Multimorbidity associated with functional independence among community-dwelling older people: a cross-sectional study in Southern China

This study was based on a cross-sectional community health survey in the Shenzhen city of Guangdong province in Southern China. The samples in this survey were selected using a multistage and stratified random sampling method, consisting of family members drawn from five percent of the total households in this municipality. In the present study, we used the available information from people aged ≥60 years of age. Of 2919 participants, 214 (7.33%) subjects were excluded due to incomplete or inconsistent data from questionnaires. Finally, a total of 2705 older adults were included in our analysis.

The presence of chronic diseases was determined via self-report, as a yes/no response to the question stem, “has a doctor ever diagnosed that you had…” [21]. The 17 diseases investigated in this study included: hypertension, chronic pain, diabetes mellitus, hyperlipidemia, bone diseases, gastroenteritis, heart disease, gout, peripheral vascular disease, chronic kidney disease, spleen and gallbladder diseases, pulmonary disease, stroke, cancer, multiple sclerosis, dementia and mental disorder. Multimorbidity was defined as the presence of two or more of the 17 chronic diseases in an individual.

Functional status was assessed by the Functional Independence Measure (FIM) in this paper. The FIM score is defined as the level of assistance required for an individual to perform activities of daily living indicating the burden of caring for them [22]. The scale includes 18 items, of which 13 items are physical domains based on the Barthel Index and 5 items are cognition items. Each item is scored from 1 to 7, based on the level of independence, where 1 represents total assistance and 7 indicates complete independence [23]. Possible scores range from 18 to 126, with higher scores indicating more independence. To our knowledge, the 18-item FIM instrument has been found to be reliable and well validated [24, 25]. And the Cronbach’ Alpha of FIM via Reliability Analysis was 0.928 in this paper. Considering the non-normal distribution of the data and the high percentage of well-functioning participants, we dichotomized functional status, the dependent variable, into “completely independent” (=126 scores) and “not completely independent” (<126 scores) according to their FIM scores. Finally, all participants were categorized into two groups: those completely independent, and those needing some help or assistance with activities of daily living.

Descriptive statistics were calculated for all measures. Means and standard deviations (SD) were presented for continuous variables, while frequency and percentage were used for categorical variables. To analyze the relationship between multimorbidity and functional independence, several logistic regression models were run.

First, we evaluated the association between the number of chronic diseases and functional independence after adjusting for demographic characteristics and other possible confounding factors. Subsequently, based on the results-of this preliminary analysis, multimorbidity and its relationship with functional independence were explored, in depth.

The spectrum of 17 chronic diseases was displayed by calculating the prevalence of each disease individually. We chose prevalent chronic medical conditions with a prevalence >10%, and analyzed their correlations using contingency tables to generate the most prevalent multimorbidity pairs (the combination of two chronic diseases) and conducted a logistic regression analysis for each of the top 10 multimorbidity pairs by adjusting for other confounding factors.

To analyze how health conditions are grouped together, we used an exploratory factor analysis with principal components using a tetrachoric correlation matrix [26], which allows that a chronic condition can load onto several factors or multimorbidity groups. The Kaiser-Meyer-Olkin measure of sampling and the Bartlett’s test of sphericity were performed to investigate sampling adequacy for conducting factor analysis. The rotation method was the oblique direct oblimin, which allows factors to be associated with each other. Subjects were assigned to a group if they were diagnosed with at least 2 of the diseases included in the group. Multimorbidity groups overlap when a person is assigned to two or more groups.

Then, to explore the impact of multimorbidity groups in functional independence, we conducted a logistic regression analysis adjusting for confounding factors. Additionally, another logistic regression model was performed to assess how overlapping groups affected functional status.

Within a sample of Chinese older adults, we found that the number of chronic conditions (≥2 diseases) was independently associated with lower functional independence. Based on this, we wanted to understand how different multimorbidity patterns affect functional independence. We found that hypertension and chronic pain account for the biggest loss in functional independence, whether alone or co-occuring. Their associations with functional independence persisted, even after adjusting for confounders. Of the five multimorbidity groups, the bones and pain group as well as the cardiovascular and metabolic groups were identified as having a significant risk of reduced functional independence. Interestingly, we detected that the number of overlap multimorbidity groups was also greatly related to functional status in individuals.

