Temporal trends in the long term risk of progression of mild cognitive impairment: a pooled analysis

Background: Mild cognitive impairment (MCI) is a condition that carries a substantial risk of dementia. The exact magnitude of that risk is uncertain because of the variations in the definition of MCI, the setting (such as memory clinic versus community) and, equally importantly, the duration of follow-up. Recently, a number of long term studies have been published with observation periods of 5 years of longer. Methods: In this quantitative review, 15 long term studies were examined and compared with the results from studies using shorter periods of observation, focusing on the annual conversion rate (ACR) of MCI to dementia. Results: The report identified six long term clinical studies conducted in specialist settings and nine long term population studies conducted in the community. Across all cohort studies with completed follow-up, the mean ACR to dementia was 4.2% (95% CI 3.9% to 4.6%). This was lower than the rate reported in studies of less than 5 years' duration. The cumulative conversion rate averaged 31.4% in this sample. The proportion converting to dementia (and Alzheimer's disease) declined with longer observation periods, suggesting that risk of progression diminishes with time. Conclusion: A 10-15% ACR only appears to hold true in clinical samples monitored over a short observation period. Recruitment of older individuals from specialist centres, particularly involving those who complain of cognitive difficulties (subjective memory complaints) will tend to favour high conversion rates. In the first few years of follow-up, many of those with the most adverse risk profile will tend to progress, dropout or die, leaving a cohort of less vulnerable sufferers. However, an inverse temporal relationship was also manifest in those who completed long term follow-up, suggesting other factors are involved such as sampling issues or heterogeneity in MCI itself.