Many people travel to mountainous regions for work and recreation. In Nepal alone, over 130,000 foreigners visit each year to complete trekking and mountaineering activities of which half may get acute mountain sickness (AMS). However, general illnesses such as diarrhea and upper respiratory symptoms, and also psychological disturbances, contribute to ill health experienced at altitude.[3-5]
The intrusive nature of such general illnesses is likely to limit work capacity and enjoyment. There is also a substantial risk of having to be evacuated from expeditions due to such illnesses, and a small but real risk of such illnesses eventually resulting in death. Furthermore, conditions such as diarrhea, upper respiratory symptoms, and anxiety may be of considerable relevance to AMS since the conditions share many of the same symptoms (eg, nausea).[8, 9] Not only does this hamper diagnosis, but some authors have speculated that upper respiratory symptoms, diarrhea, or high anxiety may be causally related to AMS or share a common mechanism.[5, 10, 11]
However, whether exposure to high altitude environments per se actually increases incidence of diarrhea, upper respiratory symptoms, and anxiety remains unclear. Detailed description of these illnesses is lacking, and how these illnesses interact together is also unknown. Thus, the aim of the present investigation was to describe physical and mental health during a typical high altitude expedition. This study also aimed to explore relationships between illnesses and commonly implicated physiological factors, such as arterial oxygen saturation, heart rate, and fluid intake. Our hypotheses were that general physical (upper respiratory symptoms, diarrhea) and mental (anxiety) health would deteriorate with increasing altitude, and that presence of any illness symptoms or altered physiological parameters would increase AMS.
- Top of page
- Declaration of Interests
This study is the first to use a longitudinal multiple regression analysis of daily illnesses and mental disturbances recorded during a relatively large expedition to high altitude. AMS affected almost half of the expedition participants, with up to one quarter having AMS on any day. However, AMS incidence alone underestimated the total illness symptom burden: all the participants also had upper respiratory symptoms, two thirds had loose stools and one third had diarrhea, and almost everyone reported mild anxiety. Upper respiratory symptoms increased as altitude was gained, and anxiety was also increased on certain days at high altitude. Detailed description of illnesses revealed that the variable contributing most to AMS symptom burden was difficulty sleeping. However, difficulty sleeping was also the least sensitive of the AMS symptoms to altitude change. The variables that contributed most to upper respiratory symptom burden were nasal and cough related, and these variables were the most sensitive of the upper respiratory symptoms to altitude change. Throughout the expedition, participants with increased AMS symptoms had poorer physical and mental health, higher heart rate, and lower fluid intake. Upper respiratory symptoms, heart rate, arterial oxygen saturations, and fluid intake also predicted AMS symptoms the following day, and thus, these predictor variables were consistent with being causally related to AMS. However, contrary to our hypotheses, this study found no increase in diarrhea with altitude, and no causal effect of diarrhea and anxiety on AMS.
The incidence of AMS in the present study is consistent with previous studies using similar ascent profiles, as recently reviewed. Although a landmark early study suggested no association between upper respiratory infections and AMS incidence, subsequent studies provided data consistent with a greater number of respiratory symptoms and diarrhea being associated with a greater number of symptoms and severity of AMS. Nevertheless, conclusive evidence that general illness caused AMS was still lacking. The present study thus extends previous findings by providing empirical support, using a longitudinal regression design that upper respiratory symptoms increase with altitude and are associated with AMS. Of course individuals may not be able to differentiate between symptoms of upper respiratory symptoms and AMS, as evidenced by the reporting of “AMS” symptoms at low altitude. This highlights that misdiagnosis may occur and incorrect treatment may be administered. Nevertheless, previous authors have suggested that upper respiratory symptoms may predispose to AMS.[5, 10] The exact cause for this relationship remains unclear, but if any upper respiratory symptoms are due to infection, then one plausible mechanism is that an immune response such as inflammation may increase AMS,[5, 29] although such a mechanism remains to be proven.
