Altitude sickness, or mountain sickness, is caused by climbing or walking to a high altitude too quickly without giving the body enough time needed to adjust to lower oxygen levels and air pressure changes.
On Sunday, it turned fatal for 34-year-old Melbourne woman Maria Strydom, who was an experienced climber scaling Mount Everest with her husband on a seven week expedition.
In their climbing group was Dutchman Eric Arnold, who also died of altitude sickness on Friday while climbing the world's highest peak.
An Indian climber who had fallen sick and was being helped down the mountain has also died.
RELATED READING:
Everest safety under scrutiny as third climber dies
Wangchu Sherpa of the Trekking Camp Nepal agency in Kathmandu said on Monday that Subhash Paul died overnight as he was being helped down the mountain by Sherpa guides.
Two other Indian members of Paul's team have been missing since Saturday. Another team member, an Indian woman who had fallen sick, was resting at a camp at a safer altitude.
The deaths have been the first fatalities on Everest this year.
Altitude sickness can appear suddenly after reaching a certain height and can easily become fatal, say experts. Source: AAP
A 'silent killer'
Alpinist of 15 years, Steve Turner, has been to India and Nepal six times, where he has been a group climb leader.
He said acute mountain sickness can be a "silent killer", and that ignoring symptoms and climbing higher can become fatal.
"Because of the lack of oxygen and stuff people expect to feel a bit tired and lethargic but when headaches become severe migraines, that's your body saying you're going too fast, too quickly," Mr Turner said.
Related reading
Melbourne's Maria Strydom dead on Everest
There is the risk people will slip into comas in their sleep, which will lead to their condition deteriorating quickly according to Mr Turner.
"Once that happens, death can follow ... If you black out or pass out, or lapse into a coma, you're generally in hospital for a week and then on the next plane home," he said.
He said climbers live by a maxim of "climb high and sleep low".
This allows them to sleep at a better altitude, while ascending lightly the next day because they have dropped off their gear several hundred metres higher.
What actually is it?
Altitude sickness is a broad term generally covering a range of symptoms, said mountaineering expert Alan Arnette.
He said these include high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE), which are forms of acute mountain sickness (AMS) and can occur at the same time.
Anyone who goes high enough without letting their body acclimatise to the changes may be affected.
HAPE is when there is fluid in the lungs, which cause climbers to become breathlessness, even while resting. People may experience coughing fits and develop a fever.
HACE occurs when there is fluid in the brain, which may interfere with people's rational thought processes. It causes people to become confused, clumsy and stumble.
Symptoms to look out for
Signs may vary but are usually characterised by extreme fatigue, confusion and trouble breathing.
People may feel a loss of energy, and become dizzy.
But this could become less pronounced over one or two days as the body starts to adapt to the higher altitude.
If it worsens, the person may start to lose their coordination and have trouble walking.
They may also get a severe headache, feel nauseous and vomit, and have changes in their skin tone.
Severe forms are considered an emergency and people need to be brought to lower heights.
Treatment
The best way to treat altitude sickness is to get oxygen to the person by coming down to a lower height of elevation where their body has already adjusted to. Mr Arnette said while there are drugs to treat symptoms, descending is the only cure.
"The best way to minimise altitude related illness is to allow the body to slowly adjust to the altitude through a series of ever higher climbs, which can take anywhere from three to six weeks on Everest," he said.
However there is no way to eliminated it altogether he said.
Mr Turner said oxygen tanks or portable altitude chambers can mimic the conditions of lower altitudes to help stabilise someone, but people should still ultimately climb down to lower levels.
"If some people push through that pain barrier it can lead to a mild form of AMS so they should go down and recover for a few hours."
Additional reporting by AAP
