Chronic Mountain Sickness (CMS), also known as Monge’s disease, is a state of failed acclimatization to the low-oxygen environment found at high elevations. This condition affects individuals who have lived at high altitudes, generally above 8,200 feet (2,500 meters), for many months or years. Unlike Acute Mountain Sickness (AMS), which occurs shortly after rapid ascent, CMS develops gradually, often over decades, in a small percentage of long-term residents. The syndrome is characterized by a maladaptive physiological response to chronic environmental hypoxia. It results from the body’s inability to maintain a proper balance between oxygen delivery and blood composition despite persistently low oxygen availability.
Physiological Drivers of Chronic Mountain Sickness
The underlying mechanism of CMS involves a dysfunctional attempt by the body to compensate for low oxygen content. The most prominent feature is excessive erythrocytosis (EE), the overproduction of red blood cells (RBCs). While a modest RBC increase is a normal acclimatization response, this response becomes exaggerated and pathological in CMS patients.
This excessive RBC concentration leads to a significant rise in blood viscosity, causing the blood to become unusually thick. Thicker blood flows less efficiently through small vessels, impairing circulation and causing poor blood flow to vital organs. The increased viscosity also raises the risk of complications, such as thrombosis and stroke.
The failure of the body’s ventilatory control also contributes significantly. Normally, low oxygen stimulates the hypoxic ventilatory response (HVR), increasing the rate and depth of breathing. However, in people with CMS, the HVR is often blunted or inadequate, meaning they do not breathe enough to correct the low blood oxygen (hypoxemia) driving the excessive RBC production.
This sustained hypoxemia creates a vicious cycle: low oxygen stimulates the bone marrow to produce more RBCs, which thickens the blood, further slowing circulation and exacerbating the oxygen deficiency. Genetic factors also play a role in susceptibility, with specific gene variations identified in populations like Andeans that may predispose individuals to this maladaptive response.
Recognizing the Clinical Signs
The symptoms of CMS stem directly from hypoxemia and hyperviscosity, leading to a progressive decline in health and functional capacity. Patients often report fatigue and a decreased ability to exercise or perform physical tasks. These complaints are frequently accompanied by persistent headaches and dizziness, signs of impaired blood flow and oxygen delivery to the brain.
Visible signs associated with the excessive red blood cell count include cyanosis, a bluish discoloration of the lips, skin, and nail beds due to high concentrations of deoxygenated hemoglobin. Cardiovascular effects are also common, manifesting as breathlessness and palpitations. The thick blood places a strain on the heart, often resulting in pulmonary hypertension (elevated blood pressure in the lung arteries).
Neurological and sleep disturbances reflect chronic oxygen deprivation affecting the central nervous system. Sufferers may experience insomnia, disrupted sleep patterns, or excessive daytime sleepiness. Other neurological complaints include tinnitus (ringing in the ears), paresthesia (a tingling sensation), and mental confusion or cognitive impairment.
Treatment Strategies and Interventions
The most effective intervention for Chronic Mountain Sickness is permanent descent to a lower altitude, ideally below 5,000 feet (1,500 meters). Relocation removes the stimulus for excessive red blood cell production, allowing the body’s physiology to normalize over weeks or months. However, descent is often impractical for native residents who have deep community and economic ties to their high-altitude homes.
For individuals who must remain at high altitude, medical management focuses on mitigating symptoms and addressing excessive erythrocytosis. A primary pharmacological treatment is the carbonic anhydrase inhibitor acetazolamide. This medication increases blood acidity, stimulating the respiratory drive and improving ventilation. The resulting increase in blood oxygen saturation blunts the pathological drive for excessive red blood cell production.
Another intervention for short-term relief is therapeutic phlebotomy, the periodic removal of a measured amount of blood. This procedure rapidly reduces the hematocrit level (the percentage of blood volume occupied by red blood cells) and temporarily lowers blood viscosity. While phlebotomy offers immediate symptomatic relief, it is not a sustainable long-term solution. It can lead to iron deficiency, which the body may attempt to correct by increasing red blood cell production again.
Additional therapies include long-term oxygen supplementation, which directly addresses hypoxemia, or other medications that improve ventilation or pulmonary circulation. These medical treatments are generally considered palliative, managing symptoms but not curing the disease. Sustained relief from CMS ultimately requires a reduction in the chronic hypoxic exposure that initiated the condition.
Diagnostic Criteria and Long-Term Prognosis
The diagnosis of Chronic Mountain Sickness is established using clinical assessment and laboratory findings, typically involving a standardized scoring system. The primary laboratory finding is excessive erythrocytosis, defined by elevated hemoglobin concentrations (usually 21 g/dL or higher for men and 19 g/dL or higher for women). This must be accompanied by a minimum score based on the presence and severity of clinical symptoms, such as headache, dizziness, cyanosis, and sleep disturbances.
The severity of the disease is assessed using these scoring systems, which differentiate CMS from the normal, adaptive acclimatization response. A diagnosis of CMS implies a state of maladaptation that carries significant health risks. The long-term outlook for individuals who remain at high altitude is guarded, as the disease is progressive and can be life-threatening.
The chronic strain on the heart and the heightened risk of blood clots from hyperviscosity increase the chances of fatal complications like heart failure and stroke. While medical management can control the disease and improve quality of life, it does not offer a cure. Sustained management and access to effective medical care remain necessary to mitigate the serious risks associated with this acquired condition.