Health Professionals

Travel Health Information Sheets

November 2007

Altitude Illness

Introduction

Altitude illness is a term used to describe a spectrum of illness associated with ascent to altitudes usually higher than 2,500 metres. It can be divided into three syndromes, acute mountain sickness (AMS), which is the most common, high-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE) [1].

Risk for travellers

It is not possible to predict the susceptibility of a traveller to AMS, and physically fit travellers are not necessarily at lower risk. The best indicator of how altitude will affect a traveller is previous experience at altitude, but even this may be unreliable.

Important risk factors are altitude gained, rate of ascent, altitude achieved, altitude at which a traveller sleeps, and level of exertion. Approximately 50% of trekkers in Nepal who walked to altitudes above 4,000m for five or more days developed AMS [2], whilst 84% of those who flew directly to 3,860m were affected [3]. Thus, rapid ascent without a period of acclimatisation puts a traveller at higher risk.

Cause

There are several physiologic contributions to the development of AMS. Hypoxia is one of the main physiological alterations during ascent to high altitude. The percentage of oxygen in the air remains the same at altitude, however, the partial pressure drops. It is this pressure which drives oxygen into the bloodstream, therefore a decrease results in lower oxygen levels in the blood. The body’s response to lower oxygen levels is to increase the breathing rate.

One of the consequences of an increased breathing rate is the expiration of more carbon dioxide producing a respiratory alkalosis. Carbon dioxide in the blood helps to control the rate of breathing, and a low level slows the breathing again, leading to further hypoxia. Over several days the kidneys will increase excretion of bicarbonate bringing the system back in balance.

Signs and symptoms

AMS is the most common syndrome, and usually occurs at altitudes of 2,500-3,500m (8,200-11,500ft) but can occur at lower altitudes between 1,500–2,500m (5,000-8,200ft) [1]. Symptoms of AMS typically occur 6-12 hours after arrival at altitude [4], but can begin more than 24 hours after ascent.

Initial symptoms include headache, fatigue, loss of appetite, nausea and sleep disturbance. These symptoms usually resolve within one to two days if further ascent does not occur.

AMS progresses in less than 10% of cases to the more severe HACE where travellers experience lethargy, confusion and ataxia in addition to the symptoms of AMS.

HAPE typically occurs in the first two to four days after arrival at altitudes higher than 2,500m [4]. HAPE is not necessarily preceded by AMS. Initial symptoms of HAPE include shortness of breath with exertion, and a dry cough, progressing to shortness of breath at rest. The cough may become productive with blood-stained sputum. HAPE is frequently accompanied by symptoms of HACE. 

Anyone with symptoms of HAPE or HACE should descend immediately. Both HACE and HAPE can progress rapidly and death is the likely consequence if a descent is not made as soon as the symptoms are recognised.

Treatment

Although mild AMS is unpleasant, it is usually self-limiting, resolving spontaneously over several hours or days if no further ascent is made. Acclimatisation may take between three days and three weeks depending on several factors including the altitude to be attained. Paracetamol, aspirin or ibuprofen can be used to relieve headache, and anti-emetics may be used for nausea. Acetazolamide (Diamox®) may also be used for treatment but the onset of its effect can be delayed. A person with AMS should never be left unattended in case symptoms worsen.

If no improvement occurs, or symptoms worsen, an immediate descent by at least 500-1,000m should be made.

The main principle of treatment of severe AMS, HACE or HAPE is immediate descent. Oxygen by face mask can help to relieve symptoms. Nifedipine and dexamethasone can be useful in the treatment of HAPE, and dexamethasone can relieve symptoms of HACE. These drugs are not routinely recommended for all travellers to carry to altitude, but are usually reserved for climbing expeditions to extreme altitudes and administered by persons with extensive experience in the management of high altitude illness. Portable hyperbaric chambers may also be used by expeditions.

Prevention

It is not always possible to prevent altitude illness, especially if an itinerary involves flying directly to a high altitude destination. Nevertheless, severe consequences of altitude should be avoidable.

The most important prevention of AMS is adequate acclimatisation and regular rest days. Travel to altitudes above 3,500m immediately from sea level should be avoided whenever possible. Following a short period of acclimatisation to 3,000m further ascent should be gradual with no more than a 300-500m increase in sleeping altitude per day, with a rest day every three days. If symptoms of AMS develop, no further ascent should be made until recovered, and a rapid descent should be made if signs of severe AMS occur.

Acetazolmide (Diamox®) has been extensively used and studied as prevention for AMS, although it is unlicensed for treatment or prevention of AMS. Acetazolamide should not be considered as an alternative to adequate acclimatisation and gradual ascent, and its routine use before ascent should be avoided. Acetazolamide will hasten acclimatisation, and may help to relieve the symptoms of AMS but has a delayed onset of 12-24 hours when used in treatment.

If travellers are to use acetazolamide, trial doses of 125mg twice daily for two days should be taken prior to travel. Assuming there are no adverse events it should then be commenced one to two days prior to ascent to 3,500m and then continued for at least two more days after reaching the highest altitude. A dose of 125mg twice daily is likely to be effective and to be associated with fewer adverse events than higher doses [5, 6]. However, this dose has not been extensively studied in comparison with higher doses.

