White matter and hypoxic hypobaria in humans

Occupational and domestic exposure associations with cerebral small vessel disease and vascular dementia: A systematic review and meta-analysis

INTRODUCTION

The prevalence of cerebral smallvessel disease (SVD) and vascular dementia according to workplace or domestic exposure to hazardous substances is unclear.

METHODS

We included studies assessing occupational and domestic hazards/at-risk occupations and SVD features. We pooled prevalence estimates using random-effects models where possible, or presented a narrative synthesis.

RESULTS

We included 85 studies (n = 47,743, mean age = 44·5 years). 52/85 reported poolable estimates. SVD prevalence in populations exposed to carbon monoxide was 81%(95% CI = 60-93%; n = 1373; results unchanged in meta-regression), carbon disulfide73% (95% CI = 54-87%; n = 131), 1,2-dichloroethane 88% (95% CI = 4-100%, n = 40), toluene 82% (95% CI = 3-100%, n = 64), high altitude 49% (95% CI = 38-60%; n = 164),and diving 24% (95% CI = 5-67%, n = 172). We narratively reviewed vascular dementia studies and contact sport, lead, military, pesticide, and solvent exposures as estimates were too few/varied to pool.

DISCUSSION

SVD and vascular dementia may be associated with occupational/domestic exposure to hazardous substances. CRD42021297800.

Early Human Pathophysiological Responses to Exertional Hypobaric Decompression Stress

INTRODUCTION: Consistent blood biomarkers of hypobaric (altitude) decompression stress remain elusive. Recent laboratory investigation of decompression sickness risk at 25,000 ft (7620 m) enabled evaluation of early pathophysiological responses to exertional decompression stress.METHODS: In this study, 15 healthy men, aged 20-50 yr, undertook 2 consecutive (same-day) ascents to 25,000 ft (7620 m) for 60 and 90 min, breathing 100% oxygen, each following 1 h of prior denitrogenation. Venous blood was sampled at baseline (T0), immediately after the second ascent (T8), and next morning (T24). Analyses encompassed whole blood hematology, endothelial microparticles, and soluble markers of cytokine response, endothelial function, inflammation, coagulopathy, oxidative stress, and brain insult, plus cortisol and creatine kinase.RESULTS: Acute hematological effects on neutrophils (mean 72% increase), eosinophils (40% decrease), monocytes (37% increase), and platelets (7% increase) normalized by T24. Consistent elevation (mean five-fold) of the cytokine interleukin-6 (IL-6) at T8 was proinflammatory and associated with venous gas emboli (microbubble) load. Levels of C-reactive protein and complement peptide C5a were persistently elevated at T24, the former by 100% over baseline. Additionally, glial fibrillary acidic protein, a sensitive marker of traumatic brain injury, increased by a mean 10% at T24.CONCLUSIONS: This complex composite environmental stress, comprising the triad of hyperoxia, decompression, and moderate exertion at altitude, provoked pathophysiological changes consistent with an IL-6 cytokine-mediated inflammatory response. Multiple persistent biomarker disturbances at T24 imply incomplete recovery the day after exposure. The elevation of glial fibrillary acidic protein similarly implies incomplete resolution following recent neurological insult.Connolly DM, Madden LA, Edwards VC, D'Oyly TJ, Harridge SDR, Smith TG, Lee VM. Early human pathophysiological responses to exertional hypobaric decompression stress. Aerosp Med Hum Perform. 2023; 94(10):738-749.

Brain structure and neurocognitive function in two professional mountaineers during 35 days severe normobaric hypoxia

BACKGROUND

Animal studies have elicited therapeutic potential of severe ambient hypoxia for neurodegenerative diseases. However, uncertainties exist relative to individual (mal-)adaption mechanisms of the brain to hypoxia. To investigate the effects of hypoxia on cognitive performance and cerebral morphology, two healthy professional mountaineers (participants A and B) conducted a 35-day study with 14 consecutive days of exposure to oxygen concentrations between 8% and 8.8%.

METHODS

Participants were examined at seven time points by cerebral magnetic resonance imaging (MRI) and at 27 time points by a test battery covering a spectrum of cognitive domains. Blood neuron specific enolase and neurofilament light chain levels were analyzed before, during and after hypoxia.

RESULTS

While cognitive performance was largely unaffected by hypoxic conditions, morphological MRI changes were evident. White matter volumes increased (max.: A: 4.3% ± 0.9%; B: 4.5% ± 1.9%) while grey matter volumes (A: -1.5% ± 0.8%; B: -2.5% ± 0.9%) and cerebrospinal fluid volumes (A: -2.7% ± 2.4%; B: -5.9% ± 8.2%) decreased. Furthermore, the number (A: 11 to 17; B: 26 to 126) and volumes (A: 140%; B: 285%) of white matter hyperintensities increased in hypoxia but had returned to baseline after a 3.5-month recovery phase. Diffusion weighted imaging of the white matter indicated cytotoxic edema formation. Biochemical markers of brain injury remained grossly negative.

