Elderly patients with greater variability in BP readings performed worse in cognitive tests than others, regardless of average BP, the BMJ study found.
European researchers said erratic BP may damage blood vessels in the brain, starving key regions of oxygen. They called for further research into whether reducing BP variability can lower this risk.
Significant swings in BP over time have previously been linked to damage to white matter in the brain, thicker carotid artery walls and atherosclerosis.
Researchers from the UK, Ireland and the Netherlands used data on 5,461 participants involved in a trial of the effect of statins on vascular events.
Patients were aged 75 years on average and all had at least one major cardiovascular risk factor, including hypertension, diabetes or cigarette smoking. Half had cardiovascular disease. Patients' BP was measured every three months for just over three years. A total of 553 participants also underwent brain MRI.
Patients with larger differences between quarterly BP readings had worse results in tests for attention, cognitive speed and memory. This was independent of average BP and other cardiovascular risk factors.
Larger swings in systolic and diastolic BP were linked to lower hippocampal volume and cortical infarcts, while higher diastolic BP differences were associated with cerebral microbleeds.
Researchers proposed three reasons why variable BP may be linked to cognitive impairment. They may have a common cause; variable BP may disrupt blood flow to key organs; or fluctuations may restrict blood flow to parts of the brain, causing brain damage, they said.
They concluded that 'higher visit-to-visit BP variability independent of average BP might be a potential risk factor with worse cognitive performance in older subjects at high risk of cardiovascular disease'.
They added: 'This observation merits further interventional studies to determine whether reducing variability in BP can decrease the risk of cognitive impairment in old age.'