Altitude sickness or mountain sickness is caused by having less oxygen in your bloodstream at higher altitudes compared with sea level. It can be serious, and even life-threatening. It occurs when you climb or walk to a high altitude too quickly and don’t give your body enough time to adjust to the lower oxygen levels in the atmosphere – a result of the lower atmospheric pressure at high altitudes compared to sea level.
Most people can ascend to a level of 2000 metres in one day without getting sick. About 20% of people develop mild altitude sickness at 2500 metres, which increases to 40% by 3000 metres and 75% by 4500 metres. Mild altitude sickness is quite common in visitors staying at ski resorts, who often mistakenly think they are suffering from a hangover or a viral illness. While there is nowhere high enough to cause altitude sickness in Australia, you may well experience it if you travel to high areas overseas.
Types of altitude sickness and their symptoms
Acute mountain sickness (AMS)
This is the mildest form of altitude sickness. Generally, symptoms develop within 6–10 hours of ascending and tend to resolve as you acclimatise to the new altitude. If you stay at the same altitude symptoms usually resolve in 1 or 2 days below 3000 m, but take longer at higher altitudes. However, in some cases symptoms may worsen and potentially develop into more dangerous forms of altitude sickness where fluid builds up in your lungs and/or brain. If your symptoms don’t resolve or get worse you should descend at least 500 metres.
Symptoms of AMS include having a headache along with any of these symptoms:
- nausea and vomiting
- loss of appetite
- feeling tired and weak
- feeling dizzy and lightheaded
- having trouble sleeping.
Severe altitude sickness
More serious forms of altitude sickness generally occur at altitudes above 2500 metres when fluid starts to accumulate in the lungs (high-altitude pulmonary oedema – HAPE) and/or in the brain (high-altitude cerebral oedema – HACE). Both conditions can occur at the same time and although uncommon, these forms of altitude sickness can be fatal. If you start to experience any of the symptoms for HAPE or HACE you should immediately start a slow, accompanied descent to lower altitudes.
Symptoms of HAPE
HAPE usually develops within 4 days of ascending but coma and death can occur within hours of ascending for some people. It is the main cause of death in people who develop altitude sickness. Symptoms include:
- severe breathing difficulties – eventually even at rest
- decreased performance
- frequent dry cough
- pink sputum due to the presence of blood
- low-grade fever (your temperature is elevated but below 38.5 degrees centigrade)
- palpitations (rapid heartbeat)
- loss of consciousness.
Symptoms of HACE
- Severe headache
- Lack of coordination – you may become clumsy and stumble
- Loss of consciousness.
Periodic breathing and high altitudes
Also known as Cheyne–Stokes breathing, periodic breathing is common at high altitudes. It involves alternate periods of deep and shallow breathing, and breathing may even cease for between 3 and 15 seconds.
When it occurs at altitude, periodic breathing is not a medical problem on its own. It is just the body’s physiological response to having lower-than-normal oxygen levels in the atmosphere. Your body reacts to this by making you breathe faster and deeper. This in turn, causes a quick build-up of carbon dioxide in your blood, which feeds back to your respiratory system causing your breathing to slow down or even stop for a short while. When your body senses a further drop in oxygen levels in the blood, it is kick-started into breathing faster and deeper again. And so the cycle continues.
Periodic breathing can interfere with your sleep if the periods of no breathing wake you up or make you sleep restlessly. If this becomes a problem, taking the prescription medicine acetazolamide (Diamox) may help.
There is no way of predicting who will, or will not, suffer from altitude sickness. Surprisingly, increased age, lack of fitness and illnesses such as asthma do not seem to make people more prone to the problem. In fact, altitude sickness is more common in young adults, maybe because they tend to be more active and adventurous than older people, and may not be limited by their fitness.
Having had altitude sickness before does not mean that you will necessarily get it again; however, you will be more susceptible. Similarly, having been okay at high altitude previously does not guarantee that you will be okay next time.
Most people with chronic illnesses can safely travel to high altitudes, but some diseases make high altitude travel dangerous. Such diseases include sickle cell anaemia and severe lung or heart disease. If you have a chronic disease, talk to your doctor before undertaking high-altitude travel.
Also, if you have diabetes your blood glucose meter may not work properly at high altitudes. Check with the manufacturer before you use it at high altitudes.
Treating altitude sickness
Acute mountain sickness
Drinking plenty of fluids and taking pain killers for your headache may help if you have AMS. If you have been prescribed sleeping tablets, stop taking them until you have recovered.
You can also treat mild-to-moderate symptoms with acetazolamide (Diamox) until you acclimatise, which may help your breathing. If you have side effects with acetazolamide or are allergic to it, you may be given a corticosteroid called dexamethasone instead.
You shouldn’t consider further ascent until your symptoms have resolved and ascend gradually when you do to prevent further episodes. If your symptoms don’t resolve within a few days you should seek medical advice – you may need to descend to a lower altitude.
Severe altitude sickness
If you develop severe altitude sickness you will need to descend immediately and be given oxygen. As an emergency stop-gap, you may have to be put in a portable altitude chamber or bag (such as a Gamow bag). Air is pumped into the chamber or bag to artificially create a higher atmospheric pressure, which provides the same relief as descending to a lower altitude. Although this may buy you time, it is not a substitute for descent.
You may also be given dexamethasone and/or the high blood pressure treatment, nifedipine.
The best way to avoid altitude sickness is to ascend slowly to your destination. This allows your body time to acclimatise to the decreasing oxygen supply. A good approach is to:
- Sleep at an altitude of 2500–3000 metres for the first 2 or 3 nights so your body can acclimatise.
- Ascend at a rate of 300–500 metres per day thereafter, and don’t overexert yourself. Go at a pace that suits you, even if it means only ascending 100 metres a day. Alternatively, you can ascend at a faster rate by leaving your equipment at the preferred altitude and then descending to a lower altitude to sleep. The next day you can retrieve your equipment and proceed upwards, repeating the ‘climb-high but sleep-low’ process until you reach your destination.
- Plan a day of rest for every 1000 metres you climb.
Keep well hydrated
Make sure you drink plenty of water and avoid alcohol. Dehydration is a contributing factor to altitude sickness. Much water is lost in the breath in cold, high altitudes, and also by evaporation through sweat, especially when undergoing activities such as skiing or trekking. Drinking your usual amount of coffee or tea is fine though.
If it is not possible to acclimatise – for example, if you are taking a short flight from sea-level to 3000 metres or more – acclimatisation can be helped by taking the medicine acetazolamide (Diamox). Take it 1 day before, or on the day of your planned trip, and continue to take it for 2 to 3 days at altitude or until you have reached the highest altitude you are going to. You can also take a mild painkiller like aspirin to prevent a headache.
If you have previously experienced HAPE, you may be prescribed other medicines, including nifedipine, dexamethasone, an asthma medicine called salmeterol (Serevent), or medicines usually used to treat impotence called tadalafil (Cialis) and sildenafil (Viagra). Your doctor can give you advice on what is appropriate for you.