Lifelong exposure to high-altitude hypoxia in humans is associated with improved redox homeostasis and structural-functional adaptations of the neurovascular unit

Tracking neural activity patterns during rapid high-altitude transitions

Rapid adaptation to dynamic changes in the environment is critical for human survival. Extensive studies have observed human behavior and brain activity in a stable environment, but there is still a lack of understanding of how our brain's functional activity drives behavioral changes when the natural environment changes. Here, we used a virtual environment platform named the hypobaric hypoxia chamber to investigate how human neural oscillations and related behaviors are affected by changes in barometric pressure and oxygen levels at different altitudes. We found that physiological compensations occurred in the hypobaric hypoxic environment followed by an increase in altitude, resulting in faster response times in working memory tasks. High-density EEG analysis revealed a significant decrease in the alpha band at high altitudes, while delta band activity gradually increased with altitude. Moreover, a predictive model based on differences in brain regions across frequency bands identified the left supramarginal gyrus and left lingual gyrus as two hub regions strongly associated with hypoxia-related behavioral changes, and activations in the pallidum and amygdala could effectively decode the specific altitude at which humans are located. Our study underscores the potential of hypobaric hypoxia chambers as a powerful tool for dynamic high-altitude research and provides novel insights into how altitude-related changes shape human cognition and brain activity.

Cerebral blood flow in Andean children and adolescents living above 5,000 m

A number of indigenous populations have resided at high-altitude for generations, resulting in various phenotypical adaptations promoting successful high-altitude adaptation. Although many of these adaptations have been investigated in adults, little is known regarding how children residing at high-altitudes adapt, particularly with regards to the cerebrovasculature. Under hypoxic environments, compensatory changes in cerebral blood flow (CBF) are necessary to couple oxygen delivery to metabolic demand in the face of reduced oxygen availability. In this study, we aimed to evaluate regional and global cerebral blood flow (CBF) in Andean children and adolescents living in the highest city in the world at 5,100 m. Eighteen Andeans (ages 6-17 yr) living in La Rinconada, Peru (5,100 m) were compared with sex-, age-, size-, and maturity-matched high-altitude Sherpa (3,800 m) living in the Khumbu valley of Nepal (n = 18) and lowlanders (44 m) living at sea-level in Cardiff, Wales (n = 18). Volumetric measurements of CBF were assessed using duplex ultrasound of the internal carotid and vertebral arteries to assess regional and global CBF. End-tidal gases and oxygen saturation were measured in all groups, while hemoglobin concentration was assessed in Andeans. Despite Andeans living under a more severe hypoxic environment, global CBF was similar between Andeans (687.01 ± 138.49 mL/min), Sherpa (711.27 ± 110.27 mL/min), and lowlanders (704.88 ± 59.23 mL/min). In contrast, vertebral artery blood flow was 24% lower in Andeans (72.93 ± 31.60 mL/min) compared with lowlanders (96.09 ± 19.23 mL/min). The similar global CBF in Andean children might be achieved through elevated hemoglobin concentration. However, lower posterior perfusion in Andeans requires further investigation to determine whether it represents an adaptive or maladaptive response.NEW & NOTEWORTHY We have, for the first time, quantified volumetric regional and global cerebral blood flow in indigenous Andean children and adolescents living above 5,000 m in the highest city in the world. Compared with Sherpa living at moderate altitude (3,800 m), and lowlanders residing at sea level, Andeans present with similar global cerebral blood flow, but lower posterior flow despite being more hypoxemic. Similar to adults, differences in high hemoglobin concentration may drive this pattern of cerebral blood flow.

Phosphodiesterase inhibition restores hypoxia-induced cerebrovascular dysfunction subsequent to improved systemic redox homeostasis: A randomized, double-blind, placebo-controlled crossover study

To what extent sildenafil, a selective inhibitor of the type-5 phosphodiesterase modulates systemic redox status and cerebrovascular function during acute exposure to hypoxia remains unknown. To address this, 12 healthy males (aged 24 ± 3 y) participated in a randomized, placebo-controlled crossover study involving exposure to both normoxia and acute (60 min) hypoxia (FiO 2  = 0.14), followed by oral administration of 50 mg sildenafil and placebo (double-blinded). Venous blood was sampled for the ascorbate radical (A•-: electron paramagnetic resonance spectroscopy) and nitric oxide metabolites (NO: ozone-based chemiluminescence). Transcranial Doppler ultrasound was employed to determine middle cerebral artery velocity (MCAv), cerebral delivery of oxygen (CDO 2 ), dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity to hypo/hypercapnia (CVRCO2HYPO/HYPER). Cortical oxyhemoglobin (cO2Hb) and oxygenation index (OI) were assessed using pulsed continuous wave near infra-red spectroscopy. Hypoxia decreased total plasma NO (P = 0.008), CDO 2 (P = <0.001) and cO2Hb (P = 0.005). In hypoxia, sildenafil selectively reduced A•- (P = 0.018) and MCAV (P = 0.018), and increased dCA metrics of low-frequency phase (P = 0.029) and CVRCO2HYPER (P = 0.007) compared to hypoxia-placebo. Collectively, these findings provide evidence for a PDE-5 inhibitory pathway that enhances select aspects of cerebrovascular function in hypoxia subsequent to a systemic improvement in redox homeostasis and independent of altered vascular NO bioavailability.

