Optic Nerve Sheath Diameter Is Not Related to High Altitude Headache: A Randomized Controlled Trial

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.

Retinal microvasculature is a potential biomarker for acute mountain sickness

Increased cerebral blood flow resulting from altered capillary level autoregulation at high altitudes leads to capillary overperfusion and then vasogenic cerebral edema, which is the leading hypothesis of acute mountain sickness (AMS). However, studies on cerebral blood flow in AMS have been mostly restricted to gross cerebrovascular endpoints as opposed to the microvasculature. This study aimed to investigate ocular microcirculation alterations, the only visualized capillaries in the central neural system (CNS), during early-stage AMS using a hypobaric chamber. This study found that after high altitude simulation, the optic nerve showed retinal nerve fiber layer thickening (P=0.004-0.018) in some locations, and the area of the optic nerve subarachnoid space (P=0.004) enlarged. Optical coherence tomography angiography (OCTA) showed increased retinal radial peripapillary capillary (RPC) flow density (P=0.003-0.046), particularly on the nasal side of the nerve. The AMS-positive group had the largest increases in RPC flow density in the nasal sector (AMS-positive, Δ3.21±2.37; AMS-negative, Δ0.01±2.16, P=0.004). Among multiple ocular changes, OCTA increase in RPC flow density was associated with simulated early-stage AMS symptoms (beta=0.222, 95%CI, 0.009-0.435, P=0.042). The area under the receiver operating characteristics curve (AUC) for the changes in RPC flow density to predict early-stage AMS outcomes was 0.882 (95%CI, 0.746-0.998). The results further confirmed that overperfusion of microvascular beds is the key pathophysiologic change in early-stage AMS. RPC OCTA endpoints may serve as a rapid, noninvasive potential biomarker for CNS microvascular changes and AMS development during risk assessment of individuals at high altitudes.

A Narrative Review of Point of Care Ultrasound Assessment of the Optic Nerve in Emergency Medicine

Point of care ultrasound (POCUS) of the optic nerve is easy to learn and has great diagnostic potential. Within emergency medicine, research has primarily focused on its use for the assessment of increased intracranial pressure, but many other applications exist, though the literature is heterogeneous and largely observational. This narrative review describes the principles of POCUS of the optic nerve including anatomy and scanning technique, as well as a summary of its best studied clinical applications of relevance in emergency medicine: increased intracranial pressure, idiopathic intracranial hypertension, optic neuritis, acute mountain sickness, and pediatric intracranial pressure assessment. In many of these applications, sonographic optic nerve sheath diameter (ONSD) has moderately high sensitivity and specificity, but the supporting studies are heterogeneous. Further studies should focus on standardization of the measurement of ONSD, establishment of consistent diagnostic thresholds for elevated intracranial pressure, and automation of ONSD measurement.

Optic nerve sheath diameter changes at high altitude and in acute mountain sickness: meta-regression analyses

BACKGROUND/AIMS

To assess changes in optic nerve sheath diameter (ONSD) at high altitude and in acute mountain sickness (AMS).

METHODS

Cochrane Library, EMBASE, Google Scholar and PubMed were searched for articles published from their inception to 31st of July 2020. Outcome measures were mean changes of ONSD at high altitude and difference in ONSD change between subjects with and without AMS. Meta-regressions were conducted to investigate the relation of ONSD change to altitude and time spent at that altitude.

RESULTS

Eight studies with 248 participants comparing ONSD from sea level to high altitude, and five studies with 454 participants comparing subjects with or without AMS, were included. ONSD increased by 0.14 mm per 1000 m after adjustment for time (95% CI: 0.10 to 0.18; p<0.01). Restricted cubic spline regression revealed an almost linear relation between ONSD change and time within 2 days. ONSD was greater in subjects with AMS (mean difference=0.47; 95% CI: 0.14 to 0.80; p=0.01; I²=89.4%).

CONCLUSION

Our analysis shows that ONSD changes correlate with altitude and tend to increase in subjects with AMS. Small study number and high heterogeneity are the limitations of our study. Further large prospective studies are required to verify our findings.

