Multimorbidity phenotypes in ankylosing spondylitis and their association with disease activity and functional impairment: Data from the prospective study of outcomes in ankylosing spondylitis cohort

Objectives: To examine the association of multimorbidity phenotypes at baseline with disease activity and functional status over time in ankylosing spondylitis (AS). Methods: Patient-reported AS morbidities (comorbidities, N = 28 and extra-musculoskeletal manifestations, EMMs, N = 3) within 3 years of enrollment with a prevalence ≥1 %, were included from the Prospective Study of Outcomes in Ankylosing Spondylitis (PSOAS) cohort. We defined multimorbidity as ≥2 morbidities (MM2+) and substantial multimorbidity as ≥5 morbidities (MM5+). Multimorbidity clusters or phenotypes were identified using K-median clustering. Disease activity (ASDAS-CRP) and functional status (BASFI) measures were collected every 6 months. Generalized estimating equation method was used to examine the associations of multimorbidity counts and multimorbidity clusters with measures of disease activity and functional status over time. Results: Among 1,270 AS patients (9,885 visits) with a median follow-up of 2.9 years (IQ range: 1.0–6.8 years), the prevalence of MM2+ and MM5+ was 49 % and 9 % respectively. We identified five multimorbidity clusters: depression (n = 321, 25 %), hypertension (n = 284, 22 %), uveitis (n = 274, 22 %), no morbidities (n = 238, 19 %), and miscellaneous (n = 153, 12 %). Patients in the depression cluster were more likely to be female and had significantly more morbidities and worse disease activity and functional status compared to those with no morbidities. Conclusion: Approximately 49 % of AS patients in the PSOAS cohort had multimorbidity and five distinct multimorbidity phenotypes were identified. In addition to the number of morbidities, the type of morbidity appears to be important to longitudinal outcomes in AS. The depression cluster was associated with worse disease activity and function.