History and exam
Key diagnostic factors
common
presence of risk factors
Key risk factors include high altitude, rapid ascent, resident at low altitude and recurrence due to individual susceptibility.[31]
headache
ataxia
Common in high-altitude cerebral edema (HACE).[48]
Identified by observing the individual walk heel-to-toe for a short distance and complete a 180-degree turn.
In severe cases ataxia may prevent individuals from standing up or even sitting upright.
Has also been found to be present in up to 60% of patients with high-altitude pulmonary edema (HAPE).[4]
change in mental state: for example, tired, irritable, confused, forgetful, irrational
In the early stages of HACE, signs can often be subtle and are therefore easily missed.
At first, HACE patients may appear tired, irritable, confused, forgetful, or prone to bouts of irrational behaviour. Completing simple tasks may be problematic: tying shoe laces, using cutlery, or writing a diary may be a lengthy process. On questioning, patients may be unable to recall the time, day, date, or location. Conventional tests that assess basic arithmetic (subtracting 7 from 100) and memory (being given a name, age, and address and asked to recall at a later stage of the examination) are often useful in identifying the early stages of HACE.
A mental state assessment should be normal in AMS.
abnormal tone, power, and reflexes
Abnormalities in tone and power can also occur in patients with high-altitude cerebral edema.
There may be neck stiffness.
Reflexes are usually brisk and clonus is sometimes present. Extensor plantar reflexes are common.[4]
Other diagnostic factors
common
nausea, vomiting, and loss of appetite
Nausea, vomiting, and loss of appetite are most common, while abdominal pain and change in bowel habit tends not to occur.
fatigue and weakness
Inability to perform simple tasks; reduced exercise capacity.
dizziness or lightheadedness
Early symptom of acute mountain sickness (AMS).
difficulty sleeping
On ascending to altitude, unacclimatised lowlanders typically complain that they take longer to get to sleep, wake frequently, often have unpleasant dreams, and do not feel refreshed in the morning.[63]
visual disturbance
Increasingly common as the condition progresses.
shortness of breath
cough with or without sputum
In the early stages of HAPE the cough is often dry before becoming loose and productive as the patient deteriorates.
Sputum is usually frothy and either white or pink, turning to frank blood in only the most severe of cases.
Most sufferers prefer to lie with their head elevated as the cough is often worse when lying flat. Occurs in 52% to 70% of those with HAPE.[4][64]
rales
Audible in those with AMS, HAPE, and HACE.
In HAPE, they are usually audible in both lung fields and tend to be concentrated in mid and lower zones.[55]
peripheral oedema
This tends to occur in the periorbital area after sleep and around the ankles and wrists following exertion.[8]
accentuated pulmonary second sound
In patients with HAPE an accentuated pulmonary second sound may be heard, indicating the presence of pulmonary hypertension.[55]
pyrexia
elevated respiratory rate
Rarely, HAPE can present without any evidence of shortness of breath. However, an elevated resting respiratory rate is often a very useful early indication of the condition. In severe cases respiratory rate can exceed 40 breaths per minute at rest, making even the mildest exertion impossible.[53]
elevated heart rate
Resting heart rate (HR) can be increased in high-altitude illness. In severe cases of high-altitude pulmonary edema, resting HR can exceed 140 beats per minute. However, in most cases HR is much lower, varying between 90 and 120 beats per minute.[53]
low arterial oxygen saturation
It is essential that arterial oxygen saturation (SaO2) measurements of healthy well-acclimatised individuals are used as a benchmark when interpreting these results; SaO2 measurements are often lower than expected in even the healthiest of individuals at altitude.
