Background Several factors known to increase the risk of NL are at play in AS patients, namely clinical and subclinical bowel inflammation, altered calcium metabolism due to inflammatory cytokines, bone-remodeling and spinal immobility.
Objectives To estimate the rates of nephrolithiasis (NL) in Swedish patients with a diagnosis of ankylosing spondylitis (AS) compared to matched comparator subjects from the general Swedish population.
Methods This is a prospective population-based national cohort study based on linkage of data from The Swedish Patient Register, The Swedish Register of Total Population and Population Changes, The Swedish Cause of Death Register, The Swedish Register of Education and The Swedish Biologics Register (2001-2009).
The rate of NL in AS patients was compared to population comparator subjects matched on age, sex, and date of study entry. Cox regression models were used to calculate Hazard Ratios (HR) with adjustment for comorbidities and anti-TNF treatment.
Results A total of 8,572 AS patients and 39,639 matched general population comparators were included in the study contributing 49,258 person-years (py) respectively 223,985 py. Mean age at study entry was 46 years (inter quartile range 36-56 years) and 65% were male.
The univariate HR of NL in AS patients compared to general population comparators was 2.44 (95%CI 2.09 to 2.84). Predictors of NL within the AS group included anti-TNF treatment (HR 1.6; 95%CI 1.2 to 2.1), prior diagnosis of IBD (HR 2.3; 95%CI 1.7 to 3.3) and prior diagnosis of NL (HR 16.4;95%CI 11.5 to 23.4). After adjustment for these confounders AS patients still had a significantly increased risk of NL diagnosis (HR 2.08; 95%CI 1.77 to 2.44) and for AS patients with anti-TNF treatment this risk was even higher (HR 3.02;95%CI 2.32 to 3.93) when compared to general population subjects. As can be seen in the figure, male sex was associated with a significantly higher risk of NL in both AS patients and general population comparators.
Conclusions The risk of NL in AS patients is more than two-fold increased and seems to correlate with disease severity of AS. NL is suggested as a novel extra-articular manifestation in AS patients. IBD, previous history of NL, and male sex were identified as other predictors of NL in the AS population.
Acknowledgements We are indebted to all colleagues and staff at our university departments, especially Pernilla Nilsson and Jonas Eriksson. This study was supported by grants from Lund University, The Skane region, Österlund and Kock Foundations, and Reumatikerförbundet.
Disclosure of Interest : None declared