DISCUSSION

Severe sustained normobaric hypoxia was tolerated in highly selected individuals which may pave the way for future translational studies of the therapeutic potential of hypoxia in neurodegenerative diseases.

High altitude is associated with pTau deposition, neuroinflammation, and myelin loss

Mammals are able to adapt to high altitude (HA) if appropriate acclimation occurs. However, specific occupations (professional climbers, pilots, astronauts and other) can be exposed to HA without acclimation and be at a higher risk of brain consequences. In particular, US Air Force U2-pilots have been shown to develop white matter hyperintensities (WMH) on MRI. Whether WMH are due to hypoxia or hypobaria effects is not understood. We compared swine brains exposed to 5000 feet (1524 m) above sea level (SL) with 21% fraction inspired O₂ (FiO₂) (Control group [C]; n = 5) vs. 30,000 feet (9144 m) above SL with 100% FiO₂ group (hypobaric group [HYPOBAR]; n = 6). We performed neuropathologic assessments, molecular analyses, immunohistochemistry (IHC), Western Blotting (WB), and stereology analyses to detect differences between HYPOBAR vs. Controls. Increased neuronal insoluble hyperphosphorylated-Tau (pTau) accumulation was observed across different brain regions, at histological level, in the HYPOBAR vs. Controls. Stereology-based cell counting demonstrated a significant difference (p < 0.01) in pTau positive neurons between HYPOBAR and C in the Hippocampus. Higher levels of soluble pTau in the Hippocampus of HYPOBAR vs. Controls were also detected by WB analyses. Additionally, WB demonstrated an increase of IBA-1 in the Cerebellum and a decrease of myelin basic protein (MBP) in the Hippocampus and Cerebellum of HYPOBAR vs. Controls. These findings illustrate, for the first time, changes occurring in large mammalian brains after exposure to nonhypoxic-hypobaria and open new pathophysiological views on the interaction among hypobaria, pTau accumulation, neuroinflammation, and myelination in large mammals exposed to HA.

Digoxin Ameliorates Glymphatic Transport and Cognitive Impairment in a Mouse Model of Chronic Cerebral Hypoperfusion

The glymphatic system plays a pivotal role in maintaining cerebral homeostasis. Chronic cerebral hypoperfusion, arising from small vessel disease or carotid stenosis, results in cerebrometabolic disturbances ultimately manifesting in white matter injury and cognitive dysfunction. However, whether the glymphatic system serves as a potential therapeutic target for white matter injury and cognitive decline during hypoperfusion remains unknown. Here, we established a mouse model of chronic cerebral hypoperfusion via bilateral common carotid artery stenosis. We found that the hypoperfusion model was associated with significant white matter injury and initial cognitive impairment in conjunction with impaired glymphatic system function. The glymphatic dysfunction was associated with altered cerebral perfusion and loss of aquaporin 4 polarization. Treatment of digoxin rescued changes in glymphatic transport, white matter structure, and cognitive function. Suppression of glymphatic functions by treatment with the AQP4 inhibitor TGN-020 abolished this protective effect of digoxin from hypoperfusion injury. Our research yields new insight into the relationship between hemodynamics, glymphatic transport, white matter injury, and cognitive changes after chronic cerebral hypoperfusion.

Hypoxic Hypoxia and Brain Function in Military Aviation: Basic Physiology and Applied Perspectives

Acute hypobaric hypoxia (HH) is a major physiological threat during high-altitude flight and operations. In military aviation, although hypoxia-related fatalities are rare, incidences are common and are likely underreported. Hypoxia is a reduction in oxygen availability, which can impair brain function and performance of operational and safety-critical tasks. HH occurs at high altitude, due to the reduction in atmospheric oxygen pressure. This physiological state is also partially simulated in normobaric environments for training and research, by reducing the fraction of inspired oxygen to achieve comparable tissue oxygen saturation [normobaric hypoxia (NH)]. Hypoxia can occur in susceptible individuals below 10,000 ft (3,048 m) in unpressurised aircrafts and at higher altitudes in pressurised environments when life support systems malfunction or due to improper equipment use. Between 10,000 ft and 15,000 ft (4,572 m), brain function is mildly impaired and hypoxic symptoms are common, although both are often difficult to accurately quantify, which may partly be due to the effects of hypocapnia. Above 15,000 ft, brain function exponentially deteriorates with increasing altitude until loss of consciousness. The period of effective and safe performance of operational tasks following exposure to hypoxia is termed the time-of-useful-consciousness (TUC). Recovery of brain function following hypoxia may also lag beyond arterial reoxygenation and could be exacerbated by repeated hypoxic exposures or hyperoxic recovery. This review provides an overview of the basic physiology and implications of hypoxia for military aviation and discusses the utility of hypoxia recognition training.

