Study population
The Rugao Longevity and Ageing Study (RuLAS) is an observational, prospective and community-based cohort study [17]. The baseline survey was conducted from November to December 2014 (Wave 1). A total of 1788 adults aged 70–84 years were recruited. Then, three follow-up surveys were conducted in April 2016 (Wave 2), November 2017 (Wave 3) and December 2019 (Wave 4). This study was approved by the Human Ethics Committee of the School of Life Sciences of Fudan University. Informed consent was obtained from each participant.
In December 2019, SD-OCT was added to perform at wave 4 of the cohort. A total of 2200 older adults were recruited in wave 4. SD-OCT was available for 1721 subjects (78.22%). Subjects who did not undergo SD-OCT (n = 479) or had upgradable OCT due to poor quality scans (n = 538) were excluded. A total of 1183 subjects who had complete retinal thickness data were recruited in wave 4. Among the 1183 subjects, 480 subjects completed both three visits (December 2014, November 2017 and December 2019) and had cognitive information. In addition, subjects with retinal related disease were excluded, including glaucoma (n = 4), age-related macular degeneration (n = 4), diabetic retinopathy (n = 3), pathological myopia (n = 16) and other retinal disease (n = 7). At last, 446 participants with three repeated measurements of cognitive function were included and analyzed in our study (Fig. 1).
Outcomes
In our study, cognitive function was evaluated by the revised Hasegawa’s dementia scale (HDS-R) [17], which comprising of orientation, memory, attention/calculation and verbal fluency [18]. HDS-R has been widely accepted in Asian populations in clinical and epidemiological surveys for the assessment of cognitive impairment [19]. Previous studies observed that HDS-R was similar to Mini-Mental State Examination and had better diagnostic accuracy for screening AD [20]. In addition, the HDS-R was more robust to demographic influence (such as level of education, age and gender) than MMSE [20, 21]. In conclusion, it was a brief and reliable tool to assess global cognitive function in Asian [22, 23]. In our study, individuals who scored higher than 21.5 were defined as normal cognition, while who scored 21.5 or below were defined as CD [17, 24, 25]. Cognitive function was assessed in November 2014, November 2017 and December 2019.
Individuals were categorized into four subgroups according to cognitive function in three repeated assessments among 5-years follow-up [26]. Individuals with consistently normal cognition (Normal-Normal-Normal) in both three visits were categorized into consistently normal cognition group (N = 159); Individuals with consistently CD in both three visits (CD-CD-CD) were categorized into persistently CD group (N = 134); Individuals who progressed to CD from normal cognition within three visits (Normal-Normal-CD or Normal-CD-CD) were categorized into progressed to CD group (N = 70). Individuals who returned to CD from normal cognitive (Normal-CD-Normal, or CD-Normal-CD), or returned to normal cognitive from CD in any two visits (CD-Normal-Normal, or CD-CD-Normal) were categorized into reverting or fluctuating CD group (N = 83).
SD-OCT scan
SD-OCT scanning was performed without pupil dilation using the spectral domain OCT-HS100 (Canon Inc, Tokyo, Japan) with a macular 3D scans over a 10 × 10 mm area (1024 A-Scan × 128 B-Scan). The Early Treatment Diabetic Retinopathy Study (ETDRS) grid focused on the macular was used for OCT measurements (Fig. 2). The ETDRS grid consisted of three concentric circles of 1, 3, and 6 mm diameters. The 3- and 6-mm circles were each divided into superior, inferior, nasal and temporal quadrants. Macular retinal nerve fiber layer (mRNFL) and ganglion cell layer-inner plexiform layer (GC-IPL) were segmented.
The right eye was used for measurements if the Signal Strength Index was ≥ 5. Otherwise, the left eye was used. The exclusion criteria included: (1) with signal strength index of both eyes < 5; (2) any ocular disorders, including pathological myopia, age-related macular degeneration, diabetic retinopathy, glaucoma, vitreoretinal interface abnormalities (e.g. epiretinal membrane, macular hole), and other eye pathology; (3) a history of clinical stoke, uncontrolled diabetes and hypertension. The thickness were automatically segmented, and manually corrected was conducted by two masked examiners (Shen H and Gong W) if necessary. The mean thickness was calculated.
Covariates
Demographic, physiologic and clinical data were collected from the RuLAS. Demographic data included age, gender, married status, educational status, smoking and alcohol consumption. Physiologic variables, including body mass index (BMI), high-density lipoprotein (HDL), low-density lipoprotein (LDL), triglyceride, serum creatinine, fasting blood glucose (FBG) and High-sensitivity C-reactive protein (Hs-CRP) were also measured. Clinical variables, including, major cardiovascular disease (CVD), diabetes mellitus, depressive status (assessed by the 15-item Geriatric Depression Scale [27]), hypertension were collected. Major CVD included cerebral infarction, stroke, cerebral hemorrhage, coronary heart disease, myocardial infarction and heart failure. In addition, mobility (measured by time up and go test [28]) and grip strength was also assessed in our study.
Statistical Analyses
Firstly, we described the characteristics of participants in our study. Continuous and categorical variables were presented as mean with standard deviation (SD) and frequency (%), respectively. Group differences between the four groups were analyzed by chi-square or t test. The differences between consistently normal cognition and CD subgroups were also analyzed. Secondly, we fitted logistic regression models, treating memberships in all four group as outcome variables, with the consistently normal cognition group as reference, to assess the effect of mRNFL and GC-IPL in four models. Model 1: unadjusted; Model 2: adjusted for age, body mass index, sex, smoking, alcohol accumulation, married status and education; Model 3: adjusted for model 2 + serum fasting glucose, triglyceride, HDL, LDL, creatinine and Hs-CRP; Model 4: adjusted for model 3 + CVD, hypertension, diabetes mellitus, mobility, grip strength and depressive status. Lastly, sensitivity analyses using logistic regression models were conducted to validate the relationship in participants without CVD and/or diabetes. A p-value (two-tailed) less than 0.05 determined as statistical significant. All analyses were conducted by SPSS 22.0 or R (Version 3.6.1: www.r-project.org/).