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

Longitudinal assessment of retinal and visual pathway electrophysiology and structure after high altitude exposure

High altitude (HA) exposure induces impairments in visual function. This study was designed to dynamically observe visual function after returning to lowland and elucidate the underlying mechanism by examining the structure and function of retina and visual pathway. Twenty-three subjects were recruited before (Test 1), and one week (Test 2) and three months (Test 3) after their return from HA (4300 m) where they resided for 30 days. The clock task was used to assess visual cognition; and pattern-reversal visual evoked potential (p-VEP) and full-field electroretinogram (ff-ERG) were employed to record electrophysiological responses of retinal cells; optical coherence tomography (OCT), color doppler imaging (CDI) and magnetic resonance imaging(MRI) were used to measure structures of retina and visual pathway. In Test 2 vs. Test 1, there was increased reaction time during angle task; the amplitudes of scotopic 3.0 cd·s/m2 and scotopic 10.0 cd·s/m2 ERG a-wave and scotopic 3.0 cd·s/m2 oscillatory potential in the right eye were significantly decreased, all of which were negatively correlated with the increased reaction time during the angle task. In Test 3 vs. Test 1, there were decreased amplitude of scotopic 10.0 cd·s/m2 a-wave in the right eye and increased velocity of ophthalmic artery and ocular perfusion pressure in bilateral eyes. The VEP and visual pathway structures remained normal throughout the entire test. HA exposure caused damage to rod and cone responses in both outer and inner retina. After returning to sea level, the damaged visual cell functions gradually recovered over time, coinciding with an increase in the ocular perfusion.

Neurofilament Light Chain Is Associated With Acute Mountain Sickness

BACKGROUND

Neurological symptoms are common in acute mountain sickness (AMS); however, the extent of neuroaxonal damage remains unclear. Neurofilament light chain (NfL) is an established blood biomarker for neuroaxonal damage.

OBJECTIVE

To investigate whether plasma (p) NfL levels increase after simulated altitude exposure, correlate with the occurrence of AMS, and might be mitigated by preacclimatization.

METHODS

Healthy subjects were exposed to simulated high altitude (4500 m) by the use of a normobaric hypoxic chamber at the University of Innsbruck two times, that is, within Cycle 1 (C1) over 12 h, and within Cycle 2 (C2) for another 12 h but with a random assignment to prior acclimatization or sham acclimatization. Before each cycle (measurement [M] 1 and 3) and after each cycle (M2 and M4), clinical data (arterial oxygen saturation [SaO2], heart rate, and Lake Louise AMS score [LLS]) and plasma samples were collected. pNfL was measured using single-molecule array (Simoa) technique.

RESULTS

pNfL levels did not significantly change within each study cycle, but increased over the total study period (M1: 4.57 [3.34-6.39], M2: 4.58 [3.74-6.0], M3: 5.64, and M4: 6.53 [4.65-7.92] pg/mL, p < 0.001). Subjects suffering from AMS during the study procedures showed higher pNfL levels at M4 (6.80 [6.19-8.13] vs. 5.75 [4.17-7.35], p = 0.048), a higher total pNfL increase (2.88 [1.21-3.48] vs. 0.91 [0.53-1.48], p = 0.022) compared to subjects without AMS. An effect of preacclimatization on pNfL levels could not be observed.

CONCLUSIONS

pNfL increases alongside exposure to simulated altitude and is associated with AMS.

Altitude-Related Illness

Altitude-related illness occurs as a result of inadequate acclimatization. The mainstay of prevention is a slow, graded ascent profile which gives the body time to respond to a low-oxygen environment. The diagnosis of these conditions is often difficult in resource-limited environments, so history and a physical exam are key in identifying patients who will require descent and evacuation. Treatment modalities such as supplemental oxygen, portable hyperbaric chambers, and medications, are all temporizing measures until the patient can be safely evacuated to a lower elevation.

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.

High myopia at high altitudes

Background: Optic nerve sheath diameter (ONSD) increases significantly at high altitudes, and is associated with the presence and severity of acute mountain sickness (AMS). Exposure to hypobaria, hypoxia, and coldness when hiking also impacts intraocular pressure (IOP). To date, little is known about ocular physiological responses in trekkers with myopia at high altitudes. This study aimed to determine changes in the ONSD and IOP between participants with and without high myopia (HM) during hiking and to test whether these changes could predict symptoms of AMS. Methods: Nine participants with HM and 18 without HM participated in a 3-day trek of Xue Mountain. The ONSD, IOP, and questionnaires were examined before and during the trek of Xue Mountain. Results: The ONSD values increased significantly in both HM (p = 0.005) and non-HM trekkers (p = 0.018) at an altitude of 1,700 m. In the HM group, IOP levels were greater than those in the non-HM group (p = 0.034) on the first day of trekking (altitude: 3,150 m). No statistically significant difference was observed between the two groups for the values of ONSD. Fractional changes in ONSD at an altitude of 1,700 m were related to the development of AMS (r pb = 0.448, p = 0.019) and the presence of headache symptoms (r pb = 0.542, p = 0.004). The area under the ROC curve for the diagnostic performance of ONSD fractional changes at an altitude of 1,700 m was 0.859 for predicting the development of AMS and 0.803 for predicting the presence of headache symptoms. Conclusion: Analysis of changes in ONSD at moderate altitude could predict AMS symptoms before an ascent to high altitude. Myopia may impact physiological accommodation at high altitudes, and HM trekkers potentially demonstrate suboptimal regulation of aqueous humor in such environments.

Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update

To provide guidance to clinicians about best practices, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines for prevention, diagnosis, and treatment of acute mountain sickness, high altitude cerebral edema, and high altitude pulmonary edema. Recommendations are graded based on the quality of supporting evidence and the balance between the benefits and risks/burdens according to criteria put forth by the American College of Chest Physicians. The guidelines also provide suggested approaches for managing each form of acute altitude illness that incorporate these recommendations as well as recommendations on how to approach high altitude travel following COVID-19 infection. This is an updated version of the original WMS Consensus Guidelines for the Prevention and Treatment of Acute Altitude Illness published in Wilderness & Environmental Medicine in 2010 and the subsequently updated WMS Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness published in 2014 and 2019.

Recent Progress in the Understanding and Management of Acute Mountain Sickness: A Narrative Review

BACKGROUND

Individuals at higher altitudes may experience a decrease in blood oxygen levels, which can result in a variety of clinical illnesses, such as high-altitude pulmonary edema, high-altitude cerebral edema, and milder but more common acute mountain sickness (AMS).

OBJECTIVE

This study aims to review the current state of knowledge related to motion sickness, the risk of AMS, and pharmacological and non-pharmacological treatments for AMS.

METHODS

Several databases, including PubMed, Bentham Science, Elsevier, Springer, and Research Gate, were used to compile the data for the article following a thorough analysis of the various research findings connected to acute mountain sickness and motion sickness, along with treatments and prevention.

RESULTS

This article covers the research on mountain sickness as well as every imaginable form of conventional and alternative medicine. It contains ten medicinal plants that are useful in treating mountain sickness and various other remedies. Additionally, case studies are provided.

CONCLUSION

Therefore, the information in the paper will help travel medicine specialists better personalize their appropriate care for patients who travel to high-altitude locations. Additionally, all available antiemetic medications, serotonin agonists, nonsteroidal anti-inflammatory drugs, and herbal treatments for motion sickness were discussed. The prevention and consequences of acute mountain sickness are also covered in this study.

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.

Older Age as a Predictive Risk Factor for Acute Mountain Sickness

BACKGROUND

Older populations are increasing and comprise a substantial portion of high-altitude travelers. Aging physiology may influence susceptibility to acute mountain sickness, though prior research remains inconclusive. The goal of this study was to investigate the relationship between increasing age and acute mountain sickness.

METHODS

This study was a pooled analysis of 5 prospective randomized controlled trials conducted at White Mountain, California from 2010, 2016-2019 with identical 4-hour rapid ascent from 1242 m to overnight sojourn at 3810 m. Acute mountain sickness was defined by the 2018 Lake Louise Questionnaire criteria.

RESULTS

There were 491 participants analyzed, 234 (48%) diagnosed with acute mountain sickness and 71 (14%) with moderate acute mountain sickness. Mean age was 37 years (±13). There was no significant correlation between Lake Louise Questionnaire severity and age (r = -0.02; 95% confidence interval [CI], -0.11-0.07, P = .7), 40-year-old dichotomy (t = -0.6; 95% CI, -0.53-0.28, P = .6), or decade of life (P = .4). Logistic regression found no increased odds of acute mountain sickness for increasing age by decade of life (odds ratio [OR] 1.0; 95% CI, 0.97-1.0) or 40-year-old dichotomy (OR 1.4; 95% CI, 0.97-2.1). A history of acute mountain sickness increased odds of acute mountain sickness (OR 3.2; 95% CI, 1.5-7.7).

CONCLUSIONS

Older age was not associated with incidence nor severity of acute mountain sickness. A history of altitude illness increased odds of acute mountain sickness and should be used for pre-ascent risk stratification.

