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Acute
Mountain Sickness (AMS) Fact Sheet
Chris Imray
AMS is a potentially
serious condition that may affect all travellers
to altitudes over about 2,500m (8000ft).
Atmospheric pressure falls with increasing altitude,
and although the percentage of oxygen in the air
is constant, the partial pressure of oxygen falls
(at 5,500m or 18,000ft, the amount of oxygen available
in each breath is roughly half that at sea level).
Diagnosis and Symptoms
of AMS
Unlike many medical conditions, which require complex
tests, the diagnosis of AMS is based upon the subject’s
symptoms. The Lake Louise Self Assessment Scoring
is a simple questionaire that can be used by anyone
to diagnose AMS.
Symptoms include headache, lethargy, shortness of
breath, sleep disturbance, loss of appetite, nausea
and vomiting and may lead to ataxia (unsteadiness),
coma and death.
Risk factors for
AMS
There is no absolute way to predict an individual’s
susceptibility to AMS.
Risk factors include: Rate of ascent, altitude achieved,
previous recent exposure (reduces risk), past experience
of susceptibility.
Natural course
of AMS
Mild cases of AMS will resolve over 24-72 hours
if no further ascent is made; descent of 500m will
treat all mild cases.
Further ascent or failure to descend may precipitate
a significant deterioration in the subject’s
condition, which can lead to fluid on the lung (high
altitude pulmonary oedema [HAPE]) or fluid on the
brain (high altitude cerebral oedema [HACE]).
Prevention of AMS
- 1. Rate of ascent
AMS can be avoided by a slow rate of ascent allowing
the body to acclimatise to the lack of oxygen. Above
3000m, the average rate of ascent should not exceed
300m per 24hrs. Rest days should be used to allow
this graded rate of ascent to be achieved. Tight
inflexible schedules are potentially dangerous.
The altitude at which the individual sleeps is important,
the 300m/day ascent rule can be exceeded by day
if the person sleeps no higher than 300m above the
previous night's altitude: ‘Climb high, sleep
low’.
- 2. Drug prophylaxis
Acetazolamide is the drug of choice in the prevention
of AMS.
250 mg bd (twice daily) or 500mg slow release od
(once daily) starting 48-24hrs prior to ascent.
Recently it has been suggested that a dose of 750mg
per day (250mg tds) is more effective. Acetazolamide
should be continued for about a week (or as long
as the subject is at altitude if this is shorter)
Side affects of acetazolamide are mild and most
commonly include paraesthesia (pins & needles)
and diuresis (increased urine output). Acetazolamide
must be avoided in people allergic to sulphonamide
drugs. Rashes may occur; the drug should be stopped
if this occurs and medical advice should be sought.
Treatment of AMS
- 1. Descent
If symptoms are mild, it is usually safe to not
to ascend further and merely to rest. Simple analgesics
such as aspirin or paracetamol may be used to treat
the headache.
Moderate or severe symptoms, particularly if there
are neurological signs or pulmonary oedema, necessitate
urgent descent of at least 500m.
Sometimes the weather conditions or local terrain
may preclude descent in which case the following
treatments have been shown to be beneficial.
- 2. Oxygen
Administration of oxygen at about 6 litres/minute.
- 3. Acetazolamide
Oral acetazolamide 250 bd (twice daily) or 500mg
slow release od (once daily) starting immediately.
- 4. Dexamethasone
Dexamethasone 8 mg immediately then 4mg qds (four
times per day) oral, intravenously or IM.
- 5. Hyperbaric chamber
Portable hyperbaric chambers (Gamow or Certec bags)
can be used to treat individuals with AMS. The subject
is placed inside the chamber and the bag pumped
up until a pressure of 200mBar is achieved. The
effect of this is equivalent to about 2000m of descent.
Treatment should be continued for at least 2 hours.
The aim is to improve the subject’s condition
sufficiently so that he/she can then descend.
Lake Louise Self
Assessment Scoring for AMS
This simple self-assessment scoring system can be
used to determine whether or not an individual has
AMS. A score of 3 or more (with a headache) after
recent gain in altitude is consistent with a diagnosis
of AMS.
| Symptom |
|
Score |
| |
|
|
| Headache |
|
none at all |
0 |
| |
|
mild headache |
1 |
| |
|
moderate headache |
2 |
| |
|
severe incapacitating
headache |
3 |
| |
|
|
|
| Gastrointestinal symptoms |
|
good appetite |
0 |
| |
|
poor appetite or nausea |
1 |
| |
|
moderate nausea or vomiting |
2 |
| |
|
severe nausea or vomiting |
3 |
| |
|
|
|
| Fatigue/weakness |
|
Not tired or weak |
0 |
| |
|
Mild fatigue/weakness |
1 |
| |
|
Moderate fatigue/weakness |
2 |
| |
|
Severe fatigue/weakness |
3 |
| |
|
|
|
| Dizziness / lightheadedness |
|
none |
0 |
| |
|
mild |
1 |
| |
|
moderate |
2 |
| |
|
severe/incapacitating |
3 |
| |
|
|
|
| Difficulty sleeping |
|
slept as well as usual |
0 |
| |
|
did not sleep as well as usual |
1 |
| |
|
woke many times, poor nights sleep |
2 |
| |
|
could not sleep at all |
3 |
| |
|
|
|
| Overall, did these symptoms affect your
activities? |
|
not at all |
0 |
| |
|
mild reduction |
1 |
| |
|
moderate reduction |
2 |
| |
|
severe reduction (bedrest) |
3 |
| |
|
|
Further reading:
The High Altitude Medicine
Handbook. Radcliffe Medical Press, Inc. ISBN 81
7303 128 2
Andrew Pollard & David Murdoch
High Altitude Medicine and Physiology. 3rd Edition.
2000 ISBN 0 340 75980
Arnold 338 Euston Road, London NW1 3BH Michael Ward,
James Milledge & John West
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