Asthma in patients with psoriasis

DEAR EDITOR, We read with interest the report by Fang et al. of the relationship between psoriasis and asthma in a large retrospective case–control study from Taiwan. The study found a 1 38-fold increased risk of asthma among patients with psoriasis, with an increasing risk according to higher age of the patients. However, although adjustment for age, sex and selected comorbidities was performed, the study lacked important confounder control, particularly for smoking and body mass index. We studied the association between self-reported asthma and psoriasis in a previously well-described population of 34 781 Danish twins, aged 20–71 years, from the nationwide Danish Twin Registry. Psoriasis was identified by the question, ‘Has a doctor ever told you that you have, or have had psoriasis? ’, whereas asthma was identified by the question, ‘Do you have, or have you ever had, asthma? ’ The prevalences of psoriasis and asthma were 4 2% and 8 7%, respectively. In accordance with the study by Fang et al., we found an increased prevalence of asthma in individuals with psoriasis (10 9% vs. 8 5%), odds ratio (OR) 1 32 [95% confidence interval (CI) 1 11–1 57, P = 0 002], which remained statistically significant after adjustment for sex, age, smoking, body mass index and hospital-diagnosed chronic obstructive pulmonary disease (COPD) (OR 1 27, 95% CI 1 06–1 54; P = 0 011) (Table 1). However, adjusting also for selfreported atopic dermatitis diluted the association further (OR 1 20, 95% CI 0 99–1 45; P = 0 064), possibly due to the strong association between atopic dermatitis and asthma and/ or to recall bias and diagnostic mix-up between atopic dermatitis/other eczemas and psoriasis. Similarly, in line with the study from Taiwan, we found a higher adjusted risk of asthma in the older age group (50– 71 years) (OR 1 61, 95% CI 1 21–2 15; P = 0 001) compared with the younger age group (20–49 years) (OR 1 09, 95% CI 0 85–1 40; P = 0 51; P-value for different risks = 0 021). Furthermore, we found that the association between psoriasis and asthma was due predominantly to an increased risk of nonatopic asthma (adjusted OR 1 47, 95% CI 1 12–1 92; P = 0 005), and not atopic asthma (OR 1 19, 95% CI 0 93– 1 52; P = 0 18), using self-reported allergy to pollen, pets or dust as a proxy for atopic asthma. Finally, we found that the association between psoriasis and asthma was observed predominantly for adult-onset asthma (age at onset of asthma ≥ 18 years of age) (adjusted OR 1 40, 95% CI 1 10–1 79; P = 0 006), and not childhood-onset asthma (OR 1 06, 95% CI 0 78–1 44; P = 0 72). We also identified patients with psoriasis through the Danish National Patient Registry [International Classification of Diseases, 8th revision (ICD-8) codes 68693-4, 69609-10, 69609-19 and 69609-99; and ICD-10 code L40 0-9]. This registry records all hospitalizations and outpatient visits in Denmark. The prevalence of psoriasis by this definition was 0 6%. The risk of asthma was increased among individuals with hospital-diagnosed psoriasis compared with individuals without hospital-diagnosed psoriasis (12 5% vs. 8 7%) (OR 1 50, 95% CI 0 98–2 28; P = 0 058). This was observed particularly among the older age group (50–71 years; OR 2 12, 95% CI 1 22–3 68; P = 0 007), but less so among the younger age group (20–49 years; OR 1 07, 95% CI 0 55–2 05; P = 0 85). The risk of asthma remained statistically significantly increased in the older age group after adjustment for