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Chronic mountain sickness (CMS) is a disease that can develop during extended time living at altitude. It is also known as 'Monge’s disease' , after its first description in 1925 by Carlos Monge^[1]. The acute form of mountain or altitude sickness is acute mountain sickness, and is experienced shortly after ascent to high altitude. In contrast, chronic mountain sickness may develop after many years of living at high altitude. In medicine, high altitude is defined as over 2500 metres, but most cases of CMS occur at over 3000 m.

CMS is characterised by polycythemia (increased hematocrit) and hypoxemia which both decrease on descent from altitude. CMS is believed to arise because of an excessive production of red blood cells, which increases the oxygen carrying capacity of the blood ^[2] but may cause increased blood viscosity and uneven blood flow through the lungs (V/Q mismatch). However, CMS is also considered an adaptation of pulmonary and heart disease to life under chronic hypoxia at altitude.^[3]

The most frequent symptoms and signs of CMS are headache, dizziness, tinnitus, breathlessness, palpitations, sleep disturbance, fatigue, anorexia, mental confusion, cyanosis, and dilation of veins.^[4]

Clinical diagnosis by laboratory indicators have ranges of: Hb > 200 g/L, Hct >65%, and arterial oxygen saturation (SaO2) <85% in both genders.^[5]

Treatment involves descent from altitude, where the symptoms will diminish and the hematocrit return to normal slowly. Acute treatment at altitude involves bleeding (phlebotomy), removal of circulating blood, to reduce the hematocrit; however this is not ideal for extended periods.

1. ^ Monge CC, Whittembury J. Chronic mountain sickness. Johns Hopkins Med J. 1976 Dec;139 SUPPL:87-9. PMID 1011412
2. ^  Online calculator explaining blood oxygen content
3. ^ Zubieta-Castillo G Sr, Zubieta-Calleja GR Jr, Zubieta-Calleja L. Chronic mountain sickness: the reaction of physical disorders to chronic hypoxia. J Physiol Pharmacol. 2006 Sep;57 Suppl 4:431-42.
4. ^ Wu TY. Chronic mountain sickness on the Qinghai-Tibetan plateau. Chin Med J (Engl). 2005 Jan 20;118(2):161-8. PMID 15667803
5. ^ Chinese Medical Association for High Altitude Medicine. Recommendation for the classification and diagnostic criteria of high altitude disease in China. Chin High Alt Med J (Chin) 1996;6:2-5.