Hypobaric Decompression and White Matter Hyperintensities: An Evaluation of the NATO Standard

INTRODUCTION: In their seminal work, McGuire and colleagues reported an increased incidence of white matter hyperintensities (WMH) in a cohort of U2 pilots and hypobaric chamber personnel. WMH burden was higher in U2 pilots with previous reports of decompression sickness (DCS), and McGuire's reports have raised concerns regarding adverse outcomes in the aftermath of hypobaric exposures. Accordingly, a NATO working group has recently revised its standard recommendations regarding hypobaric exposures, including measures to mitigate the risk of WMH. Mandatory recovery time for up to 72 h between repeated exposures has been suggested on the basis of experimental evidence. However, we argue that the evidence is scarce which supports restricting repeated exposures to mitigate WMH. It is plausible that WMH is correlated with DCS and emphasis should be made on limiting the duration of exposures rather than restricting short and repeated exposures. The profiles in the NATO recommendations are meant to mitigate the risk of DCS. Still, they will potentially expose NATO Air Force and Special Operations personnel to flight profiles that can give rise to DCS incidence above 35%. Awaiting reliable data, we recommend limiting the duration of exposures and allowing for short repeated exposures.Ottestad W, Hansen TA, Ksin JI. Hypobaric decompression and white matter hyperintensities: an evaluation of the NATO standard. Aerosp Med Hum Perform. 2021; 92(1):3942.

Cognitive Impairment During Combined Normobaric vs. Hypobaric and Normoxic vs. Hypoxic Acute Exposure

INTRODUCTION: Exposure to hypoxia has a deleterious effect on cognitive function; however, the putative effect of hypobaria remains unclear. The present study aimed to evaluate cognitive performance in pilot trainees who were exposed to acute normobaric (NH) and hypobaric hypoxia (HH). Of relevance for military pilots, we also aimed to assess cognitive performance in hypobaric normoxia (HN).METHODS: A total of 16 healthy pilot trainees were exposed to 4 randomized conditions (i.e., normobaric normoxia, NN, altitude level of 440 m; HH at 5500 m; NH, altitude simulation of 5500 m; and HN). Subjects performed a cognitive assessment (KLT-R test). Cerebral oxygen delivery (cDO₂) was estimated based middle cerebral artery blood flow velocity (MCAv) and pulse oxygen saturation (S_po₂) monitored during cognitive assessment.RESULTS: Percentage of errors increased in NH (14.3 9.1%) and HH (12.9 6.4%) when compared to NN (6.5 4.1%) and HN (6.0 4.0%). Number of calculations accomplished was lower only in HH than in NN and HN. When compared to NN, cDO₂ decreased in NH and HH.DISCUSSION: Cognitive performance was decreased similarly in acute NH and HH. The cDO₂ reduction in NH and HH implies insufficient MCAv increase to ensure cognitive performance maintenance. The present study suggests negligible hypobaric influence on cognitive performance in hypoxia and normoxia.Aebi MR, Bourdillon N, Noser P, Millet GP, Bron D. Cognitive impairment during combined normobaric vs. hypobaric and normoxic vs. hypoxic acute exposure. Aerosp Med Hum Perform. 2020; 91(11):845851.

White matter and hypoxic hypobaria in humans

Occupational exposure to hypobaria (low atmospheric pressure) is a risk factor for reduced white matter integrity, increased white matter hyperintensive burden, and decline in cognitive function. We tested the hypothesis that a discrete hypobaric exposure will have a transient impact on cerebral physiology. Cerebral blood flow, fractional anisotropy of water diffusion in cerebral white matter, white matter hyperintensity volume, and concentrations of neurochemicals were measured at baseline and 24 hr and 72 hr postexposure in N = 64 healthy aircrew undergoing standard US Air Force altitude chamber training and compared to N = 60 controls not exposed to hypobaria. We observed that hypobaric exposure led to a significant rise in white matter cerebral blood flow (CBF) 24 hr postexposure that remained elevated, albeit not significantly, at 72 hr. No significant changes were observed in structural measurements or gray matter CBF. Subjects with higher baseline concentrations of neurochemicals associated with neuroprotection and maintenance of normal white matter physiology (glutathione, N-acetylaspartate, glutamate/glutamine) showed proportionally less white matter CBF changes. Our findings suggest that discrete hypobaric exposure may provide a model to study white matter injury associated with occupational hypobaric exposure.