Optic nerve sheath diameter: present and future perspectives for neurologists and critical care physicians

BACKGROUND

Estimation of intracranial pressure (ICP) may be helpful in the management of neurological critically ill patients. It has been shown that ultrasonography of the optic nerve sheath diameter (ONSD) is a reliable tool for non-invasive estimation of increased intracranial pressure (ICP) at hospital admission or in intensive care. Less is known about the estimation of increased ICP and usefulness of ONSD in the prehospital setting. The aim of this review was to elucidate both prevailing and novel applications of ONSD for neurologists and critical care physicians.

METHODS

In this review, we discuss the technique and the novel approach of ONSD measurement, the clinical applications of ONSD in neurology and critical care patients.

RESULTS

ONSD measurement is simple, easy to learn, and has diverse applications. ONSD has utility for ICP measurement in intracranial hemorrhage and ischemic stroke, meningitis and encephalitis, and idiopathic intracranial hypertension (IIH). It is also valuable for lesser known syndromes, where an increase of ICP is postulated, such as acute mountain sickness and posterior reversible encephalopathy syndrome. ONSD changes develop in inflammatory or ischemic optic neuropathies. Some papers demonstrate the usefulness of ONSD studies in symptomatic intracranial hypotension.

CONCLUSIONS

ONSD is a safe and low-cost bedside tool with the potential of screening patients who need other neuroimaging and those who may need an invasive measurement of ICP.

Interventions for preventing high altitude illness: Part 3. Miscellaneous and non-pharmacological interventions

BACKGROUND

High altitude illness (HAI) is a term used to describe a group of mainly cerebral and pulmonary syndromes that can occur during travel to elevations above 2500 metres (˜ 8200 feet). Acute mountain sickness (AMS), high altitude cerebral oedema (HACE), and high altitude pulmonary oedema (HAPE) are reported as potential medical problems associated with high altitude ascent. In this, the third of a series of three reviews about preventive strategies for HAI, we assessed the effectiveness of miscellaneous and non-pharmacological interventions.

OBJECTIVES

To assess the clinical effectiveness and adverse events of miscellaneous and non-pharmacological interventions for preventing acute HAI in people who are at risk of developing high altitude illness in any setting.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, LILACS and the World Health Organization International Clinical Trials Registry Platform (WHO ICTRP) in January 2019. We adapted the MEDLINE strategy for searching the other databases. We used a combination of thesaurus-based and free-text search terms. We scanned the reference lists and citations of included trials and any relevant systematic reviews that we identified for further references to additional trials.

SELECTION CRITERIA

We included randomized controlled trials conducted in any setting where non-pharmacological and miscellaneous interventions were employed to prevent acute HAI, including preacclimatization measures and the administration of non-pharmacological supplements. We included trials involving participants who are at risk of developing high altitude illness (AMS or HACE, or HAPE, or both). We included participants with, and without, a history of high altitude illness. We applied no age or gender restrictions. We included trials where the relevant intervention was administered before the beginning of ascent.

DATA COLLECTION AND ANALYSIS

We used the standard methodological procedures employed by Cochrane.