In contrast to upper respiratory symptoms, in the present study, diarrhea did not increase with altitude and was not causally associated with AMS. Similarly, anxiety was increased at altitude but inconsistently so, and like diarrhea was not causally associated with AMS. Although previous studies have shown relationships between diarrhea and anxiety with AMS, they could not establish whether data were consistent with causality as was tested in the present study. Possibly, diarrhea may cause symptoms such as dehydration headache rather than AMS per se, and anxiety may be a consequence, rather than a cause of AMS.
Previous authors have also suggested that arterial oxygen saturation may predict AMS susceptibility.[10, 30-32] However, arterial oxygen saturation testing has failed to gain widespread acceptance, and some authors[33, 34] have found that resting oxygen saturation may be inferior to other predictor variables of AMS, albeit often only acute exposure was investigated. The data presented herein provide empirical evidence that arterial oxygen saturation is reduced with AMS and when daily measures are obtained reveals that resting oxygen saturation also predicts future AMS symptoms. It has been speculated that such a relationship may be due to sub-clinical pulmonary edema. Similarly, elevated heart rate has been associated with AMS by some but not all authors; the current data which is supportive of the relationship is consistent with the hypothesis of altered autonomic cardiovascular control leading to AMS. Alternatively, some other factor which elevates heart rate may cause AMS symptoms, such as dehydration. Although data on hydration state and AMS is contradictory,[10, 13, 14] the current data suggest that fluid intake reduced AMS symptoms during the expedition as a whole. However, fluid intake had little effect when investigating more specific and conservative definitions of AMS, possibly because the majority of participants achieved an intake of at least 2 L per day, recently speculated as the minimum intake required to avoid AMS. On the other hand, these findings may be due to fluid intake reducing dehydration-associated headache rather than altitude-associated headache per se, a finding consistent with recent experimental studies suggesting that dehydration induces headaches of similar severity to hypoxia.
Weaknesses of the study include lack of clinician and microbiological diagnosis of illness. However, such methods to verify diagnosis of illness have recently been scrutinized and found lacking. While self-assessment may lead to underreporting of illness due to social desirability bias, controlling for this weakness would have been unlikely to improve accuracy of the health logs. Finally, this observational cohort study was non-interventional and did not include a control group. The longitudinal analysis that allowed estimation of causality and the multiple time-point baseline period at lower altitude, which was longer than accepted incubation periods for general illnesses, addressed this issue. Furthermore, the present study's control period, completed under expedition conditions and where individuals acted as their own controls, may be a stronger design than using a control group residing at low altitude but under non-expedition conditions.
In conclusion, upper respiratory symptoms and anxiety increasingly contributed to symptom burden as altitude was gained. Data were consistent with increased heart rate, decreased arterial oxygen saturation, reduced fluid intake, and upper respiratory symptoms being causally associated with AMS. These findings are of relevance to researchers investigating travel-associated illnesses common at altitude.
For those offering travel advice, the findings should be used to increase education of the potential incidence and burden of illness and mental disturbances in high-altitude environments, especially as some commercial companies seem to ignore recommended ascent rates making high altitude illness likely. The risk of diarrhea at low altitude compared to high altitude, most likely due to poor food hygiene, is important. It is also of interest that those with general AMS symptoms may have higher anxiety, and expedition leaders should be vigilant for such mental disturbances. The findings also offer alternative intervention targets to reduce risk and severity of AMS. If upper respiratory symptoms are at least in part due to infections, those visiting high altitude could use appropriate recovery strategies when performing arduous exercise, maintain good personal hygiene, ensure good nutrition, obtain adequate good quality sleep, reduce chances of infection transmission, and aggressively treat infections with appropriate medications, all of which may reduce upper respiratory symptoms and consequently alleviate AMS symptoms. Effective strategies to increase fluid intake, for example, by purifying and flavoring water, may help avoid general headache symptoms. Not only will this enhance productivity and enjoyment of altitude sojourners, but serious complications associated with these illnesses may then be reduced.