Secondary polycythaemia associated with nocturnal apnoea--a relationship not mediated by erythropoietin?

A polysomnographic study was undertaken in nine patients with unexplained polycythaemia and nine age- and sex-matched controls. Circulating plasma levels of immunoreactive erythropoietin (IrEPO) were analysed before and after sleep. Seven out of nine polycythaemia patients were found to have sleep-disordered breathing and fulfilled the criteria for the sleep apnoea syndrome. Erythrocyte volume fraction was by definition higher among polycythaemia patients, and showed a weak positive correlation with minimum oxygen saturation during sleep (P less than 0.05). However, plasma IrEPO did not differ between the two groups or between morning and evening samples within the respective groups. In a separate study, IrEPO was repeatedly analysed during sleep in a group of six patients with severe obstructive sleep apnoea and six matched controls. No correlation with severity of sleep-disordered breathing was found. None of these patients had polycythaemia, and there was no obvious diurnal variation in IrEPO levels. A nocturnal sleep study may be warranted in patients with unexplained polycythaemia. Obstructive sleep apnoea does not appear to be related to increased IrEPO levels, although polycythaemia has been reported as a relatively common finding in this disease.