Understanding Altitude Sickness

Altitude sickness is a collective term for the range of physiological responses that occur when the human body is exposed to reduced atmospheric pressure and lower oxygen availability at high altitude. In Nepal, virtually all trekking routes above 3,000 m carry some altitude sickness risk, and the routes that spend extended time above 4,000 m (EBC, Annapurna Circuit, Three Passes, Manaslu Circuit) require specific altitude management strategies to ensure safe completion.

Altitude sickness is democratic - it does not preferentially affect older trekkers, less fit trekkers, or those without prior Himalayan experience. Olympic athletes have been evacuated from Everest Base Camp for HAPE; sedentary walkers with no prior trekking experience have completed the EBC route without incident. The primary predictor of altitude sickness is the rate of ascent - how quickly the body is moved to higher altitude without adequate time for physiological adaptation. A 60-year-old who ascends slowly and takes rest days will typically acclimatise better than a 25-year-old who pushes the itinerary to save time.

The three main altitude syndromes - AMS (Acute Mountain Sickness), HACE (High Altitude Cerebral Edema), and HAPE (High Altitude Pulmonary Edema) - exist on a spectrum of severity. AMS is common and usually benign; HACE and HAPE are medical emergencies with significant mortality risk if descent is delayed.

Acute Mountain Sickness (AMS)

AMS is the most common altitude syndrome - it affects approximately 25 - 40% of trekkers at some point above 3,000 m. The Lake Louise criteria define AMS as: headache plus one of the following symptoms occurring within 6 - 12 hours of altitude gain: nausea or vomiting, fatigue or weakness, dizziness or lightheadedness, difficulty sleeping.

Recognising AMS on the trail:

Headache: The cardinal symptom - frontal or occipital (back of head) headache that persists despite ibuprofen or paracetamol
Nausea and poor appetite: Common above 4,000 m; teahouse food that was appealing at lower altitude becomes unappetising
Disturbed sleep: The characteristic altitude sleep pattern of periodic breathing (Cheyne-Stokes respiration) - cycles of deeper breathing alternating with shallow breathing or brief apnoea - causes repeated waking and fatigue
Fatigue disproportionate to effort: Normal tiredness after a day's walking is expected; fatigue that persists through rest periods suggests AMS

Management of mild AMS: Stop ascending. Rest at the current altitude. Hydrate well (dehydration worsens AMS symptoms). Treat headache with ibuprofen (600 mg) or paracetamol (1,000 mg). Most mild AMS resolves within 12 - 24 hours at stable altitude. If symptoms do not improve after 24 hours of rest at the same altitude, descend 300 - 500 m.

The critical rule: never ascend with AMS symptoms. Proceeding to higher altitude with AMS is the most common path to HACE or HAPE.

HACE - High Altitude Cerebral Edema

HACE (High Altitude Cerebral Edema) is severe AMS - the stage where fluid accumulation in the brain causes neurological symptoms beyond headache. HACE is a medical emergency requiring immediate descent.

Symptoms indicating HACE (any of these warrant emergency descent):

Ataxia (loss of coordination): The "tandem walking test" - inability to walk heel-to-toe in a straight line is a reliable field indicator of HACE. Test suspected HACE by asking the person to walk a straight line placing each foot directly in front of the other.
Altered mental status: Confusion, disorientation, unusual behaviour, lethargy, inability to perform simple tasks
Severe headache unresponsive to analgesia
Vomiting that prevents oral hydration

Treatment: Descend immediately - even 300 - 500 m descent often produces rapid improvement. Dexamethasone 8 mg immediately (then 4 mg every 6 hours) buys time for descent organisation but is not a substitute for descent. A Gamow bag (portable hyperbaric bag) can simulate descent if available. Helicopter evacuation should be called simultaneously with initiating descent - do not wait for the helicopter before descending.

Mortality from untreated HACE: 10 - 25%. With prompt descent and dexamethasone, most patients recover fully if HACE is caught before coma develops.

