Paterniti et al (Reference Paterniti, Verdier-Taillefer and Geneste2000) report welcome prospective data showing that low blood pressure precedes depression in older people rather than vice versa. However, important alternative explanations for this effect require consideration before conclusions can be drawn regarding causality.

Poor physical health and disablement have been shown to be strong risk factors for incident depression in older people (Reference Prince, Harwood and BlizardPrince et al, 1997). A large prospective community study showed that low blood pressure was associated with raised mortality but that this effect was eliminated after adjustment for comorbid physical illness (Reference Boshuizen, Izaks and van BuurenBoshuizen et al, 1998). The same may be true for depression. Paterniti et al comment that the number of chronic diseases was measured in participants and was not associated with depressive symptoms or low blood pressure. However, it is the severity of individual conditions and resulting functional limitation which are likely to be most important in depression rather than the number of different conditions. Important confounding effects may therefore have been missed. Both depression and low blood pressure may also be secondary to early cognitive decline, although this is less likely to explain results in the age range of participants for this study.

In order to develop effective strategies for the prevention of depression in later life, further research will be required to clarify causal pathways: in particular: (a) whether low blood pressure causes depression through one of the organic mechanisms discussed by the authors; (b) whether low blood pressure and depression are both secondary to early cognitive decline; and/ or (c) whether low blood pressure is a marker for other, more psychological risk factors for depression, such as the impact of poor physical health and functional limitation.