High altitude illness (HAI) is the name given to a range of conditions that can occur when an individual ascends to a high altitude. It is most likely to occur when the ascent has not allowed sufficient time to acclimatise. HAI encompasses three conditions; acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema
(HAPE). This article will explore AMS in further detail.
What is this condition?
AMS is the most common form of HAI and can be observed in 25% of individuals sleeping at 2500-3000m who usually reside at low altitude. AMS should be considered in any individual who has recently ascended to high altitude presenting with a headache and systemic symptoms such as nausea, vomiting, disturbed sleep, fatigue and lightheadedness.
Before we jump into AMS, it is important to appreciate the normal physiological response to altitude; please see our acclimatisation article for further details. The underlying pathophysiology of AMS is not fully understood, but is thought to occur as a response to hypoxia and the subsequent effects on the central nervous system. For further interest, please see this great paper by Kumar and Choi (2015) 1 regarding the hypoxia inducible factor pathway.
There are two major risk factors for developing AMS, namely the speed of ascent and the absolute elevation the person is at. A higher elevation is considered to be any environment over 1500m, with the risk of developing AMS increasing with altitudes over 2500m. Being at elevation does not always result in AMS, especially if an individual has taken appropriate
preventative measures. Conversely, some individuals may develop symptoms of AMS at lower altitudes or despite taking preventative measures.
It is also worth considering that hypoxia may exacerbate pre-existing conditions, such as angina, hypertension, COPD and congestive heart failure. Appropriate screening in pre-trip questionnaires should explore past medical history thoroughly and indicate individuals who may require a closer level of medical supervision and lower threshold of intervention.
Interestingly, it has been suggested that physical fitness is not a protective factor against the development of HAI.
One of our faculty members, Dr Francis Screech, at the summit of Mount Kilimanjaro. Due to the severe ascent profile of the Machame route, AMS is a condition which an expedition medic must be familiar with and capable of managing when providing medical cover on this expedition.
Prevention of AMS is a key component which needs to be considered by all parties involved in high altitude travel and is the responsibility of everyone, including trip organisers, expedition medics and clients.
One component of preventing AMS is appropriate route planning with a gradual rate of ascent to allow for acclimatisation; this is termed the ascent profile. The altitude at which an individual sleeps is more important than the altitude climbed when awake. It is recommended that over 3000m, an individual should sleep no higher than 500m above the previous sleeping altitude, and a rest day should be incorporated every 3 to 4 days to allow for appropriate acclimatisation.
Pharmacological measures also have a role in prevention and should be considered for all moderate-high risk activities, as shown in table 1 2 . Prophylactic medications should be started the day before ascent and can be stopped after 2 days at the highest elevation or upon descent (without AMS symptoms).
The mainstay of prophylaxis is acetazolamide, which can help to speed up the process of acclimatisation (brand name: Diamox, dosing: 125mg BD or 250mg OD in adults). Acetazolamide is thought to work by inhibiting renal carbonic anhydrase causing a bicarbonate-wasting diuresis. This loss of base results in a relative metabolic acidosis, stimulating central chemoreceptors to respond and resulting in an increased minute ventilation which speeds acclimatisation. Side effects of acetazolamide include increased urinary frequency, paraesthesia, nausea and blurry vision. It is not recommended for use in pregnancy.
Individuals who have previous allergies to acetazolamide may consider using dexamethasone, a corticosteroid (2mg QDS or 4md BD). Caution must be exercised with prolonged usage and a weaning dose may need to be considered. Remember the use of steroids can lead to hyperglycaemia in individuals with diabetes. It is also important to note that the use of dexamethasone does not speed acclimatisation but only aids symptoms. If taken without the knowledge of leaders, there is the risk of symptom masking and delayed medical intervention.
|No history of altitude illness and ascending to ≤2800m||History of AMS and ascending to 2500-2800m in 1 day||History of AMS and ascending ≥ 2800m in 1 day|
|Taking ≥ 2 days with to ascent to 2500-300m with subsequent increases of <500m in sleep elevation and an extra day for acclimatisation every 1000m||No history of AMS and ascending to >2800m in 1 day||All individuals with a history of HACE or HAPE|
|Ascending with >500m increase in sleeping elevation at altitudes ≥3000, but with an extra day for acclimatisation every 1000m||All individuals ascending to ≥ 3500m in 1 day
|Ascending with >500m increase in sleeping elevation at altitudes ≥3000, but without extra days for acclimatisation every|
|Very rapid ascents to extreme altitude (eg. <7 day ascent of Mt Kilimanjaro)|
Table 1: shows the risk category for developing AMS. Table reproduced from Luks et al, (2019)2
One of the main presenting symptoms of AMS is a headache. This usually starts within a few hours of being at altitude, although can occur up to 24 hours later. Other symptoms include loss of appetite, nausea, vomiting, fatigue, flu-like symptoms, dizziness, and sleep disturbance. Periodic breathing, most notable during sleep, occurs as a compensatory
response to the hypoxia in an attempt to increase oxygen saturation by hyperventilation. If a blood gas analyser were available at altitude, a blood gas would be expected to show a respiratory alkalosis (see Grocott et al. 3 for further information).