HAPE - High Altitude Pulmonary Edema

HAPE (High Altitude Pulmonary Edema) is the most common cause of death from altitude illness - fluid accumulation in the lungs causes progressive respiratory failure. HAPE typically develops at night (the physiological changes that worsen HAPE are more pronounced during sleep) and can progress from moderate symptoms to death within hours in susceptible individuals.

Symptoms of HAPE (medical emergency - immediate descent):

Dry cough progressing to productive cough with frothy or pink-tinged sputum: The cough develops as fluid accumulates in the alveoli
Shortness of breath at rest: Breathlessness that persists even when sitting still - not the normal breathlessness of exertion at altitude
Crackling or gurgling sound when breathing: Audible with a stethoscope or sometimes without - the "wet crackling" sound of fluid in the lungs
Low oxygen saturation (SpO2): Pulse oximetry showing SpO2 below 75% at altitude consistent with severe HAPE
Cyanosis: Blue discolouration of lips or fingernails indicating severe oxygen depletion

Treatment: Descend immediately - even 300 - 500 m can produce rapid improvement. Nifedipine 30 mg slow-release tablet reduces pulmonary artery pressure and can stabilise HAPE sufficiently for evacuation. Supplemental oxygen if available. Gamow bag if descent is impossible. Call helicopter simultaneously with initiating descent.

HAPE risk factors: prior history of HAPE (most important - individuals who have had HAPE once are at significantly elevated risk on subsequent exposures), rapid ascent, heavy exertion at altitude, certain genetic predispositions. HAPE can occur without AMS symptoms preceding it - a person who feels generally well can develop HAPE overnight without classic AMS warning signs.

Prevention Strategies

The most effective altitude sickness prevention is the correct ascent profile:

Above 3,000 m, ascend no more than 300 - 500 m per day (sleeping altitude): The key variable is sleeping altitude, not the altitude reached during the day. "Climb high, sleep low" - ascending to a higher point during the day and returning to a lower camp to sleep provides altitude stimulus without the overnight physiological burden.
Rest days every 3 days above 3,000 m: The built-in rest days at Namche (3,440 m) and Dingboche (4,410 m) on the EBC route are not optional - they are physiologically essential acclimatisation stops.
Hydration: At altitude, the dry cold air and increased respiratory rate cause rapid fluid loss. Drink 3 - 4 litres of water per day above 3,500 m. Urine should be pale yellow; dark urine indicates dehydration. Avoid alcohol in the first 2 - 3 days at any new altitude (alcohol suppresses respiration at night, worsening oxygen saturation).
Avoid unnecessary exertion on the first day at a new altitude: Arriving at a camp and immediately doing a strenuous day hike does not enhance acclimatisation - it puts additional metabolic oxygen demand on a body already stressed by altitude gain.
Pulse oximetry monitoring: A pulse oximeter (NPR 800 - 1,500 to buy in Kathmandu; USD 30 - 60 from travel medicine suppliers) allows objective tracking of SpO2. At sea level SpO2 should be 95 - 100%; at Namche (3,440 m) 88 - 95% is typical; at Gorak Shep (5,164 m) 75 - 85% is expected. Readings significantly below these ranges, or declining readings at stable altitude, are warning signs.

Diamox (Acetazolamide)

Acetazolamide (brand name Diamox) is the only medication with good evidence for both prevention and treatment of AMS. It works by inducing a mild metabolic acidosis that stimulates increased breathing - accelerating the respiratory acclimatisation response and improving nighttime oxygen saturation.

Standard prophylactic use: 125 - 250 mg twice daily, starting 1 - 2 days before ascent and continuing until 2 days after reaching maximum altitude (or until descent begins). The 125 mg dose has good evidence for AMS prevention with fewer side effects than the historically recommended 250 mg dose.

Common side effects: tingling in fingers, toes, and around the mouth (benign, extremely common - nearly universal), increased urination (the drug is a diuretic), occasionally blurred vision. Carbonated drinks taste flat on Diamox (also benign). These are normal pharmacological effects, not allergy signs.