Acetazolamide can cause nausea, a mild diuresis, and circumoral and sensations of finger tingling. More unusual side effects include rashes, flushing and thirst. It is contraindicated in those with an allergy to sulphonamides.

Specific considerations for travellers with special health needs 

  • Pulmonary problems

Persons with chronic pulmonary disease who experience dyspnoea during mild exercise at sea level are likely to experience more severe symptoms at high altitude [7]. Therefore such travellers should avoid high-altitude destinations. Those with mild or moderate pulmonary disease may be able to tolerate trips to altitude, but should be evaluated by their respiratory consultant. All travellers with pulmonary disease intending to travel to high altitude should ensure that they are in optimum health. The ‘50 meter walk’ test has traditionally been used to assess persons with pulmonary illness. The ability to walk 50 meters without distress is a crude indicator of an individual’s ability to tolerate the relative hypoxia experienced during air travel [8].

Persons with asthma often report that their symptoms improve with altitude; however, they should be advised to continue their usual medication even if their symptoms improve. Brochospasm can be triggered by cold air, and travellers should use their asthma inhaler as necessary. There is no evidence to suggest that asthma increases the risk of altitude illness.

  • Cardiovascular problems

Persons with mild angina or those who have had successful bypass surgery have travelled to altitude without ill effects. However, those with a history of cardiac failure are likely to experience problems above 3,000m [9]. Travellers with cardiac disease should discuss their plans with their cardiologist. They should also be advised to limit their activities during the first days of acclimatisation and to allow extra time to acclimatise. Adequate supplies of cardiac medication should be carried and any dosage adjustments discussed.

Studies of the effect of high altitude on blood pressure have had conflicting results. There is little evidence of adverse events associated with hypertension at altitude, and well controlled hypertension is not a contraindication to high altitude areas [10].  It is not necessary to increase the dosages of anti-hypertensive medication.

  • Pregnancy

There is a lack of data on the effects of exposures to high altitude during pregnancy. However, it appears that short exposures are tolerated well by healthy pregnant women [7], and although changes in air cabin pressure may have a transient effect on the foetus, air travel is safe for most pregnant women [11].

It is prudent to avoid travel to high altitude during the first trimester. Pregnant women who are anaemic, carrying twins or diagnosed with any condition that reduces the oxygen supply to the foetus are also advised to avoid high altitude travel. Another consideration is that travel to high altitude is usually remote and access to medical facilities in the event of an obstetric emergency will be difficult. Trekking and skiing will also increase the risk of accidents which may be harmful to the foetus. Pregnant women travelling to high altitude areas should discuss their travel plans with their obstetrician or midwife.

References

1. Dietz T, Hackett PH. Altitude. In, Keystone J, Kozarsky P, Freedman D, et al (eds.). Travel Medicine. Elsevier Ltd., 2004.

Hackett PH, Rennie D, Levine HD. The incidence, importance, and prophylaxis of acute mountain sickness. Lancet 1976;ii:1149-55

2. Murdoch DR. Altitude illness among tourists flying to 3740 meters elevation in the Nepal Himalayas. J Travel Med 1995;2:255-6

3. Basnyat B, Murdoch DR. High-altitude illness. Lancet. 2003; 361: 1967-74.

4. Barry PW, Pollard AJ. Altitude illness. BMJ 2003;326:915-9,

5. Basnyat B, Gertsch JH, Johnson EW, et al. Efficacy of low-dose acetazolamide (125 mg BID) for the prophylaxis of acute mountain sickness: a prospective, double-blind, randomized, placebo-controlled trial. High Alt Med Biol 2003;4:45-52

6. Rodway GW, Hoffman LA, Sanders MH. High-altitude-related disorders – part II: prevention, special populations, and chronic medical conditions. Heart and Lung 2004; 33: 3-12.

7. British Thoracic Society Standards of Care Committee. Managing passengers with respiratory disease planning air travel: British Thoracic Society recommendations. Thorax 2002; 57: 289-304.

8. Clarke C. High altitude medicine. Travel Med Infect Dis. 2005; 3: 189-197.

9. Pollard A, Murdoch D. The High Altitude Medicine Handbook. Third edition. 2003, Radcliffe Medical Press, Oxon.

10. Committee on Obstetric Practice. ACOG committee opinion air travel during pregnancy. Int J Gynecol Obstet 2001; 76: 338-9.

Reading list

UIAA Mountain Medicine Centre http://www.thebmc.co.uk/Feature.aspx?id=1921

Committee to Advise on Tropical Medicine and Travel. Statement on high-altitude illnesses. CCDR 2007; 33: ACS5. Available online at http://www.phac-aspc.gc.ca/publicat/ccdr-rmtc/07vol33/acs-05/index_e.html

Hackett PH, Roach RC. Current concepts: High Altitude Illness. N Engl J Med 2001; 345: 107-114

Medex. Travel at high altitude. Available online at http://www.medex.org.uk/medex_book.htm

Pollard A, Murdoch D. The High Altitude Medicine Handbook. Third edition. 2003, Radcliffe Medical Press, Oxon.