While a single resting SaO2 recording may be of little use in the diagnosis, a series of measurements showing a large difference between the patient and those who share the same ascent profile may help identify individuals who are acclimatising poorly and are therefore prone to acute mountain sickness.[50][51][52]
In HAPE, SaO2 measurements are low. At 4559 m, HAPE patients have been shown to have a mean SaO2 of 48% compared with 78% in healthy individuals.[56]
cyanosis
In the majority of cases of HAPE there is clear evidence of cyanosis. This tends to affect extremities such as the fingers, toes, and facial features.[54]
urinary incontinence or retention
Upper motor neuron sign in patients with HACE.[4]
retinal haemorrhages and papilloedema on fundoscopy
Common finding in HACE.[55]
uncommon
chest pain
cranial nerve palsies (III, IV, and VI)
Rarely, cranial nerve palsies are present in patients with HACE and tend to involve those nerves controlling eye movement (III, IV, and VI).[4]
visual and auditory hallucinations, seizures, tinnitus, vertigo, tremors, speech disturbance, and deafness
Have all been reported in HACE. However, these features are rare.[4]
Risk factors
strong
high altitude
As height is gained, the partial pressure of inspired oxygen (PIO2) falls. In those arriving at alpine huts in Western Europe, the incidence of acute mountain sickness (AMS) has been found to be 9% at 2850 m, 13% at 3050 m, and 34% at 3650 m.[8] In the Himalayas, the incidence of AMS among 150 trekkers ranged between 10% (3000 to 4000 m), 15% (4000 to 4500 m), and 51% (4500 to 5000 m).[9] The incidence of high-altitude pulmonary edema (HAPE) follows a similar pattern with an incidence of 0.1% at 3063 m rising to 5.3% at 4486 m.[4]
rapid ascent
A gradual ascent to altitude allows the body time to acclimatise.[1] During a trek from Lukla (2900 m) to Everest base camp (5400 m) those taking 6 days were 26% less likely to develop AMS than those taking just 4 days.[26] Ascending to an altitude of 3500 m over the course of 4 days reduces the incidence and severity of AMS by 41% when compared with those taking a 1-hour flight to the same altitude.[11] The incidence of HAPE in Indian soldiers transported by air to 3500 m or higher has been shown to be 5.7%; however, if this journey is completed by road the incidence falls to only 0.3%.[27]
Research shows that periods of hypoxic exposure or 'pre-acclimatisation' prior to ascending to altitude may be able to reduce the incidence of AMS. However, the duration and degree of hypoxic exposure are not yet clear.[28][29]
low-altitude residence
Residents at low altitude have been found to be at least 3 times more likely to suffer from AMS (27% versus 8%) following a rapid ascent to 3000 m than those who live permanently above 900 m. This incidence can be reduced by a 1-week stay at 1600 m, and eliminated completely by spending at least 2 months at 1800 m before ascending to 3000 m.[11]
Alternatively, frequent exposures to altitude combined with a slow ascent can lead to a 5-fold reduction in AMS.[5][30]
By allowing the body long periods of time to acclimatise, it is possible to dramatically reduce the risk of high-altitude illness.[29]
history of previous altitude illness
Although the cause of this is unclear, high-altitude illness tends to recur. A study of mountaineers climbing to 4559 m showed that those with a history of AMS were more likely to develop the condition than those without. The difference in AMS incidence was most noticeable in those who took <3 days to climb from 2000 m to 4559 m (59% versus 32%).[17][31]
weak
younger age
A limited number of studies appear to suggest that advanced age is protective against AMS. Of visitors to resorts in Colorado, 16% of those aged older than 60 developed AMS compared with 25% of the general population.[5]
Unfortunately, conflicting data are also available and so the connection with age is unclear.[32]
Although age may have an intrinsic advantage at altitude, older climbers may simply ascend more slowly, avoid excessive amounts of exercise, or attribute their symptoms to other comorbidities.[33]
exertion
Working at altitude (e.g., mining) has been associated with altitude sickness.[34] Traditionally, new arrivals to high-altitude communities have been encouraged to rest. The benefits of this have only been confirmed in a single small hypobaric chamber study where 7 participants spent a total of 20 hours at an equivalent altitude of 4572 m over 2 different days. On the first day, the participants undertook 2 hours of moderate exercise, while the next day was spent resting. The incidence and severity of AMS was considerably lower following rest compared with the exercising session.[35]
poor awareness of high-altitude illness prior to travel
A study published in 2004 demonstrated that trekkers travelling to Nepal had a greater awareness of high-altitude illness when compared with an earlier group assessed 12 years earlier (95% versus 80%). This resulted in a slower ascent to altitude, better use of medicines, and a fall in the incidence of AMS from 45% to 29% above 4000 m.[36]
existing medical condition
Individuals with specific medical conditions meeting one or more particular criteria face an elevated risk of encountering issues at high altitude.[6] These criteria include factors such as the individual’s susceptibility to hypoxemia at high altitudes, for instance if they suffer from a lung disease of considerable severity like chronic obstructive pulmonary disease. Their vulnerability to impaired ventilatory responses and potential complications arising from pulmonary vascular responses to hypoxia, such as in pulmonary hypertension, must also be considered. Hypoxia might pose a risk of complications due to the underlying medical condition, for instance rapid deterioration in those with sickle cell disease. These individuals may develop high-altitude illness at lower elevations.[6]
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