MAIN RESULTS

We included 20 studies (1406 participants, 21 references) in this review. Thirty studies (14 ongoing, and 16 pending classification (awaiting)) will be considered in future versions of this suite of three reviews as appropriate. We report the results for the primary outcome of this review (risk of AMS) by each group of assessed interventions.Group 1. Preacclimatization and other measures based on pressureUse of simulated altitude or remote ischaemic preconditioning (RIPC) might not improve the risk of AMS on subsequent exposure to altitude, but this effect is uncertain (simulated altitude: risk ratio (RR) 1.18, 95% confidence interval (CI) 0.82 to 1.71; I² = 0%; 3 trials, 140 participants; low-quality evidence. RIPC: RR 3.0, 95% CI 0.69 to 13.12; 1 trial, 40 participants; low-quality evidence). We found evidence of improvement of this risk using positive end-expiratory pressure (PEEP), but this information was derived from a cross-over trial with a limited number of participants (OR 3.67, 95% CI 1.38 to 9.76; 1 trial, 8 participants; low-quality evidence). We found scarcity of evidence about the risk of adverse events for these interventions.Group 2. Supplements and vitaminsSupplementation of antioxidants, medroxyprogesterone, iron or Rhodiola crenulata might not improve the risk of AMS on exposure to high altitude, but this effect is uncertain (antioxidants: RR 0.58, 95% CI 0.32 to 1.03; 1 trial, 18 participants; low-quality evidence. Medroxyprogesterone: RR 0.71, 95% CI 0.48 to 1.05; I² = 0%; 2 trials, 32 participants; low-quality evidence. Iron: RR 0.65, 95% CI 0.38 to 1.11; I² = 0%; 2 trials, 65 participants; low-quality evidence. R crenulata: RR 1.00, 95% CI 0.78 to 1.29; 1 trial, 125 participants; low-quality evidence). We found evidence of improvement of this risk with the administration of erythropoietin, but this information was extracted from a trial with issues related to risk of bias and imprecision (RR 0.41, 95% CI 0.20 to 0.84; 1 trial, 39 participants; very low-quality evidence). Regarding administration of ginkgo biloba, we did not perform a pooled estimation of RR for AMS due to considerable heterogeneity between the included studies (I² = 65%). RR estimates from the individual studies were conflicting (from 0.05 to 1.03; low-quality evidence). We found scarcity of evidence about the risk of adverse events for these interventions.Group 3. Other comparisonsWe found heterogeneous evidence regarding the risk of AMS when ginkgo biloba was compared with acetazolamide (I² = 63%). RR estimates from the individual studies were conflicting (estimations from 0.11 (95% CI 0.01 to 1.86) to 2.97 (95% CI 1.70 to 5.21); low-quality evidence). We found evidence of improvement when ginkgo biloba was administered along with acetazolamide, but this information was derived from a single trial with issues associated to risk of bias (compared to ginkgo biloba alone: RR 0.43, 95% CI 0.26 to 0.71; 1 trial, 311 participants; low-quality evidence). Administration of medroxyprogesterone plus acetazolamide did not improve the risk of AMS when compared to administration of medroxyprogesterone or acetazolamide alone (RR 1.33, 95% CI 0.50 to 3.55; 1 trial, 12 participants; low-quality evidence). We found scarcity of evidence about the risk of adverse events for these interventions.

AUTHORS' CONCLUSIONS

This Cochrane Review is the final in a series of three providing relevant information to clinicians, and other interested parties, on how to prevent high altitude illness. The assessment of non-pharmacological and miscellaneous interventions suggests that there is heterogeneous and even contradictory evidence related to the effectiveness of these prophylactic strategies. Safety of these interventions remains as an unclear issue due to lack of assessment. Overall, the evidence is limited due to its quality (low to very low), the relative paucity of that evidence and the number of studies pending classification for the three reviews belonging to this series (30 studies either awaiting classification or ongoing). Additional studies, especially those comparing with pharmacological alternatives (such as acetazolamide) are required, in order to establish or refute the strategies evaluated in this review.

Optic nerve oedema at high altitude occurs independent of acute mountain sickness

BACKGROUND/AIMS

The study aims to investigate changes in the optic nerve sheath diameter (ONSD) at high altitude and to assess correlation to optic disc oedema (ODE) and acute mountain sickness (AMS). This investigation is part of the Tübingen High Altitude Ophthalmology study.

METHODS

Fourteen volunteers ascended to 4559 m for 4 days before returning to low altitude. Ultrasonography of ONSD, quantification of optic disc parameters using a scanning laser ophthalmoscope and fluorescein angiography were performed at 341 m and at high altitude. Pearson's coefficient was used to correlate changes in ONSD with the optic disc and AMS. Assessment of AMS was performed using the Lake Louise (LL) and AMS-cerebral (AMS-C) scores of the Environmental Symptom Questionnaire-III. All volunteers were clinically monitored for heart rate (HR) and oxygen saturation (SpO2).

RESULTS

The mean ONSD at high altitude (4.6±0.3 mm, p<0.05) was significantly increased compared with baseline (3.8±0.4 mm) and remained enlarged throughout high-altitude exposure. This change in ONSD did not correlate with AMS (AMS-C, r=0.26, p=0.37; LL, r=0.21, p=0.48) and high-altitude headache (r=0.54, p=0.046), or clinical parameters of SpO2 (r=0.11, p=0.72) and HR (r=0.22, p=0.44). Increased ONSD did not correlate with altered key stereometric parameters of the optic disc describing ODE at high altitude (r<0.1, p>0.5).

