Abstract
Our position is that from a research perspective there are important advantages to defining AMS as a collection of symptoms that includes, always, a headache. The same is obviously not true for clinical management of altitude illness. This contrast between the needs for research versus best practice in clinical care forms the basis of our stance. In spite of several decades of sustained research efforts, the basic pathophysiology of AMS still remains elusive. We believe that a focus on high altitude headache will help unravel the puzzle of the pathophysiology of AMS.
By stipulating for research purposes that headache be included in the definition of AMS, scientists seek to increase signal-to-noise ratio in AMS subjective symptom reporting. Unfortunately, subjective perception and reporting of symptoms is currently the only way to diagnose and quantify AMS. An example of excess noise is that without obligatory headache, a research subject who had a very bad night sleep and is dizzy at the time of completing the questionnaire could have a “severe” AMS score. Yet we know from several studies that poor sleep at altitude is not necessarily related directly to AMS. And dizziness can be secondary to hypoxemia, or to hypohydration, a common, non-AMS-related phenomenon at high altitude. Similarly, nausea and vomiting can be associated with the onset of AMS, or secondary to a bad headache of any cause, especially migraine, and in the field can also be secondary to gastrointestinal infection. Thus, obligatory presence of headache in the research scoring of AMS symptoms serves an important role by increasing the likelihood that the central symptom of AMS, first reported so clearly by Ravenhill, remains the focus of research studies. It should be noted that the Environmental Symptoms Questionnaire and its AMS-C subscale do not stipulate that headache be included to reach a score generally accepted as representing AMS; in our opinion rather than an argument reducing the importance of headache in AMS, it reveals a weakness of the AMS-C score.
Another approach to this debate is to ask if removing headache from the definition of AMS would significantly help to advance research into the pathophysiology of AMS. The answer is a resounding no. For the reasons mentioned above, the mandatory inclusion of headache focuses the researcher's attention on the central symptom of AMS. Taking this view to the next level, an idea to use only the headache score as the main response variable in research studies is gaining attention.
We argue that a sound rationale exists for including headache when assessing a group of symptoms and trying to define AMS. It is also critical to advancing our understanding of the pathophysiology of AMS to understand the cause of high altitude headache. While recognized as a unique type of headache by the International Headache Society, its underlying mechanism, like that for many other headaches, remains unknown (The International Headache Society, 2004). Parallel advances in the study of headache mechanisms in general will potentially allow multiple avenues for investigation into mechanisms of high altitude headache.
While all symptoms of AMS can also be accounted for by a large variety of non-altitude causes, many of which exist in settings where AMS is studied, headache seems most reliable and can stand by itself clinically to diagnose AMS. Advanced tools exist for quantifying headache in response to interventions (Sunshine et al., 2006; Tfelt-Hansen et al.) and recent studies have proposed novel scoring approaches for high altitude headache (Roach and Kayser, 2007; Kayser et al., 2010; Wagner et al., 2007), thus increasing the research tools available to investigate AMS.
We are aware that obligatory inclusion of headache may lead to exaggeration of incidence of AMS when non-altitude sickness headache occurs, like in the case of hypoxia-triggered migraine (Schoonman et al., 2006). For this reason we encourage investigators to screen for and exclude migraineurs, and to identify persons with a family history of migraine among potential altitude-naïve research volunteers. Along these lines we further propose that AMS headache can be verified by the response to oxygen breathing. Breathing oxygen at 2 to 4 L.min−1 will usually reverse the headache pain in 30 minutes, while a headache not attenuated by oxygen breathing probably has another cause, such as a migraine triggered by hypoxia. Steps like these will serve to further improve the signal-to-noise ratio in AMS studies and help us to better understand its underlying pathophysiology.
Is it important for clinical management to define AMS as including a headache? The answer is no. Clinically we fully agree with the traditional maxim: when feeling bad after a recent gain in altitude (with or without a headache) suspect that altitude is playing a major role until proven otherwise. For practical purposes, anyone with incapacitating symptoms at high altitude regardless of AMS score or presence of a headache should be treated as if they have severe altitude illness, descent should be organized, and oxygen administered if available. For mild to moderate cases, symptom amelioration by conventional means should be implemented. A quantified AMS score is irrelevant to sound clinical management when following these guidelines (Luks et al., 2010).