Declining Incidence of Systemic Lupus Erythematosus in Norway 1999-2017: Data From a Population Cohort Identified by International Classification of Diseases, 10th Revision Code and Verified by Classification

OBJECTIVE

The goal of this study was to provide complete, robust data on annual systemic lupus erythematosus (SLE) incidence rates over nearly two decades from the Southeast Norway area (2.9 million inhabitants) and assess accuracy of SLE-specific International Classification of Diseases (ICD) codes for SLE diagnosis.

METHODS

From administrative databases, we identified all cases International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) coded as SLE during 1999 through 2017 in Southeast Norway. We manually reviewed the chart of every case ICD-10 coded as SLE to either confirm or reject SLE diagnosis. Using SLE classification criteria, we classified all cases with confirmed SLE. We estimated annual incidence rates of classified SLE, and subsets, defined by age at diagnosis, sex, and parental country of birth. The chi-square test was applied for linear time-trend analyses of incidence.

RESULTS

Among the 3,488 cases ICD-10 coded as SLE, chart reviews confirmed SLE diagnosis in 1,558 (45%), of which 797 had new-onset disease during 1999 through 2017. Annual SLE incidence rates fell during 1999 to 2017. The fall was most pronounced in female persons 50 to 59 years old at diagnosis, in whom incidence fell from 3.4 to 1.1 per 100,000 persons (P trend < 0.001). Concurrent ecological data from the study area showed a 74% reduction in prescriptions of menopausal hormone treatment. Accuracy of ICD-10 codes for incident SLE diagnosis was acceptable in juveniles and young adults (up to 20 years) but otherwise low.

CONCLUSION

In a presumably complete population-based cohort, we identified decreasing incidence of SLE, especially among female persons 50 to 59 years old. Although reasons for declining incidence are not clear, ecological data indicate a possible role of environmental factors, for example, menopausal hormone treatments.

Persisting mortality gap in systemic lupus erythematosus; a population-based study on juvenile- and adult-onset SLE in Norway 1999-2022

OBJECTIVE

To estimate mortality and survival rates of systemic lupus erythematosus (SLE) in a contemporary, population-based setting and assess potential influences by time, sex, ethnicity, classification criteria and age at diagnosis.

METHODS

We assessed mortality and survival in the Nor-SLE cohort, which includes all chart-review confirmed SLE cases resident in Southeast Norway (population 2.9 million) 1999-2017. Study end was at death, emigration, or 1 October 2022. We defined juvenile SLE by age <16 years at diagnosis. For standardized mortality rate (SMR) estimates, we applied 15 population controls per case, all matched for age, sex, residency, and ethnicity. We analyzed survival by Kaplan-Meier and risk factors by cox regression.

RESULTS

The Nor-SLE cohort included 1558 SLE cases, of whom 749 were incident and met the 2019 European Alliance of Associations for Rheumatology and American College of Rheumatology (2019-EA) classification criteria. SMR was increased to 1.8 (95% CI 1.6-2.2) in incident adult-onset SLE but did not differ between females and males. Survival rates at 5-, 10-, 15 and 20-years were lower in incident adult-onset SLE than in matched controls. In multivariable analysis, lupus nephritis associated with decreased survival, while sex did not. Separate, long-term mortality analyses in the total Nor-SLE cohort showed that SMR peaked at 7.2 (95% CI 3.3-14) in juvenile-onset SLE (n = 93) and fell gradually by increasing age at SLE diagnosis.

CONCLUSION

This study shows persistence of a mortality gap between adult-onset SLE and controls at population level and provides indications of worryingly high mortality in juvenile-onset SLE.

POS1416 LONG-TERM OUTCOME OF SYSTEMIC LUPUS ERYTHEMATOSUS (SLE); DATA FROM THE LARGE POPULATION-BASED SOUTHEAST SLE COHORT (Nor-SLE)

Background

Population-based studies on Systemic Lupus Erythematosus (SLE) patients with a verified diagnosis is considered the gold standard to find true outcomes in SLE, but few population-based SLE cohorts have follow-up over 15 years [1]. Norway is among the few countries worldwide where social and structural factors facilitate the gathering of complete population-based cohorts in rare disease like SLE due to its healthcare organization.

