To the Editor: The excellent report by Harding et al in the Diagnostic Dilemmas article in the Journal highlighted medications and conditions that may cause false positive results of biochemical tests for phaeochromocytoma.1 Another group of patients, those with obstructive sleep apnoea (OSA), may have raised urine noradrenaline levels in the absence of a phaeochromocytoma.

Of about 170 patients seen at a hypertension screening service at the Mater Adult Hospital between 1998 and 2000, six had elevated levels of urine noradrenaline and normetadrenaline up to twice the upper limit of the normal range on repeated testing. Five were obese and were proven to have significant OSA. All required at least three antihypertensive drugs for reasonable control of their blood pressure, and had normal suppression of catecholamines with clonidine.

A recent report described a series of five patients with OSA presenting as pseudophaeochromocytoma who had consistently elevated levels of noradrenaline on measurement of 24-hour urinary catecholamine levels; normetadrenaline levels were not measured.2 Noradrenaline levels became normal in all five patients after treatment with continuous positive airway pressure, and blood pressure levels improved significantly. Excess urinary noradrenaline, rather than being adrenal in origin, was thought most likely to be due to increased neuronal release of noradrenaline from small arteries and arterioles as a result of sympathetic nerve activity and synaptic overflow.

In conclusion, OSA is an important reversible cause of elevated urine noradrenaline and normetadrenaline levels in patients with resistant hypertension.

In reply: We thank Morton for his letter which highlights another group of patients in whom raised urinary noradrenaline levels exist in the absence of a phaeochromocytoma. His experience and our group of patients1 should serve as a note of caution when making the diagnosis of phaeochromocytoma. A combination of positive results of biochemical tests, along with results of anatomical and functional imaging, should serve to minimise false positive diagnoses.