Contraindications: sulfa drug allergy (Diamox is a sulfonamide - avoid if you have reacted to sulfa antibiotics). Pregnancy and kidney disease are also contraindications. Consult a physician or travel medicine specialist before departure to confirm Diamox is appropriate and obtain a prescription.

Diamox is not a "magic bullet" - it reduces AMS risk significantly but does not eliminate it. Trekkers on Diamox still need to follow proper ascent profiles and should not use the medication as justification for rushing the acclimatisation schedule.

HRA Clinics in Nepal

The Himalayan Rescue Association (HRA) operates two clinics on the EBC route staffed by volunteer physicians (typically 2 - 3 physicians per season):

Pheriche Clinic (4,371 m): Open October - November and April - May; daily altitude education talks at 15:00; consults available; emergency oxygen available
Manang Clinic (3,519 m), Annapurna Circuit: Seasonal operation; daily altitude talks; the Manang talk is a standard stop for Annapurna Circuit trekkers before the Thorong La crossing

The HRA clinics are the best resource for altitude illness assessment on the major Khumbu and Annapurna routes. They see hundreds of altitude illness patients each season and their medical staff have direct, current experience with the altitude conditions on the specific route. If you are concerned about symptoms, go to the HRA clinic rather than asking fellow trekkers or teahouse staff.

The HRA altitude education talks are free and strongly recommended for all trekkers - they cover recognition, prevention, and treatment of AMS/HACE/HAPE in the specific context of the Khumbu route and are delivered by the physicians currently working at the clinic.

Emergency Evacuation

For HACE and HAPE, helicopter evacuation is the gold standard when descent on foot is insufficient or impossible due to distance from lower altitude. Nepal helicopter rescue operators (H2O, Fishtail Air, Simrik Airlines) can reach most Khumbu and Annapurna locations within 1 - 3 hours of a call during daylight hours in good weather.

Evacuation costs: USD 3,000 - 8,000 for a single rescue mission. Travel insurance covering helicopter evacuation to a minimum altitude of 6,000 m is essential for all Nepal trekking. Without insurance, the patient's party must provide advance payment or guarantee before some operators will dispatch - an impossible situation in an emergency. Ensure your insurance policy specifically includes high-altitude trekking (many standard policies exclude altitudes above 4,000 m).

Evacuations in bad weather cannot happen - helicopter operations require visual flight rules (VFR) conditions. A HACE or HAPE patient must be descended on foot if weather grounds helicopters. This reality reinforces the importance of early recognition and early descent - waiting for a helicopter that cannot fly due to cloud or storms is a life-threatening delay.