In our study, the great majority of older adults suffered from chronic diseases and 45.5% reported the presence of two or more chronic conditions. It fits well with the range of the overall prevalence of multimorbidity from a systematic review of 9 published studies in China (6.4%–76.5%) [27]. A study with a large sample conducted in Southern China showed that the prevalence of multimorbidity among the elderly people was 47.5% [18], consistent with our results. Multimorbidity is a common worldwide phenomenon among the elderly population, and China is no exception. Additionally, gender differences were significant whether in multimorbidity or single diseases. Women had a higher prevalence of multimorbidity, and were more prone to suffer from pain [28], bone diseases and hypertension [10] than men. Women had a longer life expectancy [29], and thus, they were affected more often by chronic disease than men. On the other hand, the low hormone levels characteristic of menopause can accelerate bone loss [30] and atherosclerosis progression [31], which could partially explain the sex-specific distribution of diseases. However, sex was not found to be independently associated with functional independence in our analysis. A review of the studies also found that after adjusting for socioeconomic, health, and social relations indicators, incidence of functional disability was similar between genders [32]. The effect of sex on functional status remains to be further studied.

The first logistic regression model we ran showed that the number of multiple chronic conditions was independently associated with functional independence. Subjects with two or more chronic diseases had a higher risk of losing functional independence, as found in the previous literatures [14, 33]. A community-based follow-up study also revealed that multimorbidity increased the odds of being in the “With disability” group at follow-up [34]. Age, especially senile age, was another independent risk factor, consistent with previous studies [12, 35]. With the aging process, the older adults would inevitably experience loss of strength, osteoporosis or other degenerative changes in addition to loss of balance that may have a greater impact on functional independence. Both higher education level and better economic conditions were associated with higher functional independence, consistent with other studies [33, 36], which could be elucidated by stronger health awareness, higher income and better access to prevention, treatment and rehabilitation. Under this premise, how different types of multimorbidity affect functional independence was the focus of our analysis.

Hypertension was found to be the single most prevalent disease and was included in both of the multimorbidity pairs with statistical significance related to functional status, specifically, heart diseases and chronic pain. Compared with single diseases, multimorbidity pairs had a higher risk of low functional independence. In fact, hypertension, which is very common in China [37], can lead to left ventricular hypertrophy and progression of atherosclerosis, which are leading causes of heart failure and ischemic heart disease [38, 39]. Chamberlain et al [40] found that hypertension was one of the most prevalent diseases in heart failure patients. Previous studies have also demonstrated that the presence of chronic pain was associated with significantly increased odds of hypertension [41].

Regarding chronic diseases clusters, we detected five multimorbidity groups using factor analysis, as performed by other authors exploring multimorbidity patterns [10, 26, 42, 43]. The first group, the SGD-CKD-GE and PVD group had a high prevalence of 10.8% but had no statistically significant association with functional status after the adjustment. The second group, including dementia, mental disorders, multiple sclerosis and stroke, had a lower prevalence (0.5%) than the SGD-CKD-GE and PVD group. Although the OR of the effect on functional dependence was the highest, the significance was not statistically significant, which may be attributable to the dichotomization of the dependent variable. The third group, which included bone diseases, gout and chronic pain, was significantly associated with functional independence. Bone diseases such as osteoarthritis [32, 35], arthrosis and osteoporosis [36] had a great effect on functional disability. Komatsu et al [44] found that muscle, bone and joint pain were independently associated with lower ADL level, which was accordant with our findings. Meanwhile, chronic pain, a prevalent condition in elderly patients, was associated with greater reported difficulty in performing certain essential self-management activities [45]. Importantly, Connolly et al [46] noted that, after age, pain was one of the strongest factors associated with difficulty in ADL/IADL combined and ADL alone. However, it was not included in the previous published article by Marventano S et al [10]. Given the strong influence of chronic pain on difficulty with functional independence, early interventions for pain reduction, control, and self-management should be performed and future studies on multimorbidity should consider inclusion of chronic pain. The cardio metabolic group is also prominent with the highest prevalence among older people, followed by the bones and pain group. In China, a large investigation conducted by Wang et al [47] showed high prevalence of hypertension, diabetes and hyperlipidemia, proving them to be common disease in the elderly population. As we all know, hypertension, diabetes mellitus and hyperlipidemia are all established risk factors for cardiovascular diseases [48], being part of the so-called “metabolic syndrome” [49]. The lung and cancer group had the lowest prevalence in our population. The cancer tended to affect the respiratory tracts as the most frequent sites [10].

Considering the presence of multiple chronic diseases with various complex interactions, subjects with multimorbidity had greater difficulty in performing daily activities and had greater need for care. More importantly, we found an interesting dose-response association. Specifically, the greater the overlap between multimorbidity groups, the greater was the impact on functional independence in older adults.

However, this study has some limitations. First, this is a cross-sectional study. Further in-depth studies with a longitudinal follow-up data are warranted to examine the cause-effect relationship. Second, the lack of inclusion of other chronic conditions may underestimate the prevalence of multimorbidity. This could be the reason for the relatively low prevalence of multimorbidity. Finally, information about the severity of disease should be taken into consideration in future studies.