The cerebral venous system and hypoxia

Intermittent hypoxia training effectively protects against cognitive decline caused by acute hypoxia exposure

Intermittent hypoxia training (IHT) is a promising approach that has been used to induce acclimatization to hypoxia and subsequently lower the risk of developing acute mountain sickness (AMS). However, the effects of IHT on cognitive and cerebrovascular function after acute hypoxia exposure have not been characterized. In the present study, we first confirmed that the simplified IHT paradigm was effective at relieving AMS at 4300 m. Second, we found that IHT improved participants' cognitive and neural alterations when they were exposed to hypoxia. Specifically, impaired working memory performance, decreased conflict control function, impaired cognitive control, and aggravated mental fatigue induced by acute hypoxia exposure were significantly alleviated in the IHT group. Furthermore, a reversal of brain swelling induced by acute hypoxia exposure was visualized in the IHT group using magnetic resonance imaging. An increase in cerebral blood flow (CBF) was observed in multiple brain regions of the IHT group after hypoxia exposure as compared with the control group. Based on these findings, the simplified IHT paradigm might facilitate hypoxia acclimatization, alleviate AMS symptoms, and increase CBF in multiple brain regions, thus ameliorating brain swelling and cognitive dysfunction.

Papilledema Secondary to Barotrauma in a Young Adult With Severe Status Asthmaticus With Ventilatory Failure, Pneumothorax, and a Complex Clinical Course

Intubation and mechanical ventilation are common therapeutic interventions in intensive care unit settings. Barotrauma is a known complication of using positive pressures in a tissue defined by extra alveolar air in locations where it is not generally found in patients receiving mechanical ventilation. Several clinical manifestations of barotrauma include pneumothorax, subcutaneous emphysema, pneumoperitoneum, pneumomediastinum or pneumopericardium, air embolization, and hyperinflated left lower lobe. However, papilledema is an unreported and uncommon complication we observed in one of our patients, making it a unique presentation. We present the case of a young male patient intubated for asthma exacerbation requiring mechanical ventilation with subsequent development of papilledema. Our case report highlights the importance of knowing this rare complication of barotrauma as early commencement of lung-protective strategies will help prevent it.

Decrease in cortical vein opacification predicts outcome after aneurysmal subarachnoid hemorrhage

BACKGROUND

The pathophysiology of brain injury after aneurysmal subarachnoid hemorrhage (aSAH) remains incompletely understood. Cerebral venous flow patterns may be a marker of hemodynamic disruptions after aneurysm rupture. We hypothesized that a decrease in venous filling after aSAH would predict cerebral ischemia and poor outcome.

OBJECTIVE

To examine the hypotheses that venous filling as measured by the cortical venous opacification score (COVES) would (1) decrease after aSAH and (2) that decreased COVES would be associated with higher rates of hydrocephalus, vasospasm, delayed cerebral iscemia (DCI), and poor functional evaluation at outcome.

METHODS

In this retrospective observational cohort study of consecutive patients with aSAH admitted to our tertiary care center between 2016 and 2018, we measured the COVES at admission and at subsequent CT angiography (CTA). We collected clinical variables and compared hydrocephalus, vasospasm, DCI, and outcome at discharge in patients with decrease in COVES with patients with stable COVES.

RESULTS

A total of 22 patients were included in the analysis. COVES decreased from first CTA to second CTA in 11 (50%) patients, by an average of 1.1 points (P=0.01). Patients whose COVES decreased between admission and follow-up imaging were more likely to develop DCI (58% vs 0%, P=0.03) and have a poor outcome at discharge (100% vs 55%, P=0.03) than patients who had no change in COVES. aSAH severity was not associated with initial COVES, and there was no association between change in COVES and development of hydrocephalus or vasospasm.

CONCLUSIONS

Development of decreased venous filling on CTA is associated with poor outcome after aSAH. This association suggests that venous hemodynamics may be reflective of, or contribute to, the pathophysiological mechanisms of brain injury after aSAH. Larger prospective studies are necessary to substantiate our findings.

