Intermittent hypoxia in patients with unexplained polycythaemia.

The aetiology of polycythaemia is unclear in up to 30% of patients. Twenty patients with unexplained polycythaemia were investigated to see whether they had an intermittent hypoxic stimulus to erythropoiesis that was undetected by conventional investigations for hypoxic secondary polycythaemia. Overnight polygraphic sleep studies showed that five patients had prolonged nocturnal hypoxaemia. Their arterial oxygen saturation was below 92%, the level at which appreciable hypoxic stimulation of erythropoiesis occurs, for 26-68% of the time for which they were studied. Considerable evidence is accumulating that intermittent hypoxia is a potent stimulus to erythropoiesis, and clinicians should consider the possibility of nocturnal hypoxia in patients with unexplained polycythaemia. Appropriate investigation will lead to the correct diagnosis of polycythaemia secondary to hypoxia in some cases previously regarded as idiopathic, and treatment may then be planned accordingly.