CONCLUSION

High-altitude exposure leads to marked oedema formation of the optic nerve independent of AMS. Increased ONSD and ODE reflect hypoxia-driven oedema formation of the optic nerve at high altitude, providing important pathophysiological insight into high-altitude illness development and for future research.

Bedside Ultrasonography as an Alternative to Computed Tomography Scan for the Measurement of Optic Nerve Sheath Diameter

Background

Optic nerve sheath diameter (ONSD) as measured by optic nerve sheath ultrasonography (ONSU) is used as a surrogate marker of intracranial pressure (ICP), especially in resource-limited settings. There is a growing interest in the use of ONSU in emergency and high-altitude setups. Notwithstanding multiple studies done on this subject, there is a paucity of data regarding standardization of techniques and comparison of ONSU with computed tomography (CT).

Materials and Methods

Thirty-five patients with a diagnosis of high-altitude cerebral edema were enrolled in the study. ONSD was measured in all patients using ONSU, along visual and coronal axis, and CT scan. We repeated ONSU in these patients on days 3, 7, 10, and 15 (day of discharge). Correlation between visual and coronal axis as well as CT scan was analyzed.

Results

The correlation of visual to coronal and coronal to visual was equally significant (both correlation coefficients being R² = 0.983). Correlation of ONSD by visual axis to CT scan was better than coronal axis (correlation coefficient R² = 0.986 vs. 0.96, respectively).

Conclusion

In our study, we found a strong correlation between the visual and coronal axes. Thus, either of the two axes can be used for monitoring ICP. However, it has been found that measurements along the coronal axis are challenging, especially in the emergency setup. ONSD measured along visual axis correlated better with CT scan as compared to the coronal axis.

Interventions for preventing high altitude illness: Part 2. Less commonly-used drugs

BACKGROUND

High altitude illness (HAI) is a term used to describe a group of mainly cerebral and pulmonary syndromes that can occur during travel to elevations above 2500 metres (˜ 8200 feet). Acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE) are reported as potential medical problems associated with high altitude ascent. In this second review, in a series of three about preventive strategies for HAI, we assessed the effectiveness of five of the less commonly used classes of pharmacological interventions.

OBJECTIVES

To assess the clinical effectiveness and adverse events of five of the less commonly used pharmacological interventions for preventing acute HAI in participants who are at risk of developing high altitude illness in any setting.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, LILACS and the World Health Organization International Clinical Trials Registry Platform (WHO ICTRP) in May 2017. We adapted the MEDLINE strategy for searching the other databases. We used a combination of thesaurus-based and free-text search terms. We scanned the reference lists and citations of included trials and any relevant systematic reviews that we identified for further references to additional trials.

SELECTION CRITERIA

We included randomized controlled trials conducted in any setting where one of five classes of drugs was employed to prevent acute HAI: selective 5-hydroxytryptamine(1) receptor agonists; N-methyl-D-aspartate (NMDA) antagonist; endothelin-1 antagonist; anticonvulsant drugs; and spironolactone. We included trials involving participants who are at risk of developing high altitude illness (AMS or HACE, or HAPE, or both). We included participants with and without a history of high altitude illness. We applied no age or gender restrictions. We included trials where the relevant medication was administered before the beginning of ascent. We excluded trials using these drugs during ascent or after ascent.

DATA COLLECTION AND ANALYSIS

We used the standard methodological procedures employed by Cochrane.

