Background/Purpose: Development of comorbidity over time in patients with systemic lupus erythematosus (SLE) is not well characterized. To provide a detailed and comprehensive picture hereof, we estimated the incidence of frequent Charlson, Elixhauser and SLICC-ACR damage index (SDI) comorbidities following diagnosis in a large inception cohort of patients with SLE compared with matched population controls.

Methods: All patients aged 18 + years at first time registration  with SLE International Classification of Diseases (ICD) codes in the Danish National Patient Register (DNPR) from January 1, 1996 to July 31, 2018 and no SLE ICD codes prior to 1996 were  included (incident cases, n=3178). The date of first SLE registration was set as index date and baseline for follow-up. For each SLE case, 19 general population comparators were matched by gender and age at index date using the Danish Civil Registration System   (n=60090). ICD-coded diagnoses during outpatient and inpatient care were retrieved from the DNPR and DNPR-Psychiatry (for psychosis and depression). For osteoporosis, hypertension and diabetes, we used first date of ICD codes in the DNPR or date of first filled prescription of medication for these conditions (National Register of Medicinal Product Statistics).
We identified prevalent comorbidities at baseline and excluded these individuals from analyses of incident events. For comorbidities and mortality, incidence rates (IRs) per 1000 person-years and adjusted incidence rate ratios (IRRs) with corresponding 95% confidence intervals (95% CI) were estimated during time intervals (0-1, 1-2, 2-5, 5-10 years) after index date. Poisson regression estimated IRR adjusted for age at index date and gender.

Results: 84% of patients with SLE and general population comparators were female; mean age at baseline was 47 years. Most comorbidities studied occurred more frequently during follow up in SLE patients compared with general population controls (Figure); nearly every 95% CI for these IRRs excluded the null. For most comorbidities and mortality, adjusted IRRs were highest in the first year of follow-up and decreased over time. The highest first-year IRRs (range: 15-56) were seen for typical features of SLE, including coagulopathy, renal disease, polyneuropathy and pulmonary circulation disease. Osteoporosis also occurred frequently during the first years of SLE. The IRRs for depression were consistently increased to about 2.0 during the whole follow-up. The highest IRRs during 5-10 years of follow-up were seen for renal disease, coagulopathy and osteoporosis.

Conclusion: This study provides a unique and comprehensive overview of the occurrence and timing of comorbidity in a nationwide, incident cohort of adult Danish SLE patients. We saw the highest comorbidity rates during the first year of follow-up, which may be due to features of SLE disease presentation as well as increased surveillance. However, compared to the general population the patients also demonstrated consistently increased rates of a broad range of comorbidities featured by the SDI, Charlson and Elixhauser indices. These data may prove valuable for upcoming works on evaluating comorbidity and damage accrual in patients with SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/development-of-comorbidity-in-danish-nationwide-cohort-of-newly-diagnosed-patients-with-systemic-lupus-erythematosus/