Occurrence of cognitive impairment and dementia in the community: a 9-year-long prospective study

Objective: To determine incidence rates of non-dementia cognitive impairment, to examine the impact of attrition due to death on the observed incidence estimates, and to compare the observed andcorrectedestimatesofnon-dementiacognitiveimpairmentwithdementiaincidencerates. Methods: A total of 1,435 persons without dementia aged 75 from the Kungsholmen Project wereevaluatedforoccurrenceofdementiaover9years.Atotalof1,070cognitivelyunimpaired subjects were also followed using amnestic mild cognitive impairment (aMCI) and other cognitive impairment,nodementia(OCIND)definitions.Tocorrecttheobservedincidenceratesforattrition due to death, cognitive status for subjects lost due to death was imputed using information on previous cognitive and health status. Observed and corrected incidence rates (IR) and 95% CIs werecalculatedwiththeperson-yearsmethod,usingPoissondistribution. Results: Incidence rates per 1,000 person-years were as follows: dementia IR 70.4 (64.0 to 77.4); aMCI observed IR 11.4 (8.6 to 15.1), corrected IR 13.7 (10.3 to 18.2); OCIND observed IR 33.8 (28.7 to 39.8), corrected IR 42.1 (36.5 to 48.6). Both aMCI and OCIND incidence increased with advancing age. Observed incidence of aMCI and OCIND together was similar to that of dementia at age 75 to 79 but lower at more advanced ages. However, the cognitive impairment incidence after age 79 increased substantially when the estimates were correctedforattritionduetodeathduringfollow-up. Conclusions:Non-dementiacognitiveimpairmentiscommonandoftenunderestimatedinpopulationstudiesthatdonotadjustforattrition.