Frequently Asked Questions

What altitude does altitude sickness start in Nepal?: AMS can begin at altitudes as low as 2,500 m in susceptible individuals, but serious symptoms are rare below 3,000 m. The practical risk zone for Nepal trekkers begins at Namche Bazaar (3,440 m) and increases significantly above 4,000 m. Most AMS cases on the EBC route occur between Namche and Dingboche (3,440 - 4,410 m) when trekkers are ascending too quickly; HACE and HAPE most commonly develop above 4,500 m.
Does fitness protect against altitude sickness?: No - physical fitness provides no protection against altitude sickness. AMS, HACE, and HAPE are caused by the rate of ascent and individual physiological susceptibility, not fitness level. Olympic athletes have been evacuated for HAPE from high Himalayan expeditions. Fit trekkers may be able to push harder physically at altitude, which can paradoxically increase their risk by causing more rapid ascent. The acclimatisation schedule should be followed regardless of fitness level.
Can I predict whether I will get altitude sickness?: Prior history is the best predictor: if you have previously had AMS above a specific altitude without HACE or HAPE, you are likely to have similar symptoms at similar altitudes on future trips. Prior HAPE is the strongest predictor of future HAPE - a history of HAPE significantly elevates the risk on subsequent high-altitude exposure. A previous trip without AMS reduces (but does not eliminate) future AMS risk. There is no pre-departure test that reliably predicts altitude susceptibility.
Should I take Diamox preventatively for the EBC trek?: Diamox use is a personal medical decision that should be made in consultation with a physician or travel medicine specialist before departure. The evidence for Diamox as AMS prophylaxis is strong (it reduces AMS incidence by approximately 50% in ascent to Himalayan altitudes). Many EBC trekkers use it; many do not. The standard recommendation: if you have a history of AMS, are under time pressure for acclimatisation, or have a physician recommendation, use Diamox. If you have adequate time for acclimatisation and no prior AMS history, the choice is more ambiguous.
What is the "golden rule" of altitude trekking?: The golden rule of altitude trekking is: never ascend with AMS symptoms. If you have a persistent headache, nausea, or loss of coordination at altitude, do not continue ascending regardless of schedule pressure, peer pressure, or sunk-cost thinking about the trek investment. If symptoms do not resolve with 24 hours of rest at the same altitude, descend. This single rule, followed consistently, prevents most cases of HACE and HAPE.
Can I fly directly from Kathmandu to high altitude without acclimatising?: Flying from Kathmandu (1,400 m) to Lukla (2,860 m) is a common and generally safe altitude gain - Lukla's altitude does not require special acclimatisation for most trekkers. However, flying Kathmandu to Jomsom (2,720 m) or to Phaplu (2,413 m) also carries low risk. The problem arises when trekkers fly to altitude and immediately ascend further without acclimatisation - flying to Lukla and running to Namche in one day is the most common itinerary error that leads to AMS at Namche. Always plan at least one night at the flight-arrival altitude before ascending.
What is a Gamow bag and are they available on Nepal treks?: A Gamow bag (hyperbaric bag) is an inflatable pressure chamber that simulates descent of 1,500 - 2,500 m by increasing the air pressure inside the bag. Pumping the bag with a foot pump increases internal pressure to simulate lower altitude oxygen availability, providing temporary relief for HACE and HAPE while descent is being organised. Gamow bags are available at some HRA clinics and large teahouses at high altitude, but they are not ubiquitous on the trail. They are a temporising measure, not a treatment - the patient must still be descended.
How do I know if my headache is altitude-related or from dehydration?: Dehydration headaches are very common at altitude (the dry cold air and increased breathing rate cause rapid fluid loss) and can mimic AMS headache. The practical approach: drink 500 ml of water, take ibuprofen (600 mg), and assess in 30 - 60 minutes. If the headache responds promptly to hydration and analgesia and does not recur, it is likely dehydration-related. A persistent headache that does not respond to hydration and analgesia, or that is accompanied by nausea, should be treated as AMS and ascent should stop.
What should I do if someone in my group shows HACE symptoms?: Act immediately - do not wait until morning or until the helicopter arrives before starting descent. Wake everyone in the group, get the patient dressed and moving downhill immediately, administer dexamethasone 8 mg if available, and simultaneously call for helicopter evacuation. The principle is to descend on foot while simultaneously calling for aerial evacuation - the helicopter is a backup, not the primary treatment. Even 200 - 300 m of descent typically produces improvement in HACE symptoms. Waiting at the same altitude for a helicopter that may be delayed by weather can be fatal.
What pulse oximeter readings should concern me at EBC altitude?: SpO2 reference ranges at EBC altitudes: at Namche (3,440 m) expect 88 - 95%; at Dingboche (4,410 m) expect 82 - 92%; at Gorak Shep (5,164 m) expect 72 - 85%; at Kala Patthar (5,545 m) expect 65 - 80%. Readings significantly below these ranges - particularly below 70% at Gorak Shep or readings that are declining rather than stable over rest periods - warrant immediate medical assessment. Also note: SpO2 is lower at night during sleep and higher after controlled breathing exercises; single-point readings are less informative than trends over time.