Exploring the unifying concept of Spondyloarthritis: a latent class analysis of the REGISPONSER registry.

OBJECTIVES: The aim of our study is to identify the potential distinct phenotypes within a broad Spondyloarthritis (SpA) population.

METHODS: We conducted a cross-sectional study using the REGISPONSER registry with data from 31 specialist centres in Spain including patients with SpA who fulfilled the European Spondyloarthropathy Study Group (ESSG) criteria. A latent class analysis (LCA) was performed to identify the latent classes underlying SpA according to a set of predefined clinical and radiographic features, independently of expert opinion.

RESULTS: In a population of 2319 SpA patients, a 5 classes LCA model yielded the best fit. Classes named 'axial with spine involvement' and 'axial with isolated SIJ involvement" show a primarily axial SpA phenotype defined by inflammatory back pain and high HLA-B27 prevalence. Patients in class 'axial + peripheral' show similar distribution of manifest variables to previous classes but also have a higher likelihood of peripheral involvement (peripheral arthritis/dactylitis) and enthesitis, therefore representing a mixed (axial and peripheral) subtype. Classes 'Peripheral + psoriasis' and 'Axial + peripheral + psoriasis' are indicative of peripheral SpA (and/or PsA) with high likelihood of psoriasis, peripheral involvement, dactylitis, nail disease, and low HLA-B27 prevalence, while class 'Axial + peripheral + psoriasis' also exhibits increased probability of axial involvement both clinically and radiologically.

CONCLUSION: The identification of 5 latent classes in the REGISPONSER registry with significant overlap between axial and peripheral phenotypes is concordant with a unifying concept of SpA. Psoriasis and related features (nail disease and dactylitis) influence the phenotype of both axial and peripheral manifestations.