Mind the gap: prevalence and pathophysiology of isolated systolic hypertension: In reply

We are pleased that Dr. Bursztyn enjoyed our editorial on isolated systolic hypertension.1 We believe it highlighted the importance of a serious medical condition that is both underdiagnosed and undertreated. We would agree that the true prevalence of isolated systolic hypertension will vary between populations and also with the definition one uses. From the Framingham cohort,2 defining isolated systolic hypertension as a systolic pressure >160 and diastolic <90 mm Hg yields a prevalence of ∼14% for men and 23% for women aged more than 60 years. However, using the Joint National Committee on Detection, Prevention, Evaluation and Treatment of High Blood Pressure definition of systolic ≥140 and diastolic <90 mm Hg, Franklin et al3 found that ∼51% of subjects aged more than 50 years in the National Health and Nutrition Examination Survey study had isolated systolic hypertension. Moreover, given that the prevalence of isolated systolic hypertension increases considerably with age,2 it might be expected that a substantially larger proportion of the individuals aged over 70 years would be affected. Therefore, it is somewhat surprising that Bursztyn et al4 only reported an incidence of 5% for isolated systolic hypertension in an Israeli population. A number of explanations for this disparity exist, including the rather small sample size, 448 of the Israeli study, ethnic considerations, and perhaps the unusual method of blood pressure assessment—averaging lying and standing pressures—used by Bursztyn et al. Although peripheral vascular resistance was assessed in 23 patients with systolic hypertension in the study by Vardan et al,5 to which Dr. Bursztyn refers, no direct comparative measurements whatsoever were undertaken in age-matched normotensive subjects. Therefore, it is difficult to draw any conclusion regarding differences in peripheral vascular resistance between those with isolated systolic hypertension and those older individuals who remain normotensive. However, in a well-controlled study involving both older and younger patients with systolic hypertension, Simon et al6 demonstrated that peripheral vascular resistance was no different between patients and controls. Nevertheless, arterial stiffness was increased in the older subjects with systolic hypertension, which may, in part, explain why isolated systolic hypertension is often resistant to therapy with conventional antihypertensive agents targeted at resistance vessels. Dr. Bursztyn's final criticism centers on whether isolated systolic hypertension and essential hypertension are different entities. We stand by our assertion that they are indeed separate conditions. Isolated systolic hypertension is predominantly a disease of older individuals and is associated with arterial stiffing, whereas essential hypertension tends to occur in younger subjects.3 Stiffening of the large arteries is an age-dependent process7,8 and is accelerated in subjects with essential hypertension.9 Therefore, one might consider that individuals with essential hypertension are more likely to develop isolated systolic hypertension in later life. Although recent data suggest that systolic pressure increases with age in subjects with essential hypertension, diastolic pressure actually changes little between the third and seventh decades of life.10 Such observations do not necessarily mean that isolated systolic hypertension and essential hypertension are the same condition, just that the latter may increase the risk of the former, as indeed essential hypertension increases the risk of developing renal impairment. Moreover, although the Framingham study suggests that ∼40% of patients with isolated systolic hypertension previously had elevated diastolic pressures,2 a population-based study conducted in the UK reported a figure of only ∼16%.11 Therefore, a substantial number of elderly individuals will develop isolated systolic hypertension despite never having been hypertensive in earlier life.