Neurological Biomarker Profiles in Royal Canadian Air Force (RCAF) Pilots and Aircrew

BACKGROUND/OBJECTIVES

Military aviators can be exposed to extreme physiological stressors, including decompression stress, G-forces, as well as intermittent hypoxia and/or hyperoxia, which may contribute to neurobiological dysfunction/damage. This study aimed to investigate the levels of neurological biomarkers in military aviators to assess the potential risk of long-term brain injury and neurodegeneration.

METHODS

This cross-sectional study involved 48 Canadian Armed Forces (CAF) aviators and 48 non-aviator CAF controls. Plasma samples were analyzed for biomarkers of glial activation (GFAP), axonal damage (NF-L, pNF-H), oxidative stress (PRDX-6), and neurodegeneration (T-tau), along with S100b, NSE, and UCHL-1. The biomarker concentrations were quantified using multiplexed immunoassays.

RESULTS

The aviators exhibited significantly elevated levels of GFAP, NF-L, PRDX-6, and T-tau compared to the CAF controls (p < 0.001), indicating increased glial activation, axonal injury, and oxidative stress. Trends toward higher levels of S100b, NSE, and UCHL-1 were observed but were not statistically significant. The elevated biomarker levels suggest cumulative brain damage, raising concerns about potential long-term neurological impairments.

CONCLUSIONS

Military aviators are at increased risk for neurobiological injury, including glial and axonal damage, oxidative stress, and early neurodegeneration. These findings emphasize the importance of proactive monitoring and further research to understand the long-term impacts of high-altitude flight on brain health and to develop strategies for mitigating cognitive decline and neurodegenerative risks in this population.

[Research Progress in Stellate Ganglion Block and Regulation of Autonomic Nervous Functions]

Stellate ganglion (SG), also known as the cervical thoracic sympathetic ganglion, is formed by the fusion of the inferior cervical ganglion and the first thoracic ganglia. It is responsible for transmitting sympathetic innervation to the upper extremities, head, neck, and heart. Stellate ganglion block (SGB) involves the injection of local anesthetics on or around the surface of SG, which induces a broad autonomic nerve blocking effect in the area controlled by SG. As a nerve block technique, ultrasound-guided SGB can be used to regulate autonomic nervous functions and achieve therapeutic effects of the relevant diseases by interfering sympathetic nerve activities of SG. In this article, we summarized and reviewed the research and clinical applications of ultrasound-guided SGB in regulating autonomic nervous functions, focusing on publications from the past five years. Furthermore, we discussed the prospective development in applying SGB in the treatment of diseases associated with high-altitude environment.

Healthy Aging at Moderate Altitudes: Hypoxia and Hormesis

BACKGROUND

Aging is associated with cellular and tissue responses that collectively lead to functional and structural deterioration of tissues. Poor tissue oxygenation, or hypoxia, is involved in such responses and contributes to aging. Consequently, it could be speculated that living at higher altitude, and therefore in hypoxic conditions, accelerates aging. This assumption is indeed supported by evidence from populations residing at very high altitudes (>3,500 m). In contrast, accumulating evidence suggests that living at moderate altitudes (1,500-2,500 m) is protective rather than injurious, at least for some body systems.

SUMMARY

In this review, we critically evaluate the hypothesis that the physiological responses to mild hypoxic stress associated to life at moderate altitudes provide protection from many hypoxia-related diseases through hormesis. Hormesis means that a low dose of a stressor (here hypoxia) elicits beneficial outcomes, while a higher dose can be toxic and might explain at least in part the dose-dependent contrasting effects of hypoxia on the aging processes. The lack of well-designed longitudinal studies focusing on the role of the altitude of residence, and difficulties in accounting for potentially confounding factors such as migration, ethnicity/genetics, and socioeconomic and geoclimatic conditions, currently hampers translation of related research into uncontroversial paradigms.

