned developmental processes and may give rise to long-lasting consequences on brain's function. This review will focus on cerebrovascular dysfunction as an important early event that may lead to long-term phenotypic changes in the brain after pediatric TBI. These, in turn may be associated with accelerated brain aging and cognitive dysfunction. Finally, since no effective treatments are currently available, understanding the unique pathophysiological mechanisms of pediatric TBI is crucial for the development of new therapeutic options.

Longitudinal Brain Magnetic Resonance Imaging CO2 Stress Testing in Individual Adolescent Sports-Related Concussion Patients: A Pilot Study

Background

Advanced neuroimaging studies in concussion have been limited to detecting group differences between concussion patients and healthy controls. In this small pilot study, we used brain magnetic resonance imaging (MRI) CO₂ stress testing to longitudinally assess cerebrovascular responsiveness (CVR) in individual sports-related concussion (SRC) patients.

Methods

Six SRC patients (three males and three females; mean age = 15.7, range = 15–17 years) underwent longitudinal brain MRI CO₂ stress testing using blood oxygen level-dependent (BOLD) MRI and model-based prospective end-tidal CO₂ targeting under isoxic conditions. First-level and second-level comparisons were undertaken using statistical parametric mapping (SPM) to score the scans and compare them to an atlas of 24 healthy control subjects.

Results

All tests were well tolerated and without any serious adverse events. Anatomical MRI was normal in all study participants. The CO₂ stimulus was consistent between the SRC patients and control subjects and within SRC patients across the longitudinal study. Individual SRC patients demonstrated both quantitative and qualitative patient-specific alterations in CVR (p < 0.005) that correlated strongly with clinical findings, and that persisted beyond clinical recovery.

Conclusion

Standardized brain MRI CO₂ stress testing is capable of providing a longitudinal assessment of CVR in individual SRC patients. Consequently, larger prospective studies are needed to examine the utility of brain MRI CO₂ stress testing as a clinical tool to help guide the evaluation, classification, and longitudinal management of SRC patients.

Quantitative mapping of cerebrovascular reactivity using resting-state BOLD fMRI: Validation in healthy adults

In conventional neuroimaging, cerebrovascular reactivity (CVR) is quantified primarily using the blood-oxygenation level-dependent (BOLD) functional MRI (fMRI) signal, specifically, as the BOLD response to intravascular carbon dioxide (CO2) modulations, in units of [%ΔBOLD/mmHg]. While this method has achieved wide appeal and clinical translation, the tolerability of CO2-related tasks amongst patients and the elderly remains a challenge in more routine and large-scale applications. In this work, we propose an improved method to quantify CVR by exploiting intrinsic fluctuations in CO2 and corresponding changes in the resting-state BOLD signal (rs-qCVR). Our rs-qCVR approach requires simultaneous monitoring of PETCO2, cardiac pulsation and respiratory volume. In 16 healthy adults, we compare our quantitative CVR estimation technique to the prospective CO2-targeting based CVR quantification approach (qCVR, the "standard"). We also compare our rs-CVR to non-quantitative alternatives including the resting-state fluctuation amplitude (RSFA), amplitude of low-frequency fluctuation (ALFF) and global-signal regression. When all subjects were pooled, only RSFA and ALFF were significantly associated with qCVR. However, for characterizing regional CVR variations within each subject, only the PETCO2-based rs-qCVR measure is strongly associated with standard qCVR in 100% of the subjects (p≤0.1). In contrast, for the more qualitative CVR measures, significant within-subject association with qCVR was only achieved in 50-70% of the subjects. Our work establishes the feasibility of extracting quantitative CVR maps using rs-fMRI, opening the possibility of mapping functional connectivity and qCVR simultaneously.