The incidence of mild cognitive impairment: A systematic review and data synthesis

Abstract

Introduction: Incidence estimates of mild cognitive impairment (MCI) range widely. We obtained contemporary age-specific MCI incidence rates and examined sources of heterogeneity.

Methods: We conducted a systematic review of population-based studies from the Americas, Europe, and Australia using restrictive inclusion criteria to limit heterogeneity. Incidence was examined using 5-year age categories for MCI and amnestic/nonamnestic subtypes. Data were synthesized using quantitative and qualitative descriptive analyses and quantitative meta-analyses.

Results: Meta-analysis estimates (95% CI) of MCI incidence per 1000 person-years were 22.5 (5.1-51.4) for ages 75-79y, 40.9 (7.7-97.5) for ages 80-84y, and 60.1 (6.7-159.0) for ages 85+y. Despite restrictive inclusion criteria, considerable heterogeneity (measured by I²) remained. Meta-analysis findings and simple descriptive statistics were consistent and supported by qualitative review.

Discussion: Heterogeneity in MCI incidence estimates persisted across age-specific estimates from population samples, likely reflecting differences in populations and methods. Incidence rate ranges are important to consider with summary point estimates.

Keywords: Alzheimer's disease; Diagnosis; Incidence; Meta-analysis; Mild cognitive impairment; Systematic literature review.

Fig. 1

Process of identification and exclusion of studies for multiprong qualitative and quantitative approach. Abbreviations: aMCI, amnestic mild cognitive impairment; CIND, cognitive impairment, no dementia; MCI, mild cognitive impairment; naMCI, nonamnestic cognitive impairment; SCC, subjective cognitive complaint. *Reasons for full-text exclusions: (Phase 2.2)—disease definition not clearly described; (Phase 2.3)—study is not related to or is not focused on MCI, aMCI, or naMCI incidence, is focused solely on a type of MCI due to a specific disease (e.g., Parkinson’s disease, Lewy body disease), or does not report MCI, aMCI, or naMCI incidence; (Phase 2.4)—sampling method not clearly described; (Phase 2.6)—convenience sample or disease-based sample; (Phase 2.7)—if two studies use the same data set or patient population for the same parameter for the same time period, select the study with the longer follow-up period; (Phase 2.8)—review paper (include the cited studies instead); (Phase 2.12)—clinical trials; (Phase 2.14)—region outside of inclusion area (Americas, Europe, and Australia). ^†Reasons for multiprong analyses exclusions: age bands—estimates fell outside of our specified age bands; CIND—estimates are for cognitive impairment, no dementia instead of MCI; sex stratified—estimates are only provided stratified by sex; SCC—modified MCI criteria to exclude subjective cognitive concern as a criterion.

Fig. 2

Example of a patient funnel for decision-makers, estimating the incidence of MCI in the patient population with Alzheimer's disease pathology. Abbreviations: AD, Alzheimer's disease; MCI, mild cognitive impairment. *Refers to application of case identification correction factor to account for the patient journey.