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Altitude (or mountain) sickness refers to the symptoms that affect people at an altitude level, usually above 8,000 feet or 2,400 m.  Altitude is generally defined as

  - High Altitude: 1500 - 3500 m (5000 - 11500 ft)
  - Very High Altitude: 3500 - 5500 m (11500 - 18000 ft)
  - Extreme Altitude: above 5500 m

People facing the risk of altitude sickness would include mountain climbers, hikers, skiers, or travelers at high-altitude areas. However, there are no specific factors such as age, sex, or physical condition that correlate with susceptibility to altitude sickness. Some people are just more susceptible than others. 

1. Causes

It arises mainly from the  reduced air pressure and lower oxygen levels at high altitudes. It usually happens when there is rapid ascent to a high altitude area, because our body does not have the time to adjusting to the decreased oxygen level there. The symptoms also depend on the speed of your climb and how hard you push yourself.

Basically, air is thinner at high altitudes. As we reach higher altitudes, our body cannot take in as much oxygen as it requires. (There are fewer oxygen molecules per breath.) High altitude and lower air pressure also cause fluid to leak from the blood capillaries which can cause fluid build-up in both the lungs and the brain. This results in the manifestation of the symptoms (e.g. headache). Nonetheless, as we give our body more time to adjust to the altitude change, the symptoms will usually go away. 

It is said that most people will experience the Cheyne-Stokes Respirations once they ascend above 3,000 m (or 10,000 feet). This is a periodic breathing pattern during sleep, which is characterised first by a few shallow breathes; and then deep sighing respiratory motions, then slowing down rapidly (in some cases, even ceasing entirely for a few seconds). It then goes back to the shallow breaths again in a cycle. When the person stops breathing for a few seconds, he might become restless and wake up with a sudden feeling of suffocation. As this breathing pattern disturbs the individual's sleep, he can become exhausted  the next day. At high altitudes, this type of breathing is not considered abnormal, though doctors can prescribe acetazolamide to relieve the symptoms. 

2. Symptoms

The symptoms are usually mild, though there have also been life-threatening cases which affect the nervous system, lungs, muscles and heart. Typical symptoms, which can manifest from a few hours to a few days that you have been at a high altitude, are

  • Throbbing headache, which gets worse during the night and when you wake up

  • Dizziness or light-headedness

  • Fatigue

  • Insomnia or awakening frequently at night

  • Loss of appetite

  • Nausea or vomiting

  • Easily out of breath with light activity

    More severe altitude sickness would have symptoms such as 

    • Bluish discoloration of the skin (cyanosis)

    • Chest tightness or congestion

    • Cough

    • Coughing up blood

    • Gray or pale complexion

    • Inability to walk in a straight line, or to walk at all

    • Difficulty in breathing

    In very severe cases of altitude sickness, the person may cough up frothy sputum. This could be a sign of a high altitude pulmonary oedema (HAPO) where fluid collects in the lungs, interfering with the supply of oxygen to the body. Moreover, the brain can also swell (know as high altitude cerebral oedema (HACO), making it difficult for the individual to coordinate his physical movements. If left untreated, death may occur.

    Oxygen levels

    3. Treatment 

    As altitude sickness is easier to treat in the early stages, it is important to identify the symptoms quickly and take necessary remedial action. If you feel unwell at a high altitude, it is better to assume that it is altitude sickness and stop the ascent. If you only have mild symptoms, you may be able to stay at that altitude and let your body acclimatise to the altitude. When doing so, just rest, drink water and do not engage in any heavy physical activities.

    (i) The best treatment for altitude sickness is to descend to a lower altitude as quickly as possible. Getting to a lower altitude will help to alleviate or eliminate the symptoms. 

    (ii) If you cannot descend to a lower altitude, do not ascend any further. Limit your physical activity and drink lots of water.

    (iii) Provide extra oxygen to the individual to alleviate the symptoms.

    (iv) Acetazolamide (Diamox) may be given to help improve breathing and reduce mild symptoms. 

    (v) People with severe altitude sickness may need to be immediately admitted to a hospital.

    It usually takes one to three days for the body to acclimatise to a higher altitude. During this time, the individual's breathing becomes deeper, the number of red blood cells in the blood available to carry oxygen increases, the lungs become better supplied with oxygen-rich blood, and oxygen is released more efficiently in the tissues.

    4. Prevention
    There is no foolproof way to prevent altitude sickness as it often depends on the individual's condition and circumstances. Some general tips are
    • Pace your ascent to higher altitudes. Do not ascend at a rapid pace. Try to space out your ascent so that you can get acclimatised to the rising altitudes at a more comfortable pace. While it might be more time-consuming, it is better than being stricken with altitude sickness. 
    • If you have to fly into high-altitude cities, try to avoid large meals, smoking and alcohol before your arrival. These will only exacerbate your symptoms later.
    • Upon arrival at high-altitude regions, limit your physical activity in the first few days. Rest and drink lots of water. (It is normal to urinate more than usual at a high altitude as the body's chemistry and fluid balance changes during acclimatisation. If you are not,  you may be dehydrated or not acclimatising well.)
    • If possible, sleep at an altitude that is lower than the altitude you were at during the day. 
    • Eat a high-carbohydrate diet while at high altitude.
    • Talk to your doctor about the option of taking acetazolamide (Diamox).
    Altitude sickness