Acute mountain sickness: Do different time courses point to different pathophysiological mechanisms?

Acute mountain sickness (AMS) is a syndrome of nonspecific symptoms (i.e., headache, anorexia, nausea, vomiting, dizziness, and fatigue) that may develop in nonacclimatized individuals after rapid exposure to altitudes ≥2,500 m. In field studies, mean AMS scores usually peak after the first night at a new altitude. Analyses of the individual time courses of AMS in four studies performed at 3,450 m and 4,559 m revealed that three different patterns are hidden in the above-described overall picture. In 41% of those who developed AMS (i.e., AMS-C score >0.70), symptoms peaked on day 1, in 39%, symptoms were most prominent on day 2, and in 20%, symptoms were most prominent on day 3. We suggest to name the different time courses of AMS type I, type II, and type III, respectively. Here, we hypothesize that the variation of time courses of AMS are caused by different pathophysiological mechanisms. This assumption could explain why no consistent correlations between an overall assessment of AMS and single pathophysiological factors have been found in a large number of studies over the past 50 yr. In this paper, we will briefly review the fundamental mechanisms implicated in the pathophysiology of AMS and discuss how they might contribute to the three different AMS time courses.