Analysis of intracranial pressure pulse waveform in studies on cerebrospinal compliance: a narrative review

Continuous monitoring of mean intracranial pressure (ICP) has been an essential part of neurocritical care for more than half a century. Cerebrospinal pressure-volume compensation, i.e. the ability of the cerebrospinal system to buffer changes in volume without substantial increases in ICP, is considered an important factor in preventing adverse effects on the patient's condition that are associated with ICP elevation. However, existing assessment methods are poorly suited to the management of brain injured patients as they require external manipulation of intracranial volume. In the 1980s, studies suggested that spontaneous short-term variations in the ICP signal over a single cardiac cycle, called the ICP pulse waveform, may provide information on cerebrospinal compensatory reserve. In this review we discuss the approaches that have been proposed so far to derive this information, from pulse amplitude estimation and spectral techniques to most recent advances in morphological analysis based on artificial intelligence solutions. Each method is presented with focus on its clinical significance and the potential for application in standard clinical practice. Finally, we highlight the missing links that need to be addressed in future studies in order for ICP pulse waveform analysis to achieve widespread use in the neurocritical care setting.

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.

Sleeping with Elevated Upper Body Does Not Attenuate Acute Mountain Sickness: Pragmatic Randomized Clinical Trial

PURPOSE

Acute mountain sickness commonly occurs following ascent to high altitude and is aggravated following sleep. Cephalad fluid shifts have been implicated. We hypothesized that sleeping with the upper body elevated by 30º reduces the risk of acute mountain sickness.

METHODS

In a pragmatic, randomized, observer-blinded field study at 4554 meters altitude, we investigated 134 adults aged 18-70 years with a Lake Louise score between 3 and 12 points on the evening of their arrival at the altitude. The individuals were exposed to sleeping on an inflatable cushion elevating the upper body by 30º or on a sham pillow in a horizontal position. The primary endpoint was the change in the Acute Mountain Sickness-Cerebral (AMS-C) score in the morning after sleeping at an altitude of 4554 meters compared with the evening before. Sleep efficiency was the secondary endpoint.

RESULTS

Among 219 eligible mountaineers, 134 fulfilled the inclusion criteria and were randomized. The AMS-C score increased by 0.250 ± 0.575 in the control group and by 0.121 ± 0.679 in the intervention group (difference 0.105; 95% confidence interval, -0.098-0.308; P = .308). Oxygen saturation in the morning was 79% ± 6% in the intervention group and 78% ± 6% in the control group (P = .863). Sleep efficiency did not differ between groups (P = .115).

CONCLUSIONS

Sleeping with the upper body elevated by 30° does not lead to relevant reductions in acute mountain sickness symptoms or hypoxemia at high altitude.

Acute mountain sickness: Do different time courses point to different pathophysiological mechanisms?

Acute mountain sickness (AMS) is a syndrome of nonspecific symptoms (i.e., headache, anorexia, nausea, vomiting, dizziness, and fatigue) that may develop in nonacclimatized individuals after rapid exposure to altitudes ≥2,500 m. In field studies, mean AMS scores usually peak after the first night at a new altitude. Analyses of the individual time courses of AMS in four studies performed at 3,450 m and 4,559 m revealed that three different patterns are hidden in the above-described overall picture. In 41% of those who developed AMS (i.e., AMS-C score >0.70), symptoms peaked on day 1, in 39%, symptoms were most prominent on day 2, and in 20%, symptoms were most prominent on day 3. We suggest to name the different time courses of AMS type I, type II, and type III, respectively. Here, we hypothesize that the variation of time courses of AMS are caused by different pathophysiological mechanisms. This assumption could explain why no consistent correlations between an overall assessment of AMS and single pathophysiological factors have been found in a large number of studies over the past 50 yr. In this paper, we will briefly review the fundamental mechanisms implicated in the pathophysiology of AMS and discuss how they might contribute to the three different AMS time courses.

Possible Role of Glymphatic System of the Brain in the Pathogenesis of High-Altitude Cerebral Edema

In this article, we suggest that the glymphatic system of the brain can play an important role in the pathogenesis of high-altitude cerebral edema (HACE). Water enters the intercellular space of the brain primarily through aquaporin-4 (AQP-4) water channels, the main component of the glymphatic system, whereas acetazolamide, pharmacological agent used in the prevention of HACE, is the blocker of the AQP-4 molecule. In animal experiments, cerebral edema caused by hypobaric hypoxia was associated with an increased expression of AQP-4 by astrocytes. Also, the glymphatic system is primarily active during sleep, although sleep at high altitude is a well-known risk factor of developing HACE. All these findings support our hypothesis. We suggest that future research on the prevention and treatment of HACE should involve factors that are already known to modify activity of the glymphatic system, such as angiotensin-converting enzyme inhibitors or other pharmaceutical agents affecting noradrenergic system of the brain, body posture during sleep, anatomy of the veins draining the cranial cavity, and the influence of physical activity before and during exposure to high altitude, especially in relation to sleep.