MAIN RESULTS

We included eight studies (334 participants, 9 references) in this review. Twelve studies are ongoing and will be considered in future versions of this review as appropriate. We have been unable to obtain full-text versions of a further 12 studies and have designated them as 'awaiting classification'. Four studies were at a low risk of bias for randomization; two at a low risk of bias for allocation concealment. Four studies were at a low risk of bias for blinding of participants and personnel. We considered three studies at a low risk of bias for blinding of outcome assessors. We considered most studies at a high risk of selective reporting bias.We report results for the following four main comparisons.Sumatriptan versus placebo (1 parallel study; 102 participants)Data on sumatriptan showed a reduction of the risk of AMS when compared with a placebo (risk ratio (RR) = 0.43, CI 95% 0.21 to 0.84; 1 study, 102 participants; low quality of evidence). The one included study did not report events of HAPE, HACE or adverse events related to administrations of sumatriptan.Magnesium citrate versus placebo (1 parallel study; 70 participants)The estimated RR for AMS, comparing magnesium citrate tablets versus placebo, was 1.09 (95% CI 0.55 to 2.13; 1 study; 70 participants; low quality of evidence). In addition, the estimated RR for loose stools was 3.25 (95% CI 1.17 to 8.99; 1 study; 70 participants; low quality of evidence). The one included study did not report events of HAPE or HACE.Spironolactone versus placebo (2 parallel studies; 205 participants)Pooled estimation of RR for AMS was not performed due to considerable heterogeneity between the included studies (I² = 72%). RR from individual studies was 0.40 (95% CI 0.12 to 1.31) and 1.44 (95% CI 0.79 to 2.01; very low quality of evidence). No events of HAPE or HACE were reported. Adverse events were not evaluated.Acetazolamide versus spironolactone (1 parallel study; 232 participants)Data on acetazolamide compared with spironolactone showed a reduction of the risk of AMS with the administration of acetazolamide (RR = 0.36, 95% CI 0.18 to 0.70; 232 participants; low quality of evidence). No events of HAPE or HACE were reported. Adverse events were not evaluated.

AUTHORS' CONCLUSIONS

This Cochrane Review is the second in a series of three providing relevant information to clinicians and other interested parties on how to prevent high altitude illness. The assessment of five of the less commonly used classes of drugs suggests that there is a scarcity of evidence related to these interventions. Clinical benefits and harms related to potential interventions such as sumatriptan are still unclear. Overall, the evidence is limited due to the low number of studies identified (for most of the comparison only one study was identified); limitations in the quality of the evidence (moderate to low); and the number of studies pending classification (24 studies awaiting classification or ongoing). We lack the large and methodologically sound studies required to establish or refute the efficacy and safety of most of the pharmacological agents evaluated in this review.

Interventions for preventing high altitude illness: Part 1. Commonly-used classes of drugs

BACKGROUND

High altitude illness (HAI) is a term used to describe a group of cerebral and pulmonary syndromes that can occur during travel to elevations above 2500 metres (8202 feet). Acute hypoxia, acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE) are reported as potential medical problems associated with high altitude. In this review, the first in a series of three about preventive strategies for HAI, we assess the effectiveness of six of the most recommended classes of pharmacological interventions.

OBJECTIVES

To assess the clinical effectiveness and adverse events of commonly-used pharmacological interventions for preventing acute HAI.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), LILACS and trial registries in January 2017. We adapted the MEDLINE strategy for searching the other databases. We used a combination of thesaurus-based and free-text terms to search.

SELECTION CRITERIA

We included randomized-controlled and cross-over trials conducted in any setting where commonly-used classes of drugs were used to prevent acute HAI.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures as expected by Cochrane.

MAIN RESULTS

We included 64 studies (78 references) and 4547 participants in this review, and classified 12 additional studies as ongoing. A further 12 studies await classification, as we were unable to obtain the full texts. Most of the studies were conducted in high altitude mountain areas, while the rest used low pressure (hypobaric) chambers to simulate altitude exposure. Twenty-four trials provided the intervention between three and five days prior to the ascent, and 23 trials, between one and two days beforehand. Most of the included studies reached a final altitude of between 4001 and 5000 metres above sea level. Risks of bias were unclear for several domains, and a considerable number of studies did not report adverse events of the evaluated interventions. We found 26 comparisons, 15 of them comparing commonly-used drugs versus placebo. We report results for the three most important comparisons: Acetazolamide versus placebo (28 parallel studies; 2345 participants)The risk of AMS was reduced with acetazolamide (risk ratio (RR) 0.47, 95% confidence interval (CI) 0.39 to 0.56; I² = 0%; 16 studies; 2301 participants; moderate quality of evidence). No events of HAPE were reported and only one event of HACE (RR 0.32, 95% CI 0.01 to 7.48; 6 parallel studies; 1126 participants; moderate quality of evidence). Few studies reported side effects for this comparison, and they showed an increase in the risk of paraesthesia with the intake of acetazolamide (RR 5.53, 95% CI 2.81 to 10.88, I² = 60%; 5 studies, 789 participants; low quality of evidence). Budenoside versus placebo (2 parallel studies; 132 participants)Data on budenoside showed a reduction in the incidence of AMS compared with placebo (RR 0.37, 95% CI 0.23 to 0.61; I² = 0%; 2 studies, 132 participants; low quality of evidence). Studies included did not report events of HAPE or HACE, and they did not find side effects (low quality of evidence). Dexamethasone versus placebo (7 parallel studies; 205 participants)For dexamethasone, the data did not show benefits at any dosage (RR 0.60, 95% CI 0.36 to 1.00; I2 = 39%; 4 trials, 176 participants; low quality of evidence). Included studies did not report events of HAPE or HACE, and we rated the evidence about adverse events as of very low quality.