KEY MESSAGES

Deeper investigations are required to understand the impact of altitude-related hypoxia on age-related diseases and to develop molecular markers of ageing/senescence in humans that are linked to hypoxia. However, the presented emerging evidence supports the view that hypoxia conditioning has the potential to improve life quality and expectancy.

Assessment of Dynamic Cerebral Autoregulation During Long-Term Exposure to High Altitude in Normal Subjects by Ultrasonography

PURPOSE

To evaluate changes in dynamic cerebral autoregulation (CA) during short-term and long-term exposure to high altitude with ultrasonography, and also study the sex differences in the response of CA to altitude.

METHODS

We assessed the differences in dynamic CA and measured with Doppler ultrasound of the bilateral internal carotid artery (ICA), vertebral artery (VA), and middle cerebral artery (MCA) and the values of basic information within 48 hours and at 2 years after arrival at Tibet in 65 healthy Han young Chinese volunteers, meanwhile, we compared the resistance index (RI) and pulsatility index (PI) of the right MCA at inhale oxygen 8 minutes when a newcomer with 2 years after arrival at Tibet.

RESULTS

With 2 years of altitude exposure, the SaO2 of all subjects was above 90%, the mean PEF, DAP, and HR values decreased, HGB increased compared within 48 hours in same-gender groups. Comparisons of cerebral hemodynamics between before 2 years and after 2 years within male and female groups, the mean RI and PI values of bilateral MCA after 2 years were significantly higher than before 2 years, at the same time, the mean RI and PI values of bilateral ICA were significant differences (P < .05) between male groups, with regard to female groups, showed that the mean RI and PI values of bilateral VA were significant differences (P < .05). Comparisons of Right MCA hemodynamics between after oxygen uptake 8 minutes and 2 years, the mean RI and PI values were no significant difference within male and female groups (P > .05).

CONCLUSIONS

Acute mountain sickness could result from an alteration of dynamic autoregulation of cerebral blood flow, but the impaired autoregulation may be corrected with the extension of time, furthermore, the response of CA to altitude in males and females are different.

Altitude illnesses

Millions of people visit high-altitude regions annually and more than 80 million live permanently above 2,500 m. Acute high-altitude exposure can trigger high-altitude illnesses (HAIs), including acute mountain sickness (AMS), high-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE). Chronic mountain sickness (CMS) can affect high-altitude resident populations worldwide. The prevalence of acute HAIs varies according to acclimatization status, rate of ascent and individual susceptibility. AMS, characterized by headache, nausea, dizziness and fatigue, is usually benign and self-limiting, and has been linked to hypoxia-induced cerebral blood volume increases, inflammation and related trigeminovascular system activation. Disruption of the blood-brain barrier leads to HACE, characterized by altered mental status and ataxia, and increased pulmonary capillary pressure, and related stress failure induces HAPE, characterized by dyspnoea, cough and exercise intolerance. Both conditions are progressive and life-threatening, requiring immediate medical intervention. Treatment includes supplemental oxygen and descent with appropriate pharmacological therapy. Preventive measures include slow ascent, pre-acclimatization and, in some instances, medications. CMS is characterized by excessive erythrocytosis and related clinical symptoms. In severe CMS, temporary or permanent relocation to low altitude is recommended. Future research should focus on more objective diagnostic tools to enable prompt treatment, improved identification of individual susceptibilities and effective acclimatization and prevention options.

The effects of long-term high-altitude exposure on cognition: A meta-analysis

Long-term high altitudes (HA) exposure's impact on cognition has yielded inconsistent findings in previous research. To address this, we conducted a meta-analysis of 49 studies (6191 individuals) to comprehensively evaluate this effect. Moderating factors such as cognitive task type, altitude (1500-2500 m, 2500-4000 m, and above 4000 m), residential type (chronic and lifelong), adaptation level and demographic factors were analyzed. Cognitive tasks were classified into eight categories: perceptual processes, psychomotor function, long-term memory, working memory, inhibitory control, problem-solving, language, and others. Results revealed a moderate negative effect of HA on cognitive performance (g = -.40, SE =.18, 95% CI = -.76 to -.05). Psychomotor function and long-term memory notably experience the most pronounced decline, while working memory and language skills show moderate decreases due to HA exposure. However, perceptual processes, inhibitory control, and problem-solving abilities remain unaffected. Moreover, residing at altitudes above 4000 m and being a HA immigrant are associated with significant cognitive impairment. In summary, our findings indicate a selective adaptation of cognitive performance to HA conditions.

