What Is Masked Hypertension?

THE JOURNAL OF CLINICAL HYPERTENSION 171 The gradual acceptance of ambulatory blood pressure monitoring (ABPM) as an improved method for classifying the blood pressure status of patients with suspected hypertension has been driven by the publication of a series of studies that have shown that ABPM gives a better prediction of risk than conventional clinic measurements. The official recognition by the Centers for Medicare and Medicaid Services (CMS), the federal body that approves new technologies, that ABPM is clinically useful and reimbursable for the diagnosis of white coat hypertension means that we now have two independent techniques for measuring blood pressure in clinical practice—conventional clinic measurement and ABPM (home monitoring is not yet officially approved for any reimbursement, although its use has been endorsed by the sixth report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure [JNC VI] and other august bodies). This means that we can define hypertension independently by each of the two methods. We thus have four potential groups of patients who are: 1) normotensive by both methods (true normotensives); 2) hypertensive by both (true, or sustained hypertensives); 3) hypertensive by clinic measurement and normotensive by ambulatory measurement (white coat hypertensives); and 4) normotensive by clinic measurement and hypertensive by ambulatory measurement (Figure). From a clinical point of view, the first two groups are easy to deal with, since both methods give the same classification. Of more interest are the groups where there is disagreement. The third group, usually referred to as white coat hypertensives, or less frequently as isolated office hypertensives, has been extensively studied, and is generally accepted as being at relatively low risk of cardiovascular morbidity,1 a view which is consistent with the concept that ambulatory pressure gives a better prediction of risk than clinic pressure. However, it must be admitted that not everyone shares the view that white coat hypertension is benign, particularly because there are studies that have shown that it may be associated with some degree of target organ damage. This issue will be dealt with in a subsequent report of this series. Up to now, little attention has been given to the fourth group, who have been given the awkward titles of “reverse white coat hypertension” or “white coat normotension.” If it were true that the ambulatory pressure gives the better classification of risk, it would imply that these people should be regarded as being genuinely hypertensive, as argued below. We have recently proposed2 that the phenomenon should be called “masked hypertension,” on the grounds that the hypertension is not detected by the routine methods. But what evidence is there that this group deserves recognition as a discrete entity, as opposed to being made up of people who happened to have an unusually high ambulatory pressure or a low clinic pressure on that particular occasion? There are potentially several questions that could be asked to decide this issue. First, the phenomenon of masked hypertension would be more credible if it could be shown that it is reproducible on repeat testing. So far as we are aware, this issue has not been examined. What Is Masked Hypertension?