AUTHORS' CONCLUSIONS

Our assessment of the most commonly-used pharmacological interventions suggests that acetazolamide is an effective pharmacological agent to prevent acute HAI in dosages of 250 to 750 mg/day. This information is based on evidence of moderate quality. Acetazolamide is associated with an increased risk of paraesthesia, although there are few reports about other adverse events from the available evidence. The clinical benefits and harms of other pharmacological interventions such as ibuprofen, budenoside and dexamethasone are unclear. Large multicentre studies are needed for most of the pharmacological agents evaluated in this review, to evaluate their effectiveness and safety.

Noninvasive Brain Physiology Monitoring for Extreme Environments: A Critical Review

Our ability to monitor the brain physiology is advancing; however, most of the technology is bulky, expensive, and designed for traditional clinical settings. With long-duration space exploration, there is a need for developing medical technologies that are reliable, low energy, portable, and semiautonomous. Our aim was to review the state of the art for noninvasive technologies capable of monitoring brain physiology in diverse settings. A literature review of PubMed and the Texas Medical Center library sites was performed using prespecified search criteria to identify portable technologies for monitoring physiological aspects of the brain physiology. Most brain-monitoring technologies require a moderate to high degree of operator skill. Some are low energy, but many require a constant external power supply. Most of the technologies lack the accuracy seen in gold standard measures, due to the need for calibration, but may be useful for screening or monitoring relative changes in a parameter. Most of the technologies use ultrasound or electromagnetic radiation as energy sources. There is an important need for further development of portable technologies that can be operated in a variety of extreme environments to monitor brain health.

Optic Nerve Sheath Diameter Increase on Ascent to High Altitude: Correlation With Acute Mountain Sickness

OBJECTIVES

Elevated optic nerve sheath diameter on sonography is known to correlate with increased intracranial pressure and is observed in acute mountain sickness. This study aimed to determine whether optic nerve sheath diameter changes on ascent to high altitude are associated with acute mountain sickness incidence.

METHODS

Eighty-six healthy adults enrolled at 1240 m (4100 ft), drove to 3545 m (11,700 ft) and then hiked to and slept at 3810 m (12,500 ft). Lake Louise Questionnaire scores and optic nerve sheath diameter measurements were taken before, the evening of, and the morning after ascent.

RESULTS

The incidence of acute mountain sickness was 55.8%, with a mean Lake Louise Questionnaire score ± SD of 3.81 ± 2.5. The mean maximum optic nerve sheath diameter increased on ascent from 5.58 ± 0.79 to 6.13 ± 0.73 mm, a difference of 0.91 ± 0.55 mm (P = .09). Optic nerve sheath diameter increased at high altitude regardless of acute mountain sickness diagnosis; however, compared to baseline values, we observed a significant increase in diameter only in those with a diagnosis of acute mountain sickness (0.57 ± 0.77 versus 0.21 ± 0.76 mm; P = .04). This change from baseline, or Δ optic nerve sheath diameter, was associated with twice the odds of developing acute mountain sickness (95% confidence interval, 1.08-3.93).

CONCLUSIONS

The mean optic nerve sheath diameter increased on ascent to high altitude compared to baseline values, but not to a statistically significant degree. The magnitude of the observed Δ optic nerve sheath diameter was positively associated with acute mountain sickness diagnosis. No such significant association was found between acute mountain sickness and diameter elevation above standard cutoff values, limiting the utility of sonography as a diagnostic tool.

