Time Trends in the Prevalence of Neurocognitive Disorders and Cognitive Impairment in the United States: The Effects of Disease Severity and Improved Ascertainment

Background:

Trends in the prevalence of cognitive impairment (CI) based on cognitive assessment instruments are often inconsistent with those of neurocognitive disorders (ND) based on Medicare claims records.

Objective:
We hypothesized that improved ascertainment and resulting decrease in disease severity at the time of diagnosis are responsible for this phenomenon.

Methods:
Using Medicare data linked to the Health and Retirement Study (1992–2012), we performed a joint analysis of trends in CI and ND to test our hypothesis.

Results:
We identified two major contributors to the divergent directions in CI and ND trends: reductions in disease severity explained more than 60% of the differences between CI and ND prevalence over the study period; the remaining 40% was explained by a decrease in the fraction of undiagnosed individuals.

Discussion:
Improvements in the diagnoses of ND diseases were a major contributor to reported trends in ND and CI. Recent forecasts of CI and ND trends in the U.S. may be overly pessimistic.