Hypoxia Sensing and Responses in Parkinson's Disease

Parkinson's disease (PD) is associated with various deficits in sensing and responding to reductions in oxygen availability (hypoxia). Here we summarize the evidence pointing to a central role of hypoxia in PD, discuss the relation of hypoxia and oxygen dependence with pathological hallmarks of PD, including mitochondrial dysfunction, dopaminergic vulnerability, and alpha-synuclein-related pathology, and highlight the link with cellular and systemic oxygen sensing. We describe cases suggesting that hypoxia may trigger Parkinsonian symptoms but also emphasize that the endogenous systems that protect from hypoxia can be harnessed to protect from PD. Finally, we provide examples of preclinical and clinical research substantiating this potential.

Tau Protein Alterations Induced by Hypobaric Hypoxia Exposure

Tauopathies are a group of neurodegenerative diseases whose central feature is dysfunction of the microtubule-associated protein tau (MAPT). Although the exact etiology of tauopathies is still unknown, it has been hypothesized that their onset may occur up to twenty years before the clear emergence of symptoms, which has led to questions about whether the prognosis of these diseases can be improved by, for instance, targeting the factors that influence tauopathy development. One such factor is hypoxia, which is strongly linked to Alzheimer's disease because of its association with obstructive sleep apnea and has been reported to affect molecular pathways related to the dysfunction and aggregation of tau proteins and other biomarkers of neurological damage. In particular, hypobaric hypoxia exposure increases the activation of several kinases related to the hyperphosphorylation of tau in neuronal cells, such as ERK, GSK3β, and CDK5. In addition, hypoxia also increases the levels of inflammatory molecules (IL-β1, IL-6, and TNF-α), which are also associated with neurodegeneration. This review discusses the many remaining questions regarding the influence of hypoxia on tauopathies and the contribution of high-altitude exposure to the development of these diseases.

High altitude hypoxia and oxidative stress: The new hope brought by free radical scavengers

Various strategies can be employed to prevent and manage altitude illnesses, including habituation, oxygenation, nutritional support, and medication. Nevertheless, the utilization of drugs for the prevention and treatment of hypoxia is accompanied by certain adverse effects. Consequently, the quest for medications that exhibit minimal side effects while demonstrating high efficacy remains a prominent area of research. In this context, it is noteworthy that free radical scavengers exhibit remarkable anti-hypoxia activity. These scavengers effectively eliminate excessive free radicals and mitigate the production of reactive oxygen species (ROS), thereby safeguarding the body against oxidative damage induced by plateau hypoxia. In this review, we aim to elucidate the pathogenesis of plateau diseases that are triggered by hypoxia-induced oxidative stress at high altitudes. Additionally, we present a range of free radical scavengers as potential therapeutic and preventive approaches to mitigate the occurrence of common diseases associated with hypoxia at high altitudes.

Acute hypoxia impairs posterior cerebral bioenergetics and memory in man

Hypoxia has the potential to impair cognitive function; however, it is still uncertain which cognitive domains are adversely affected. We examined the effects of acute hypoxia (∼7 h) on central executive (Go/No-Go) and non-executive (memory) tasks and the extent to which impairment was potentially related to regional cerebral blood flow and oxygen delivery (CDO2 ). Twelve male participants performed cognitive tasks following 0, 2, 4 and 6 h of passive exposure to both normoxia and hypoxia (12% O2 ), in a randomized block cross-over single-blinded design. Middle cerebral artery (MCA) and posterior cerebral artery (PCA) blood velocities and corresponding CDO2 were determined using bilateral transcranial Doppler ultrasound. In hypoxia, MCA DO2 was reduced during the Go/No-Go task (P = 0.010 vs. normoxia, main effect), and PCA DO2 was attenuated during memorization (P = 0.005 vs. normoxia) and recall components (P = 0.002 vs. normoxia) in the memory task. The accuracy of the memory task was also impaired in hypoxia (P = 0.049 vs. normoxia). In contrast, hypoxia failed to alter reaction time (P = 0.19 vs. normoxia) or accuracy (P = 0.20 vs. normoxia) during the Go/No-Go task, indicating that selective attention and response inhibition were preserved. Hypoxia did not affect cerebral blood flow or corresponding CDO2 responses to cognitive activity (P > 0.05 vs. normoxia). Collectively, these findings highlight the differential sensitivity of cognitive domains, with memory being selectively vulnerable in hypoxia. NEW FINDINGS: What is the central question of this study? We sought to examine the effects of acute hypoxia on central executive (selective attention and response inhibition) and non-executive (memory) performance and the extent to which impairments are potentially related to reductions in regional cerebral blood flow and oxygen delivery. What is the main finding and its importance? Memory was impaired in acute hypoxia, and this was accompanied by a selective reduction in posterior cerebral artery oxygen delivery. In contrast, selective attention and response inhibition remained well preserved. These findings suggest that memory is selectively vulnerable to hypoxia.