An alternative diagnosis to consider is high altitude headache (HAH). This can occur as a symptom of AMS, but can occur in isolation, without the systemic symptoms described above. A HAH is usually described as mild-moderate intensity, dull in nature and exacerbated with exertion or head movement. As suggested by Carod-Artal 4 , a diagnosis of HAH may be considered at elevations of over 2500m and must not be attributable to other causes. Diagnosis can be confirmed by administration of supplemental oxygen, 2-4 litres per minute; symptoms of HAH should improve within 15 minutes. Management includes NSAIDs and paracetamol. Please see an excellent review of this condition by Carod-Artal 4 for further information.
The diagnosis of AMS is based upon clinical presentation. Symptoms beginning over 48 hours after remaining at the same altitude should prompt consideration of other conditions. AMS may be considered a harbinger of more serious HAI: red flags include breathlessness with a productive cough, which may indicate developing HAPE and neurological symptoms
such as severe headache, change in mental status and cerebellar ataxia, which should prompt consideration of development of HACE. If available, administration of supplemental oxygen of 2-4L per minute via nasal cannula should rapidly improve headache and other symptoms, therefore supporting a diagnosis of AMS.
Please note, the Lake Louise Score (LLS) was initially developed by researchers as a tool to quantify the severity of AMS. It is not recommended for use in clinical practice as a tool to diagnose or classify severity of AMS.
The following advice has been taken from the Wilderness Medical Society’s clinical practice guidelines (2019) 2 – see the link below for full information. Please note, doses given are for adults.
AMS is usually self-limiting, with symptoms resolving within 24-48 hours as acclimatisation occurs. Mild symptoms can be treated with conservative measures, such as avoidance of further ascent, hydration and limiting physical activity. Simple analgesia such as paracetamol or ibuprofen can be used for headaches and antiemetics can be used for
nausea or vomiting.
If moderate-severe symptoms are present, in addition to conservative measures, pharmacological treatments should be given. Conversely to preventative measures, dexamethasone is the mainstay of treatment, aiming to alleviate symptoms (dose: 8mg IM, IV or PO, then 4mg every 6 hours until symptoms resolve). Acetazolamide could also be considered for treatment, with the aim of speeding up acclimatisation (dose: 250mg BD). These medications can be used together, but hyperglycaemia may occur. Side effects of both medications include gastrointestinal symptoms and may overlap with the symptoms of AMS.
The definitive treatment for AMS is descent, but this is not always required unless symptoms are incapacitating or worsening despite conservative measures and pharmacological management. Symptoms are expected to improve with 300-1000m of decrease in altitude. If available, supplemental oxygen can be used for symptom control if descent isn’t feasible, or can be used to aid descent. Sats should be targeted to >90% with awareness given to the amount of supplemental oxygen available. Remember to never let anyone descend alone.
If untreated, AMS can progress to HAPE +/- HACE which can be fatal.
Differentials to consider
- High altitude headache
- Caffeine withdrawal
- Carbon monoxide poisoning
Take home messages
- Prevention is key through a combination of careful route planning and consideration
of pharmacological prophylaxis.
- Early recognition is vital, as is awareness of the development of more serious
conditions such as HACE or HAPE.
- If symptoms of AMS appear, an individual should not ascend to any higher altitude
until symptoms have resolved.
Are you interested in learning more about AMS and other high altitude conditions?
If so, why not check out our Altitude Medicine Course? Whilst you’re there, why don’t you take a look at our other courses too?
- Arterial Blood Gases and Oxygen Content in Climbers on Mount Everest by Grocott et al. (2009)
Access it here: https://www.nejm.org/doi/full/10.1056/nejmoa0801581
- Hypoxia-Inducible Factors and Cancer by Jun et al. (2017)
This is a fascinating paper which discusses a potential link between the cancer mortality rate in individuals living at higher altitude versus lower altitude.
Access it here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5607450/
- 1: Kumar, H., Choi, D-K., (2015). Hypoxia Inducible Factor Pathway. [online].
Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4600544/#:~:text=Hypoxia%20inducib
- 2: Luks, AM., Auerbach, PS., Freer, L., Grissom CK., Keyes, LE., McIntosh, SC.,
Rodway, GW., Schoene, RB., Zafren, K., Hackett, PH., (2019). Wilderness Medical
Society Practice Guidelines for the Prevention and Treatment of Acute Altitude
Illness: 2019 Update. [online]. Available at: https://www.wem.academy/wp-content/uploads/2020/01/wms-guidelines-2019-ams.pdf
- 3: Grocott, MP., Martin, DS., Levett, DZH., McMorrow, R., Windsor. J., Montgomery,
HE., (2009). Arterial Blood Gases and Oxygen Content in Climbers on Mount
Everest. [online]. Available at: https://www.nejm.org/doi/full/10.1056/nejmoa0801581
- 4: Carod- Artal, FJ., (2014). High Altitude Headache and Acute Mountain Sickness.
[online]. Available at: https://www.elsevier.es/en-revista-neurologia-english-edition–
- Peacock, AJ. (1998). Oxygen at high altitude. [online].
Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/
- Prine, TS., Thurman, J., Huebner, K., (2022). Acute Mountain Sickness. [online].
Available at: https://www.ncbi.nlm.nih.gov/books/NBK430716/