Association Between Socioeconomic Status and Comorbidities Among Patients With Rheumatoid Arthritis

Results of a Nationwide Cross-Sectional Survey

Anna Shin; Seunghwan Shin; Ji Hyoun Kim; You-Jung Ha; Yun Jong Lee; Yeong Wook Song; Eun Ha Kang

Disclosures

Rheumatology. 2019;58(9):1617-1622. 

In This Article

Discussion

The present study confirms a high prevalence of comorbidities across multi-organ systems in RA patients,[1] and shows that SES is an important risk factor of common comorbidities in RA patients aged <63 years. Among three SES components, income level was most strongly associated with RA-related comorbidities including mental (depression, depressive mood and suicide ideation), cardiometabolic (diabetes, obesity and hypertriglyceridemia) and musculoskeletal comorbidities (osteoarthritis). Low education was associated with suicide ideation in those aged <63 years. We did not find any significant association between SES levels and comorbidities among those aged ≥63 years.

Studies of SES effect on RA comorbidities have been largely limited to depression.[9–11] Depression has been one of the most common RA-related comorbidities, found in up to 15–17% of RA patients.[1,12] Jacobi et al. reported a 2.6-times risk of depression associated with low SES among RA patients.[10] Consistent with previous studies,[1,10,12] our study showed that ~10% of RA patients had depression and that the age-adjusted odds for depression was 2-fold increased associated with low income.

The most notable finding of our study is an increased risk of cardiometabolic risk factors (diabetes, obesity and hypertriglyceridemia) and musculoskeletal disease (osteoarthritis) in association with low vs high income even in relatively young RA patients. The widespread effect of low income on various organ systems is in line with clustered comorbidities observed in RA patients of low SES.[13]

Previous studies had reported an increased prevalence of cardiometabolic risk factors in RA vs non-RA patients or in a low vs high SES population.[2,14] Our study shows that such risk would be further elevated in the co-presence of RA and low SES. Moreover, considering that low SES is a strong predictor of high RA activity,[6,7] our findings are consistent with the previous knowledge that the risk of metabolic syndrome is elevated in the proportion to RA activity.[15] Our results call for particular attention in light of an exceptionally high cardiovascular risk in RA patients.[2] Because the cardiometabolic risk factors found in our study to aggregate in low SES patients are modifiable by treatment, SES should be actively considered in RA management.

Low income is a risk factor for osteoarthritis development.[16] An increased risk of secondary osteoarthritis has been noted in RA of higher activity or longer duration.[17,18] Our study shows that osteoarthritis is as prevalent as 11% in relatively young patients who would actively participate in occupational activity, and that low income/education confers a 2-fold increased risk of osteoarthritis among RA patients. Because the function of RA patients is already in jeopardy,[7] comorbid osteoarthritis would cause a high disease burden in low SES patients.

Several mechanisms underlying the association can be suggested. First, RA-related comorbidities are largely mediated by chronic inflammation. In particular, both cardiometabolic risk factors and secondary osteoarthritis are tightly associated with chronic inflammation.[2,17,18] Because low SES has been consistently shown to associate with high RA activity,[6,7] the association can be partly explained by high disease activity expected in the low-income group. Second, unhealthy behaviors like smoking, drinking and physical inactivity are known to mediate the association between low SES and cardiometabolic comorbidities.[19,20] However, we only found a mild decrease in vigorous exercise rates in the low vs high income group but no significant difference in drinking or smoking habits. Third, hampered healthcare access has been associated with poor health outcome in low SES populations.[7] However, we did not find such access inequalities, possibly due to universal healthcare coverage and a negligible deductible imposed on RA beneficiaries in Korea. However, higher, albeit non-significant, rates of patients with low vs high income reported failed access to medical care when needed (23.8% vs 18.1%). Despite this report, there was a trend of more frequent outpatient clinic visits among low income patients (Supplementary Table S4, available at Rheumatology online). These findings potentially suggest a higher burden of health problems in the low-income group, to whom healthcare services were insufficient to meet their health conditions. Overall, our results indicate that insufficient healthcare service use and high RA activity among low SES patients might jointly explain different comorbidity distributions according to SES.

Important strengths and limitations of this study deserve comments. First, we present a systematic overview of SES effect on comorbid conditions of various organ systems in RA patients. Second, our database is based on nationwide surveys covering 9 years, providing high generalizability. Third, relevant mechanisms of association between low SES and poor comorbid status were considered. However, due to the cross-sectional nature of the study, causal evidence is lacking between low SES and comorbidity risk. In addition, the study's reach was limited by age stratification, which might have overlooked the relatively less dominant role of SES compared with the aging process in the elderly.

In conclusion, relatively young RA patients with low SES were more susceptible to multiple comorbidities than those with high SES in Korea. Such inequalities of RA-related comorbidities were most prominent in mental, cardiometabolic and musculoskeletal systems. Acknowledging different risks of comorbidities according to SES would be essential to provide optimal care and to improve health outcomes in patients with RA.

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