Ventilatory and cerebrovascular regulation and integration at high-altitude

Ascent to high-altitude elicits compensatory physiological adaptations in order to improve oxygenation throughout the body. The brain is particularly vulnerable to the hypoxemia of terrestrial altitude exposure. Herein we review the ventilatory and cerebrovascular changes at altitude and how they are both implicated in the maintenance of oxygen delivery to the brain. Further, the interdependence of ventilation and cerebral blood flow at altitude is discussed. Following the acute hypoxic ventilatory response, acclimatization leads to progressive increases in ventilation, and a partial mitigation of hypoxemia. Simultaneously, cerebral blood flow increases during initial exposure to altitude when hypoxemia is the greatest. Following ventilatory acclimatization to altitude, and an increase in hemoglobin concentration-which both underscore improvements in arterial oxygen content over time at altitude-cerebral blood flow progressively decreases back to sea-level values. The complimentary nature of these responses (ventilatory, hematological and cerebral) lead to a tightly maintained cerebral oxygen delivery while at altitude. Despite this general maintenance of global cerebral oxygen delivery, the manner in which this occurs reflects integration of these physiological responses. Indeed, ventilation directly influences cerebral blood flow by determining the prevailing blood gas and acid/base stimuli at altitude, but cerebral blood flow may also influence ventilation by altering central chemoreceptor stimulation via central CO₂ washout. The causes and consequences of the integration of ventilatory and cerebral blood flow regulation at high altitude are outlined.

Wearable physiological sensors and real-time algorithms for detection of acute mountain sickness

This is a minireview of potential wearable physiological sensors and algorithms (process and equations) for detection of acute mountain sickness (AMS). Given the emerging status of this effort, the focus of the review is on the current clinical assessment of AMS, known risk factors (environmental, demographic, and physiological), and current understanding of AMS pathophysiology. Studies that have examined a range of physiological variables to develop AMS prediction and/or detection algorithms are reviewed to provide insight and potential technological roadmaps for future development of real-time physiological sensors and algorithms to detect AMS. Given the lack of signs and nonspecific symptoms associated with AMS, development of wearable physiological sensors and embedded algorithms to predict in the near term or detect established AMS will be challenging. Prior work using [Formula: see text], HR, or HRv has not provided the sensitivity and specificity for useful application to predict or detect AMS. Rather than using spot checks as most prior studies have, wearable systems that continuously measure SpO2 and HR are commercially available. Employing other statistical modeling approaches such as general linear and logistic mixed models or time series analysis to these continuously measured variables is the most promising approach for developing algorithms that are sensitive and specific for physiological prediction or detection of AMS.

Lessons from altitude: cerebral perfusion insights and their potential translational clinical significance

What is the topic of this review? The long-held assumption that transcranial Doppler middle cerebral artery velocity is a surrogate for cerebral blood flow has been questioned in certain circumstances, particularly where tissue oxygenation changes. What advances does it highlight? Cerebral venous outflow restriction appears to be implicated in the development of high-altitude cerebral oedema. Rapid ascent to high altitude commonly results in acute mountain sickness and, on occasion, potentially fatal high-altitude cerebral oedema. The exact pathophysiological mechanisms behind these syndromes remain to be determined. One of the main theories to explain the development of acute mountain sickness is an increase in intracranial pressure. Vasogenic (extracellular water accumulation attributable to increased permeability of the blood-brain barrier) and cytotoxic (intracellular) oedema have also been postulated as potential mechanisms that underlie high-altitude cerebral oedema. Recently published findings derived from a very challenging field study (obtained at altitudes of up to 7950 m), substantiated by sea-level hypoxic magnetic resonance angiography studies, have given new insights into the maintenance of cerebral blood flow at altitude. This report provides new perspectives and potential mechanisms to account for the maintenance of cerebral oxygen delivery at high and extreme altitude. In particular, the long-held assumption that transcranial Doppler middle cerebral artery velocity is a surrogate for cerebral blood flow has been shown to be incorrect in certain circumstances. The emerging evidence for a potential third mechanism, namely the restrictive venous outflow hypothesis, in the development of high-altitude cerebral oedema, over and above the accepted vasogenic and cytotoxic hypotheses, is also appraised.