  Acute mountain sickness (AMS) is a common problem among visitors at high
altitude, and may progress to life-threatening pulmonary and cerebral oedema in
a minority of cases. International consensus defines AMS as a constellation of
subjective, non-specific symptoms. Specifically, headache, sleep disturbance,
fatigue and dizziness are given equal diagnostic weighting. Different
pathophysiological mechanisms are now thought to underlie headache and sleep
disturbance during acute exposure to high altitude. Hence, these symptoms may
not belong together as a single syndrome. Using a novel visual analogue scale
(VAS), we sought to undertake a systematic exploration of the symptomatology of
AMS using an unbiased, data-driven approach originally designed for analysis of
gene expression. Symptom scores were collected from 293 subjects during 1110
subject-days at altitudes between 3650m and 5200m on Apex expeditions to
Bolivia and Kilimanjaro. Three distinct patterns of symptoms were consistently
identified. Although fatigue is a ubiquitous finding, sleep disturbance and
headache are each commonly reported without the other. The commonest pattern of
symptoms was sleep disturbance and fatigue, with little or no headache. In
subjects reporting severe headache, 40% did not report sleep disturbance. Sleep
disturbance correlates poorly with other symptoms of AMS (Pearson r = 0.31 vs
headache). These results challenge the accepted paradigm that AMS is a single
disease process and describe at least two distinct syndromes following acute
ascent to high altitude. This approach to analysing symptom patterns has
potential utility in other clinical syndromes.
