On 4 August, investigators of the Alzheimer Cohorts Consortium (ACC) published an article in Neurology, presenting the 27-year time trends in dementia incidence in Europe and the US. Analysing the combined data from 7 population cohorts, the investigators found that the incidence rate of dementia has declined by 13% per decade over the last 25 years.
The incidence rate of a disease measures the frequency with which it occurs over a specified time period, and is linked to the risk of developing the disease in question. Assessment of the incidence rate of dementia is complicated by the fact that many population-based cohort studies don’t have a sufficiently large number of participants to accurately quantify changes in incidence over time. To address this issue, the ACC investigators analysed the data from 7 different EU and US cohorts, totaling 49,202 individuals aged 65 years and above. At least 15 years of follow-up data was available for each participant, including detailed clinical information and whether they had been diagnosed with Alzheimer’s disease or dementia.
Of the 49,202 participants, 4253 incident cases of dementia were diagnosed. As expected, the incidence of dementia increased with age, averaging 65 diagnoses per 1000 people aged between 85-89. Interestingly, analysis of the incidence time trends revealed an average decline of 13% per decade in the dementia incidence rate. The decrease was larger in men (24%) than in women (8%), although there was some variation between cohorts and geographical locations. In practice, this means that the chance of a 75-year old man developing dementia in his remaining lifetime is currently 18%, compared to a 25% risk for 75-year old men in 1995.
The authors suggest that if this trend of decline continues, 15 million fewer people than anticipated will develop dementia by 2040 – although the annual number of new cases of dementia will still number in the tens of millions, increasing over time. Further studies are now required to identify the underlying causes that account for the decreased dementia incidence rate over time.
