Using the modified LLS criteria, we found a prevalence of AMS of 26.7% in mountaineers ascending to moderate-high altitudes in the Pyrenees region. To our knowledge, this is the first study that analyses the prevalence and risk factors of AMS in this region.
There is less information regarding the prevalence of AMS at moderate to high altitudes (around 3,000 m) in comparison with high to extreme altitudes. Our impression is that the prevalence found in our study is higher than the expected but it is difficult to compare our results with previous studies performed at similar heights because of differences in geographic locations and studied populations. AMS occurred in 25% of visitors to moderate altitudes (1,920-2,956 meters) in the Rocky Mountains of Colorado [7]. The prevalence of AMS in other studies performed at Mount Fuji [9] and Western and Eastern Alps [10] was 29.5% (3,776 meters), 34.9% (3,817 meters) and 38.0% (3,454 meters), respectively. It is also important to highlight that these studies were conducted before the LLS was modified in 2018 and, to our knowledge, no studies regarding AMS prevalence have been published using this modified score. Other factors could explain the differences in AMS prevalence at similar altitudes (different hut locations, different weather conditions, different levels of mountaineer experience, etc.) but the explanation to them is beyond the scope of this study.
We have also found different risk factors for AMS, many of them already described in the literature that deserve discussion.
Men and women appear to be equally at risk for AMS although some observational studies suggest a slightly higher risk for women [7]. We also found a higher risk for AMS in women but the statistical significance of this difference was borderline (p value = 0.07).
Regarding pre-existing medical conditions we only found that OSAS was associated with a greater risk for AMS in the univariate analysis. Several studies have shown that obesity and nocturnal hypoxemia are risk factors for the development of AMS. Patients with OSAS and significant arterial desaturation at sea level would be expected to have more profound arterial desaturation during apneic periods at high altitude, but there are no data on this issue [11].
The most relevant risk factor (with a higher odds ratio) for developing AMS in our study was the intensity of exertion perceived by mountaineers. Those individuals who reported heavy exertion had more than 2 fold risk of developing AMS. This finding has also been found in other studies and could be explained due to the stress caused in the autonomic nervous system by additional hypoxia that generates an intense exercise [3, 10]. The role of exercise in this study must be discussed. We found a high proportion of subjects being fatigued (half of them moderately to severe) and it is difficult to assess whether these symptoms are secondary to AMS or indicate the effect of exercise. Furthermore, Moore et al have recently drawn attention to the inclusion of fatigue in the LLS. These authors suggest that fatigue may contribute to increase false positive AMS diagnoses and propose to remove this symptom from the score [12]. For this reason, we believe the reported AMS scores would have been lower without the inclusion of fatigue and the prevalence of AMS would have been lower.
Moreover, self-assessed bad physical condition was found to be another significant risk factor for AMS. This can be explained, in part, by the fact that physical condition is related to the level of exertion during the ascent, suggesting that low fitness climbers do not tolerate the unusual exertion of mountaineering, or they appear to workout excessively. The results regarding this issue in the literature are inconclusive, with some studies finding that physical condition is a risk factor for AMS [7, 10] while others not [4, 13]. This variability between studies may be explained because self-reported physical condition is difficult to objectively evaluate with considerable variability between individuals.
We also found that history of HAI on previous exposures was also a risk factor for suffering AMS (almost more than two fold risk). Based on our findings and previous studies, a strong relationship seems to exist between a self-report of previous HAI and the risk of subsequent development of AMS. There are different theories regarding individual susceptibility to HAI, but it is likely to be derived from both genetic and environmental variables. The genetic influence of AMS remains an active area of investigation with no identified specific genetic predisposition [2, 4, 7, 13–15].
Finally, we also found that NSAIDs consumption in the previous 48 hours was associated with an increased risk for AMS. However, caution should be applied when interpreting this apparently confusing result. We do not believe that taking an analgesic places one at a greater risk for subsequent AMS. It is likely that many of our participants who used NSAIDs did so after they started experiencing symptoms associated with AMS in an effort to alleviate those symptoms. The findings that LLS was higher and AMS was more severe in those taking NSAIDs supports this hypothesis. The same finding and interpretation has also been reported in previous studies [13]. On the other hand, the use of NSAIDs was not associated with a previous history of AMS, so it seems unlikely that their use was preventive to avoid symptoms.
It is commonly accepted that residing at an altitude above 800 meters from sea level offers protection against AMS [7, 16]. In our study, we found a higher proportion of participants living below 800 meters from sea level in the ones who suffered AMS compared with those who not, but these differences did not reach statistical significance (p = 0.09). This may be explained, in part, because only 10% of participants were living above 800 meters from the sea level.
This study has some limitations that must be taken into account. First, data were collected in person and on-line and this could to contribute to a participant selection bias. Despite this, we did not found differences in the prevalence of AMS between the two types of data collection. Second, the individuals who participated in the study did so voluntarily (non-probabilistic method), allowing perhaps to select individuals with “different” characteristics to those who decide not to participate (more motivation, less severity of symptoms, etc.). Third, the results presented here have performed in a specific area of the Pyrenees and probably cannot be generalized.