Optic Nerve Ultrasound Evaluation in Animals and Normal Subjects

In recent years, ultrasonographic measurement of the optic nerve sheath diameter (ONSD) has been widely used to identify the presence of increased intracranial pressure (ICP). Intracranial hypertension is a life-threatening condition that can be caused by various neurological and non-neurological disorders, and it is associated to poor clinical results. Ultrasonography could be used to qualitatively and efficiently detect ICP increases, but to reach this purpose, clear cut-off values are mandatory. The aim of this review is to provide a wide overview of the most important scientific publications on optic nerve ultrasound normal values assessment published in the last 30 years. A total of 42 articles selected from PubMed medical database was included in this review. Our analysis showed that ocular ultrasonography is considered to be a valuable diagnostic tool, especially when intracranial hypertension is suspected, but unfortunately this research provided conflicting results that could be due to the different ultrasound protocols. This is mainly caused by the use of B scan alone, which presents several limitations. The use of B-scan coupled with the standardized A-scan approach could give more accurate, and reliable ultrasound evaluation, assuring higher data objectivity.

Predictive Capacity of Pulmonary Function Tests for Acute Mountain Sickness

Small, Elan, Nicholas Juul, David Pomeranz, Patrick Burns, Caleb Phillips, Mary Cheffers, and Grant S. Lipman. Predictive capacity of pulmonary function tests for acute mountain sickness. High Alt Med Biol. 22: 193-200, 2021. Background: Pulmonary function as measured by spirometry has been investigated at altitude with heterogenous results, though data focused on spirometry and acute mountain sickness (AMS) are limited. The objective of this study was to investigate the capacity of pulmonary function tests (PFTs) to predict the development of AMS. Materials and Methods: This study was a blinded prospective observational study run during a randomized controlled trial comparing acetazolamide, budesonide, and placebo for AMS prevention on White Mountain, CA. Spirometry measurements of forced expiratory volume in one second (FEV₁), forced vital capacity (FVC), and peak expiratory flow were taken at a baseline altitude of 1,250 m, and the evening of and morning after ascent to 3,810 m. Measurements were assessed for correlation with AMS. Results: One hundred three participants were analyzed with well-matched baseline demographics and AMS incidence of 75 (73%) and severe AMS of 48 (47%). There were no statistically significant associations between changes in mean spirometry values on ascent to high altitude with incidence of AMS or severe AMS. Lake Louise Questionnaire scores were negatively correlated with FVC (r = -0.31) and FEV₁ (r = -0.29) the night of ascent. Baseline PFT had a predictive accuracy of 65%-73% for AMS, with a receiver operating characteristic of 0.51-0.65. Conclusions: Spirometry did not demonstrate statistically significant changes on ascent to high altitude, nor were there significant associations with incidence of AMS or severe AMS. Low-altitude spirometry did not accurately predict development of AMS, and it should not be recommended for risk stratification.

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.

Ultrasound in Austere Environments

Ultrasonography is a noninvasive, reliable, repeatable, and inexpensive technology that has dramatically changed the practice of medicine. The clinical use of portable ultrasound devices has grown tremendously over the last 10 years in the fields of intensive care, emergency medicine, and anesthesiology. In this review we present the various ways that handheld portable ultrasound devices can be used in austere environments. The purpose of this review is to consider the wide-ranging applications for providers going into the austere environment, which include pulmonary, ocular, vascular, and trauma evaluations, the postdisaster setting, and the role of ultrasonography in tropical diseases. This review is not meant to be a comprehensive how-to guide for each study type, but an overview of some of the more common wilderness applications. This review also focuses on the limitation of each study type. The goal is to help wilderness medicine providers feel more comfortable incorporating ultrasonography as part of their tool kit when heading into austere environments.

High altitude trekking after lung transplantation: a prospective study using lung ultrasound to detect comets tails for interstitial pulmonary edema in lung transplant recipients and healthy volunteers

The intensity of physical activity which can be tolerated after lung transplantation and the tolerance to prolonged exercise at high altitude are poorly investigated. Lung ultrasound comet tails have been used in the diagnosis of interstitial pulmonary edema and high pulmonary altitude edema. The aim was to assess the number of lung ultrasound comet tails and to monitor changes in the optic nerve sheath diameter (ONSD) during a climb to the top of Mount Kilimanjaro in 10 lung transplant recipients and 10 healthy controls at three different altitude levels: 1360, 3505, 4900 m. Lung transplant recipients showed a constant increase in comet tail scores with altitude, whereas control subjects only showed an increase at the highest measurement point. Differences between groups (transplant versus control) reached significance only after the first ascend: 0.9 (95% CI: -0.41; 2.21) vs. 0.1 (95% CI: -0.12; 0.32) (P = 0.2; 1360 m), 2.33 (95% CI: 0.64; 4.02) vs. 0.3 (95% CI: -0.18; 0.78) (P = 0.04; 3505 m), and 4.11 (95% CI: 0.13; 0.34) vs. 2.9 (95% CI: 0.49; 5.31) (P = 0.15; 4900 m); ONSD increased significantly in both groups from 3.53 (95% CI: 0.34; 0.66) at 1360 m to 4.11 (95% CI: 0.36; 0.71) at 4900 m (P < 0.05). Lungs of transplant recipients are able to adapt to altitude and capable of performing prolonged exercise at high altitude after slow ascend.

