Study design and participants
A sample of urban older adults was obtained from a cross-sectional survey. The survey was conducted in Jinjiang District, one of the twelve districts in Chengdu, Sichuan Province in China. Chengdu is an important central city in western China, ranking eighth in Gross Domestic Product (GDP) among all cities in China, generating 1,217.02 billion yuan in 2016. Jinjiang District also belongs to one of the central districts of Chengdu, ranking fifth in GDP (among all districts in China) with 83.46 billion yuan in 2016. In this area, 21.62% of people were aged 60 years and over in 2016. A multi-stage cluster sampling method was applied to ensure that study participants were selected from various socio-economic sectors, which made it more representative of the population. The eleven towns in Jinjiang District were divided into three levels based on income level – low, middle, and high. Six residential areas were selected randomly from each group, with a total of 18 residential areas. Then we randomly selected three to six buildings from each residential area and investigated all older adults meeting the inclusion criteria. The data was collected from October 2016 to March 2017.
The inclusion criteria for participants were as below: 1) Permanent residents (length of residence ≥ 12 months); and 2) 60 years old and over. Participants were excluded if they: 1) Were unable to complete the survey because of severe visual or hearing impairments, serious physical illness or weakness; 2) Had a history of traumatic brain injuries or psychiatric illnesses that affect cognitive function; 3) Might have major depressive disorders, which was defined as the Chinese self-reported version geriatric depression inventory (GDI-SR) score ≥ 3 ; and 3) Were demented or had MCI without memory impairment. In total, 617 randomly selected older adults participated this survey. Among the selected, 13 participants had visual or hearing impairments, 20 refused to answer questions, 84 participants had symptoms of depression, 39 participants gave incomplete data, and 93 participants did not meet the diagnosis of aMCI or normal cognitive function. All in all, 368 older adults were included for analysis.
The survey protocol (including the informed consent) was approved by the Medical Ethics Committee of Sichuan University. All the participants signed the informed consent forms. Qualified research assistants with medical backgrounds and community physicians administered this survey. All the research assistants and clinicians were intensively trained by psychiatrists from the Mental Health Center of West China Hospital.
All participants completed the following neuropsychological assessments: Mini-Mental State Examination (MMSE), the Clinical Dementia Rating (CDR), Auditory Verbal Learning Test (AVLT), Wechsler’s Logical Memory Task (LMT), Boston Naming Test (BNT) and Trail Making Test Part A (TMT-A).
The MMSE scale was used to assess global cognitive function . The MMSE consists of multiple questions and covers six cognitive domains: orientation (10 points), immediate memory (3 points), attention and calculation (5 points), recall ability (3 points), language (8 points), and visuospatial ability (1 point). Usually, the visuospatial ability task was classified as one of the language items. The score totals ranged from 0 to 30, with higher scores indicating better cognitive function.
The CDR is a reliable tool for staging dementia severity . It includes six cognitive categories: memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care. According to clinical scoring rules, CDR 0 = no dementia, CDR 0.5 = indicates questionable dementia, CDR 1 = mild dementia, CDR 2 = moderate dementia and CDR 3 = severe dementia. In our current study, CDR was also used to obtain information on cognitive complaints and activities of daily living. And the psychiatrists from Mental Health Center of West China Hospital were responsible for CDR rating.
AVLT is a well-recognized measure for verbal learning and memoryB. The examiner read a list of 12 unrelated Chinese words aloud three times. Immediately following each presentation and 20 minutes after the last, the participant was required to recall as many words as possible without a time limit and in any order. The immediate recall scores consisted of the number of words recalled in each trial (range 0-12) and the total number of words recalled in the 3 immediate trials (range 0-36). The delayed score consisted of the number of words recalled after the 20-min delay (range 0-12), which we refer to as the long-term delayed recall in our study. In the end, the participants were shown the word list.
LMT primarily tested participants’ logical memory . The participants were told a short story orally, which contained 20 underlining keywords. Then the examinee was asked to recall the story (immediate recall). Approximately 20 or 30 minutes later, free recall of the story was again elicited (delayed recall).
BNT was used as a test for language assessment . The theme of 30 images were asked to be named. If a participant named the item correctly, credits were given for self-corrections and it was recorded among ‘spontaneous naming scores (SN)’. If a participant gave a wrong response or gave no response within 20 seconds, the examiner provided a standard semantic cue. If a participant gave the correct answer with the cue, credit was given and was recorded under ‘naming scores after phonemic cue (CN)’.
TMT-A was used to assess the execution function of participants . It required the participants to link numbers from 1 to 25 as fast as possible while keeping the nib on the page. The amount of time consumed and the number of errors made were recorded, defined as TMT-A(s) and TMT-A error, respectively.
Social information was gathered through participants or their appropriate informants. The following data was collected by standard questionnaire: 1) Demographic data such as age, gender, educational level, marital status and income (average monthly income per person in family); 2) History of chronic diseases including hypertension, diabetes, coronary heart disease and cerebrovascular disease; 3) Daily living information (sleep disorders, smoking and drinking alcohol). Specifically, marital status consisted of two categories: “marriage” and “no marriage”. The latter was defined as being divorced, widowed, or unmarried.
Sleep disorders of older adults were assessed by the Chinese version of Pittsburgh Sleep Quality Index scale (PSQI) , which is in the reference period of “past one month”. The scale contains 19 items scored on a 3-point Likert scale and can be divided to 7 domains: subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbances, use of sleep medications and daytime dysfunction. The global PSQI scores ranged from 0 to 21, with higher total scores indicating worse sleep quality. The Chinese version of the PSQI has been confirmed to show good reliability and validity .
Diagnosis of aMCI was made based on Petersen’s criteria [33-35] : 1) Memory complaint by participants, preferably corroborated by their informants; 2) Objective memory impairment in addition to a z score ≤ -1.5 for at least one memory neuropsychological test and a CDR score of 0.5; 3) Preserved general cognitive function according to MMSE scores adjusted by educational level  (>17 for illiterate, >20 for primary school and >24 for above the middle school); 4) Intact daily living ability; and 5) Absence of dementia. Ultimately, among 368 older adults in this study, 59 were aMCI participants and 309 were participants with normal cognitive function.
The continuous variable was presented as mean ± standard deviation (SD) and categorical variables were described in terms of frequency (%). The Spearman correlation coefficient and the Spearman partial correlation coefficient controlling age, gender and educational level were used to detect differences in cognitive test results between the aMCI group and normal group (individuals in the aMCI group were assigned a value of 1 and individuals in the normal group were assigned a value of 0). A comparison of continuous data among aMCI participants and those with normal cognitive function was performed using independent-sample t-test analysis. Chi-square tests were applied to examine group differences in dichotomous variables data. For the ordinal categorical variable (educational level), the Cochran-Armitage test for trend was used to verify whether the prevalence of aMCI is higher with the lower educational level. Multiple logistic regression analysis was utilized to screen for the risk and protective factors of aMCI. The reported p values are the results of two-sided tests. P values of <0.05 were considered as statistically significant. Statistical analysis was performed using Stata version 15.1.