Objectives

To examine long-term outcome of SLE in a population-based setting and determine if immediate cause of death differs between SLE patients and the general population.

Methods

The study included all SLE patients who were resident in the Southeast region of Norway during 1999 - 2017 and met the 1997 American College of Rheumatology classification criteria for SLE. All SLE diagnosis was confirmed by chart review. SLE patients and 15 controls for each case (matched by age, gender and ethnicity) were linked to the Norwegian Cause of Death Registry. We examined survival by means of Kaplan-Meyer estimates and used log rank test to test for differences. To estimate risk of death, we performed calculations of standard mortality rate (SMR) by dividing the number of deaths on the number of years observed. The excepted number of deaths referred to the number of deaths for the matched control group. All SLE cases were included in SMR. The 95 % confidence interval (CI) of SMR was calculated with Mid-P exact test. We defined immediate cause of death as the final event directly leading to death. An International Classification of Diseases 10th revision code of I00-99 or R96 classified as death from cardiovascular disease (CVD) (except pulmonary embolism and cerebral bleeding) and of infections A00-B99, J10-18, N39, M86 or U07.

Results

We identified 1298 SLE patients in the region, of whom 673 was incident cases; all captures within one year from diagnosis. Of the incident cases, 76 (11%) died during 8434 years of follow-up (Table 1). The five-, ten-, 15- and 20-year survival for incident SLE patients (controls) was respectively 98 (98), 94 (96), 87 (94) and 82 (88) % and differed significantly first after ten years of disease duration compared to controls. Figure 1 shows 20-year survival for incident SLE patients and matched controls; stratified by gender. SMR for all SLE cases was 2.3 (95 % CI 1.5. - 4.0); female SLE 2.5 (95 % CI 1.6 – 3.9) and male SLE 1.9 (95 % CI 1.3 – 2.2). The most common immediate cause of death in SLE patients was CVD; whereof myocardial infarction (21 %) was most frequent. SLE patients died more often of CVD than controls (29 % vs. 21 %, p = 0.01) and had a tendency to more infections (23 % vs. 18 %, p = 0.07), whereof pneumonia (58 %) was most frequent.

Table 1.

Patient demographics, follow-up time and number of deaths in the total Systemic Lupus Erythematosus (SLE) cohort and in incident SLE patients.

Total SLE cohortIncident SLEFemaleMalen = 1298n = 577n = 96Of European descent, n (%)1140 (88)472 (82)86 (90)Juvenile onseta, n (%)93 (7)31 (5)6 (6)LNb, n (%)470 (36)177 (30)49 (51)Cumulative ACR criteriac, µ (SD)5.4 (1.2)5.3 (1.2)5.1(1.1)Follow-up years, total1925261601217Deaths, n (%)282 (23)54 (9)22 (23)Age at diagnosis, years µ (SD)35.5 (15.7)37.4 (15.6)44 (17.9)Disease duration at death, years µ (SD)20.4 (12.5)9.6 (5.8)10.6 (10.5)

µ: mean, n: number, SD: standard deviation

a Diagnosed before age of 16 b Lupus Nephritis defined by 1999 American College of Rheumatology classification criteria for Systemic Lupus Erythematosus c1997 American Collee of Rheumatology classification criteria for Systemic Lupus Erythematosus

Conclusion

Mortality in SLE is substantially increased. Differences in survival compared to the general population only appear after ten years of disease duration. CVD was the most common immediate cause of death and more frequent in SLE patients.

References

[1]Reppe Moe, S., Haukeland, H., Molberg, Ø., & Lerang, K. (2021). Long-Term Outcome in Systemic Lupus Erythematosus; Knowledge from Population-Based Cohorts. J Clin Med, 10(19). doi:10.3390/jcm10194306

Disclosure of Interests

None declared