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.

Hydration Status as a Predictor of High-altitude Mountaineering Performance

Background: Hydration status is a controversial determinant of athletic performance. This relationship has not been examined with mountaineering performance.

Methods: This was a prospective observational study of mountaineers who attempted to climb Denali in Alaska. Participants’ urine specific gravity (SG), and ultrasound measurements of the inferior vena cava size and collapsibility index (IVC-CI) were measured at rest prior to ascent. Upon descent, climbers reported maximum elevation gained for determination of summit success.

Results: One hundred twenty-one participants enrolled in the study. Data were collected on 111 participants (92% response rate); of those, 105 (87%) had complete hydration data. Fifty-seven percent of study participants were found to be dehydrated by IVC-CI on ultrasound, and 55% by urine SG. No significant association was found with summit success and quantitative measurements of hydration: IVC-CI (50.4% +/- 15.6 vs. 52.9% +/- 15.4, p = 0.91), IVC size (0.96 cm +/- 0.3 vs. 0.99 cm +/- 0.3, p = 0.81), and average SG (1.02 +/- 0.008 vs. 1.02 +/- 0.008, p = 0.87). Categorical measurements of urine SG found 24% more successful summiters were hydrated at 14 Camp, but this was not found to be statistically significant (p = 0.56). Summit success was associated with greater water-carrying capacity on univariate analysis only: 2.3 L, 95% confidence interval (2.1 – 2.5) vs. 2.1 L, 95% confidence interval (2 – 2.2); p < 0.01.

Conclusions: Intravascular dehydration was found in approximately half of technical high-altitude mountaineers. Hydration status was not significantly associated with summit success, but increased water-carrying capacity may be an easy and inexpensive educational intervention to improve performance.

The incidence of increased ICP in ICU patients with non-traumatic coma as diagnosed by ONSD and CT: a prospective cohort study

BACKGROUND

Unexplained coma after critical illness can be multifactorial. We evaluated the diagnostic ability of bedside Optic Nerve Sheath Diameter [ONSD] as a screening test for non-traumatic radiographic cerebral edema.

METHODS

In a prospective study, mixed medical-surgical intensive care units [ICU] patients with non-traumatic coma [GCS < 9] underwent bedside ultrasonographic ONSD measurements. Non-traumatic radiographic cerebral edema [NTRCE] was defined as > 5 mm midline shift, cisternal, sulcal effacement, or hydrocephalus on CT.

RESULTS

NTRCE was identified in 31 of 102 patients [30.4 %]. The area under the ROC curve for detecting radiographic edema by ONSD was 0.785 [95 % CI 0.695-0.874, p <0.001]. ONSD diameter of 0.57 cm was found to be the best cutoff threshold with a sensitivity 84 % and specificity 71 %, AUC 0.785 [95 % CI 0.695-0.874, p <0.001]. Using ONSD as a bedside test increased the post-test odds ratio [OR] for NTRCE by 2.89 times [positive likelihood ratio], whereas post-test OR for NTRCE decreased markedly given a negative ONSD test [ONSD measurement less than 0.57 cm]; negative likelihood ratio 0.22.

CONCLUSIONS

The use of ONSD as a bedside test in patients with non-traumatic coma has diagnostic value in identifying patients with non-traumatic radiographic cerebral edema.

Oxidative stress response to acute hypobaric hypoxia and its association with indirect measurement of increased intracranial pressure: a field study

High altitude is the most intriguing natural laboratory to study human physiological response to hypoxic conditions. In this study, we investigated changes in reactive oxygen species (ROS) and oxidative stress biomarkers during exposure to hypobaric hypoxia in 16 lowlanders. Moreover, we looked at the potential relationship between ROS related cellular damage and optic nerve sheath diameter (ONSD) as an indirect measurement of intracranial pressure. Baseline measurement of clinical signs and symptoms, biological samples and ultrasonography were assessed at 262 m and after passive ascent to 3830 m (9, 24 and 72 h). After 24 h the imbalance between ROS production (+141%) and scavenging (−41%) reflected an increase in oxidative stress related damage of 50–85%. ONSD concurrently increased, but regression analysis did not infer a causal relationship between oxidative stress biomarkers and changes in ONSD. These results provide new insight regarding ROS homeostasis and potential pathophysiological mechanisms of acute exposure to hypobaric hypoxia, plus other disease states associated with oxidative-stress damage as a result of tissue hypoxia.