Cutaneous lupus erythematosus and the association with systemic lupus erythematosus: a population-based cohort of 1088 patients in Sweden

P>Background Studies reporting the incidence of isolated cutaneous lupus erythematosus (CLE) are rare. Objectives To examine in a population-based cohort study the incidence of CLE and its subsets in Sweden. The short-term probability of receiving an additional diagnosis of systemic lupus erythematosus (SLE) is also assessed. Methods A population-based open cohort study including all patients with CLE [International Classification of Diseases (ICD) code, ICD-10: L93] in Sweden, 2005-2007. Patients (n = 1088) were identified in the Swedish National Patient Register. Results The incidence of CLE was 4 center dot 0/100 000; the female/male ratio was 3 : 1. Mean age at disease onset was 54 years. The most common subset was discoid lupus erythematosus (DLE) (80%, n = 868). A quarter of the patients (24%, n = 260) were already diagnosed with SLE at the time they were diagnosed with CLE. During the whole observation period (2005-2007), an additional 18% (n = 107) were diagnosed with SLE, the probability of receiving an additional SLE diagnosis being highest for the subacute CLE (SCLE) subset. Conclusions This is the first nationwide epidemiological study on CLE. We found the incidence of CLE to be about equal to that of SLE, and found a higher short-term probability for receiving an additional diagnosis with SLE than previously described for CLE. Subsets other than DLE and SCLE were rarely reported in our system; an update of the ICD codes for this diagnostic group could increase reporting of these more unusual cases. Our study clarifies that monitoring and follow-up are called for in this patient group due to the risk for SLE, and underscores the need for clear criteria for risk assessment in the large group of patients with CLE who also fulfil criteria for SLE.