Description of methodological issues in a trial designed to evaluate if a multi-component intervention aimed at vascular risk factors can prevent dementia.

Multi-center, open, cluster-randomized controlled clinical trial (preDIVA) including 3535 non-demented subjects aged 70–78, executed in primary practice and coordinated from one academic hospital. General practices are randomized to standard care or intensive vascular care.

Vascular care consists of 4-monthly visits to a practice nurse who monitors all cardiovascular risk factors. Hypertension, hypercholesterolemia, overweight, lack of physical exercise and diabetes are strictly controlled according to a protocol and treated in a way, tailored to the characteristics of individual participants.

Primary outcomes are incident dementia and disability; secondary outcomes are mortality, vascular events (stroke, myocardial infarction, peripheral vascular disease), cognitive decline and depression.

Between May 2006 and February 2009, 3535 subjects from 115 general practices have been included. The clusters have an average size of 31 (SD 22, range 2–114). 1658 Patients from 52 practices were randomized to the standard care condition and 1877 patients in 63 practices to the vascular care condition.

When designing a cluster-randomized trial, clustering of patient data within GP practices leads to a loss of power. This should be adjusted for in the power calculation. Since intensive vascular care will probably lead to a reduction in cardiovascular mortality, the competing risks of mortality and dementia should be taken into account.