Symptom progression in acute mountain sickness during a 12-hour exposure to normobaric hypoxia equivalent to 4500 m

The diagnosis and quantification of severity of acute mountain sickness (AMS) continue to be problematic. What symptoms should be included in a score and how to weigh any given symptom in the total score remain matter of debate. Seventy seven healthy male (n=43) and female (n=34) volunteers, aged between 18 and 42 years, were exposed to normobaric hypoxia (Fio2=12.6%≙4500 m) for 12 hours. Symptoms of AMS according to the Lake Louise Scoring system (LLS) were recorded before and after 30 min, 3, 6, 9, and 12 hours in hypoxia. AMS scores continued to increase steeply during the entire hypoxia exposure in subjects suffering from AMS (LLS>2). Headache was the predominant symptom and the severity of nausea progressed faster in subjects who left the hypoxia room prematurely (severely affected by AMS) compared to those moderately affected (LLS>2 but completing the 12-h hypoxia exposure). Whereas headache scores up to 6 hours in hypoxia were not correlated with other AMS symptoms, nausea was correlated with dizziness and fatigue (r=0.45 and 0.56, p<0.01). Cluster analysis identified three different distributions of symptom severity compatible with being very likely free of AMS (cluster 1), compatible with very likely suffering from AMS (cluster 3), and compatible with ambiguous allocation (cluster 2). In conclusion, our findings confirm that headache plus one or more of the symptoms nausea, dizziness, and fatigue of at least mild to moderate severity are required for diagnosis of AMS. The inter-relationship between nausea, dizziness, and fatigue, however, raises the question whether each of these symptoms should be given equal diagnostic weighting. The time course of symptom progression within the first hours at altitude may provide clinically important information on the severity of subsequent AMS development and will support the decision to start therapeutic intervention.