Chronic mountain sickness

Lore & Background

Chronic mountain sickness was first described in 1925 by Carlos Monge Medrano, a Peruvian doctor specializing in diseases of high altitude. Unlike acute mountain sickness, which occurs shortly after ascent, CMS may develop only after many years of living at high altitude. The disease is believed to arise from excessive production of red blood cells due to low oxygen levels, increasing blood viscosity and causing uneven blood flow through the lungs (V/Q mismatch). It has recently been correlated with increased expression of the genes ANP32D and SENP1.

Symptoms include headache, dizziness, tinnitus, breathlessness, palpitations, sleep disturbance, fatigue, loss of appetite, confusion, cyanosis, and dilation of veins. Over time, raised blood pressure in the lungs (pulmonary hypertension) can develop and may progress to heart failure (cor pulmonale). CMS is also considered an adaptation of pulmonary and heart disease to chronic hypoxia at altitude.

Treatment includes migration to low altitude, which is curative though not immediate; bloodletting (phlebotomy) combined with fluid replacement; medication with acetazolamide; and oxygen therapy with slow breathing techniques. A 2013 study found the highest CMS prevalence in Andean countries of South America and the lowest in native populations of the East African Mountains of Ethiopia.

Reader's Guide

Chronic mountain sickness represents a significant medical condition in high-altitude populations, with its first description in 1925 by Carlos Monge Medrano establishing it as a distinct disease. The article emphasizes that CMS is most common among native populations of high-altitude nations, with prevalence varying globally—highest in Andean South America and lowest in Ethiopian highlands. Its diagnostic criteria are clearly defined by laboratory values: hemoglobin ≥21 g/dL in males and ≥19 g/dL in females, hematocrit >65%, and arterial oxygen saturation <85%. The disease's legacy includes its treatment approaches: migration to low altitude is curative, while phlebotomy, acetazolamide, and oxygen therapy provide symptomatic relief. The recent correlation with genes ANP32D and SENP1 suggests a genetic component. CMS is notable for its distinction from acute mountain sickness, developing over years rather than hours, and for its characterization as both a maladaptive response and an adaptation to chronic hypoxia. The article does not provide mortality data or long-term outcomes beyond noting potential progression to pulmonary hypertension and cor pulmonale.

Did You Know?

• Chronic mountain sickness was first described in 1925 by Peruvian doctor Carlos Monge Medrano.
• Most cases of CMS occur at altitudes over 3,000 metres (9,800 ft).
• Diagnosis requires hemoglobin ≥21 g/dL in males or ≥19 g/dL in females, with hematocrit >65%.

Discovery & Historical Context

Chronic mountain sickness entered the medical record in 1925, when Carlos Monge Medrano, a Peruvian physician whose practice centred on ailments linked to high-altitude living, formally described the condition. His work drew a crucial distinction that remains central to the field: while acute mountain sickness strikes within days or weeks of a rapid ascent, the chronic form insidiously develops only after years—sometimes decades—of continuous residence at elevation. The threshold for 'high altitude' in medical terminology sits above 2,500 metres, yet the vast majority of CMS cases cluster at elevations exceeding 3,000 metres, where the partial pressure of oxygen is low enough to trigger prolonged physiological stress. The condition is not evenly distributed across the globe; it predominates among indigenous and long-settled populations of high-altitude nations, particularly in the Andes and other mountainous regions of South America. Monge Medrano's observation that the disease was a slow, cumulative response rather than an acute reaction reshaped how clinicians think about altitude-related illness and laid the groundwork for the diagnostic and therapeutic frameworks that followed.

Pathophysiology & Mechanism

At its core, chronic mountain sickness is a disorder of oxygen balance. Prolonged exposure to the thin air of high elevations prompts the body to overproduce erythrocytes in an attempt to compensate for the reduced oxygen available in the atmosphere. This overcompensation drives haematocrit to dangerous levels, thickening the blood and disrupting the even distribution of flow through pulmonary capillaries—a phenomenon known as ventilation-perfusion mismatch. Over time, the increased viscosity and altered haemodynamics place sustained strain on the pulmonary vasculature, raising blood pressure within the lungs. In the most severe trajectories, this pulmonary hypertension progresses to right-sided heart failure, a condition termed cor pulmonale. Researchers also view CMS as a maladaptive form of acclimatisation: the cardiovascular and pulmonary systems attempt to adapt to chronic hypoxia but overshoot, creating a self-reinforcing cycle of excess red-cell production and impaired gas exchange. More recently, molecular studies have linked the condition to heightened expression of the genes ANP32D and SENP1, hinting at a genetic component that may explain why some individuals at the same altitude develop the disease while others do not.

Diagnosis & Clinical Presentation

Patients with chronic mountain sickness typically present with a constellation of symptoms that can be mistaken for a range of other conditions. The most frequently reported complaints include persistent headache, dizziness, ringing in the ears, shortness of breath, a racing or pounding heartbeat, disrupted sleep, profound fatigue, diminished appetite, episodes of confusion, a bluish discolouration of the skin and mucous membranes, and visibly enlarged veins. Because these signs overlap with many other disorders, definitive diagnosis relies on laboratory thresholds established by clinical consensus. For males, a haemoglobin concentration of 21 g/dL or above is considered diagnostic; for females, the cutoff is 19 g/dL. Both sexes must also show a haematocrit exceeding 65 percent and an arterial oxygen saturation below 85 percent. These values, taken together, confirm the hallmark combination of polycythaemia and hypoxaemia that defines the disease. Importantly, the condition is most prevalent among those whose lifelong residence places them above 3,000 metres, though the formal altitude threshold for 'high altitude' in medicine begins at 2,500 metres.

Treatment & Global Distribution

Management of chronic mountain sickness ranges from the most definitive intervention to pharmacological and supportive measures. Relocating to a lower elevation remains the only true cure; once the body is exposed to the denser, oxygen-rich air near sea level, haematocrit gradually normalises, though the recovery is not instantaneous. For patients who cannot or choose not to relocate, therapeutic phlebotomy can temporarily reduce red-cell mass, and its effect is prolonged when combined with intravenous fluid replacement to maintain circulating volume. The carbonic anhydrase inhibitor acetazolamide has demonstrated efficacy by suppressing erythropoietin-driven red-cell production, thereby improving arterial oxygenation and lowering heart rate. Supplemental oxygen therapy paired with coached slow-breathing exercises offers additional symptomatic relief. Geographically, a 2013 global review found that CMS prevalence is highest among Andean populations in South America and lowest among communities native to the Ethiopian highlands, underscoring that altitude alone does not determine risk—genetic and environmental factors clearly modulate susceptibility.