The cerebral venous system and hypoxia

Most hypobaric hypoxia studies have focused on oxygen delivery and therefore cerebral blood inflow. Few have studied venous outflow. However, the volume of blood entering and leaving the skull (∼700 ml/min) is considerably greater than cerebrospinal fluid production (0.35 ml/min) or edema formation rates and slight imbalances of in- and outflow have considerable effects on intracranial pressure. This dynamic phenomenon is not necessarily appreciated in the currently taught static "Monro-Kellie" doctrine, which forms the basis of the "Tight-Fit" hypothesis thought to underlie high altitude headache, acute mountain sickness, and high altitude cerebral edema. Investigating both sides of the cerebral circulation was an integral part of the 2007 Xtreme Everest Expedition. The results of the relevant studies performed as part of and subsequent to this expedition are reviewed here. The evidence from recent studies suggests a relative venous outflow insufficiency is an early step in the pathogenesis of high altitude headache. Translation of knowledge gained from high altitude studies is important. Many patients in a critical care environment develop hypoxemia akin to that of high altitude exposure. An inability to drain the hypoxemic induced increase in cerebral blood flow could be an underappreciated regulatory mechanism of intracranial pressure.

Normobaric hypoxia and symptoms of acute mountain sickness: Elevated brain volume and intracranial hypertension

OBJECTIVE

The study was undertaken to determine whether normobaric hypoxia causes elevated brain volume and intracranial pressure in individuals with symptoms consistent with acute mountain sickness (AMS).

METHODS

Thirteen males age = (26 (sd 6)) years were exposed to normobaric hypoxia (12% O2 ) and normoxia (21% O2 ). After 2 and 10 hours, AMS symptoms were assessed alongside ventricular and venous vessel volumes, cerebral blood flow, regional brain volumes, and intracranial pressure, using high-resolution magnetic resonance imaging.

RESULTS

In normoxia, neither lateral ventricular volume (R(2)  = 0.07, p = 0.40) nor predominance of unilateral transverse venous sinus drainage (R(2)  = 0.07, p = 0.45) was related to AMS symptoms. Furthermore, despite an increase in cerebral blood flow after 2 hours of hypoxia (hypoxia vs normoxia: Δ148ml/min(-1) , 95% confidence interval [CI] = 58 to 238), by 10 hours, when AMS symptoms had developed, cerebral blood flow was normal (Δ-51ml/min(-1) , 95% CI = -141 to 39). Conversely, at 10 hours brain volume was increased (Δ59ml, 95% CI = 8 to 110), predominantly due to an increase in gray matter volume (Δ73ml, 95% CI = 25 to 120). Therefore, cerebral spinal fluid volume was decreased (Δ-40ml, 95% CI = -67 to -14). The intracranial pressure response to hypoxia varied between individuals, and as hypothesized, the most AMS-symptomatic participants had the largest increases in intracranial pressure (AMS present, Δ7mmHg, 95% CI = -2.5 to 17.3; AMS not present, Δ-1mmHg, 95% CI = -3.3 to 0.5). Consequently, there was a significant relationship between the change in intracranial pressure and AMS symptom severity (R(2)  = 0.71, p = 0.002).

INTERPRETATION

The data provide the strongest evidence to date to support the hypothesis that the "random" nature of AMS symptomology is explained by a variable intracranial pressure response to hypoxia.

Investigation of whole-brain white matter identifies altered water mobility in the pathogenesis of high-altitude headache

Elevated brain water is a common finding in individuals with severe forms of altitude illness. However, the location, nature, and a causative link between brain edema and symptoms of acute mountain sickness such as headache remains unknown. We examined indices of brain white matter water mobility in 13 participants after 2 and 10 hours in normoxia (21% O2) and hypoxia (12% O2) using magnetic resonance imaging. Using a whole-brain analysis (tract-based spatial statistics (TBSS)), mean diffusivity was reduced in the left posterior hemisphere after 2 hours and globally reduced throughout cerebral white matter by 10 hours in hypoxia. However, no changes in T2 relaxation time (T2) or fractional anisotropy were observed. The TBSS identified an association between changes in mean diffusivity, fractional anisotropy, and T2 both supra and subtentorially after 2 and 10 hours, with headache score after 10 hours in hypoxia. Region of interest-based analyses generally confirmed these results. These data indicate that acute periods of hypoxemia cause a shift of water into the intracellular space within the cerebral white matter, whereas no evidence of brain edema (a volumetric enlargement) is identifiable. Furthermore, these changes in brain water mobility are related to the intensity of high-altitude headache.