Onset of Disability According to Mild Cognitive Impairment Subtype in Community‐Dwelling Older Adults in Japan

To the Editor: Cognitive impairment is associated with disability in activities of daily living (ADLs),1 which are essential activities that a person needs to be able to perform to live independently.2 Mild cognitive impairment (MCI) is associated with the development of Alzheimer's disease (AD).3 Individuals with MCI typically have difficulties performing instrumental ADLs,4 although whether the subtypes of MCI have differing effects on disability onset is unclear. This prospective cohort study investigated the association between MCI subtype and disability onset in community-dwelling older adults in Japan. Participants were 3,459 individuals aged 65 and older who were cognitively healthy (n = 2,600) or had MCI (n = 859). The criteria for MCI, defined as having subjective memory complaints, consisted of intact general cognitive functioning (Mini–Mental State Examination score ≥ 24), age-adjusted objective cognitive impairment, no use of Japan's long-term care insurance (LTCI)5 or functional dependency (no need for supervision or external help to perform ADLs), and no clinical criteria for dementia. Age-adjusted objective cognitive impairment (age-adjusted score of ≤1.5 SDs below the mean) was identified using the National Center for Geriatrics and Gerontology Functional Assessment Tool (NCGG-FAT).6 All participants underwent a baseline assessment that included face-to-face interviews to record demographic data such as age, sex, self-rated health, number of prescribed medications, and medical history; NCGG-FAT for determining MCI and its subtypes (amnestic or nonamnestic, single or multiple domain); and walking speed. After this initial screening, participants were evaluated monthly for incident certification of need of care according to the LTCI system over a 2-year period. Every Japanese person aged 65 and older is eligible for benefits (institutional and community-based services, but not cash) in cases of physical or mental disability. Onset of disability was defined as the point at which individuals were certified as needing care under the LTCI system. During the 2-year follow-up period, 99 (2.9%) participants developed disability based on the LTCI system. The prevalence of disability onset within these 2 years in those with MCI (4.8%) was more than twice that of those who were cognitively healthy (2.2%) (P < .001). A significantly greater prevalence of disability onset in those with multiple-domain amnestic (aMCI; 7.6%) and multiple-domain nonamnestic (naMCI; 8.3%) MCI was also found (Figure 1A), and Kaplan–Meier log-rank tests further demonstrated that the probability of disability onset was significantly higher in participants with multiple-domain aMCI or multiple-domain naMCI than in those who were cognitively healthy (P < .001; Figure 1B). Furthermore, a Cox proportional hazards regression model adjusted for age, sex, hypertension, diabetes mellitus, heart disease, cancer, osteoporosis, fractures (after age 60), number of prescribed medications, walking speed, and self-rated health showed that participants with multiple-domain aMCI (hazard ratio (HR) = 1.99, 95% confidence interval (CI) = 1.01–3.94, P = .047) or multiple-domain naMCI (HR = 2.82, 95% CI = 1.35–5.90, P = .006) during the baseline assessment had a significantly greater risk of disability onset than those who were cognitively healthy. Conceptualizations of MCI typically include no significant functional decline,3 but there are discussions regarding whether this paradigm should incorporate elderly individuals with MCI who have functional impairment.7, 8 Functional decline has also been shown to be detectable in individuals with MCI 1 year before Alzheimer's disease diagnosis.9 The results of this prospective study showed that MCI is associated with disability onset, especially in older adults with multiple-domain aMCI or naMCI. The results also show that the subtypes of MCI, such as single or multiple domain, differ in their correlation with disability risk in the elderly Japanese population. Specifically, older adults with multiple-domain aMCI and naMCI had a higher risk of disability incidence than those who were considered cognitively healthy. Differing rates of progression from MCI to dementia have previously been reported, with multiple-domain aMCI the most likely subtype to be a precursor for the disease.10 The current findings expand this finding by indicating that, not only is multiple-domain MCI associated with a significantly greater risk of onset of dementia, but also that the progression for disability is similar. The most significant limitation of this study was the inability to assign causation to the onset of disability, which is something that should be addressed in future research. Mild cognitive impairment is associated with disability onset in community-dwelling older adults in Japan. Specifically, older adults with multiple-domain aMCI or naMCI are most at risk of developing disability. Conflict of Interest: The authors have no financial or any other kind of personal conflicts associated with this paper. Author Contributions: Makizako: study concept, data acquisition and interpretation, preparation of manuscript. Shimada: study concept, data interpretation, revision of manuscript. Doi, Tsutsumimoto: data acquisition and interpretation, revision of manuscript. Lee: data interpretation, revision of manuscript. Suzuki: study concept, interpretation of data. Sponsor's Role: None.