Current smoking is associated with incident ankylosing spondylitis - The HUNT population-based Norwegian health study

OBJECTIVE: Smoking contributes to progression of ankylosing spondylitis (AS). Because smoking is also a risk factor for incident rheumatoid arthritis (RA) and psoriatic arthritis, our aim was to test whether smoking habits are associated with incident AS. METHODS: Using data from the HUNT health study of the entire adult population of Nord-Trøndelag, Norway, participants in HUNT2 (1995-1997) and HUNT3 (2006-2008) were identified who reported a diagnosis of AS in HUNT3 but not in HUNT2 (n = 107). Incident AS cases were compared with AS-unaffected individuals (n = 35,278) in a case-control design. Participants with RA were excluded. RESULTS: Present smoking was significantly associated with incident self-reported AS in logistic regression adjusted for potential confounders (OR 1.99, 95% CI 1.28-3.11, p = 0.002). Previous smoking (OR 1.15, 95% CI 0.66-2.02, p = 0.62) or total pack-years at HUNT2 (OR 1.01, 95% CI 0.99-1.04, p = 0.21) were not significant. The association with present smoking remained significant in various sensitivity analyses: including only cases with high probability of true AS diagnosis (OR 1.82, 95% CI 1.03-3.19, p = 0.04); including only cases with AS reported more than 3-5 years after HUNT2 (OR 2.34, 95% CI 1.09-5.03, p = 0.029), or including only participants genotyped for HLA-B27 (94 cases and 859 controls) adjusting for genotype (OR 1.79, 95% CI 1.04-2.85, p = 0.033). Hypertension was also significantly associated with incident AS (OR from 1.65 to 2.81). CONCLUSION: In the HUNT population-based study, incident AS was associated with current smoking and hypertension. If verified in further studies, this suggests that smoking should be discouraged in those at a higher AS risk, e.g., with a family history or carrying HLA-B27