Ambulatory blood pressure monitoring in secondary hypertension

The term 'secondary hypertension' includes a variety of aetiological processes with no obvious common feature apart, perhaps, from the loss of normal blood pressure regulatory factors. It has been suggested that all forms of secondary hypertension, including renal/renovascular, accelerated phase, glucocorticoid and mineralocorticoid excess, phaeochromocytoma and toxaemia in pregnancy are associated with the loss of the normal nocturnal fall in blood pressure. The evidence for this view appears strongest for glucocorticoid-induced hypertension, but for most other forms the available data are conflicting and are mostly based on small numbers of patients without adequate control populations. Sleep is a powerful determinant of the nocturnal fall in blood pressure but few studies have addressed the quality of sleep in groups of patients who are often sick and/or hospitalized. Abnormalities in sympathetic drive (e.g. autonomic neuropathy and phaeochromocytoma) can abolish the nocturnal blood pressure reductions, and activation of the sympathetic nervous system may be a critical factor in determining the normal diurnal changes in blood pressure. As approximately 20% of patients with 'essential hypertension' have a blunted fall in nocturnal blood pressure it seems unlikely that ambulatory blood pressure could ever be useful in screening for secondary hypertension.