Background. Involvement of the axial skeleton (of sacroiliac joints and / or spine) is one of the frequent manifestations associated with psoriatic skin disease along with involvement of peripheral musculoskeletal structures (peripheral arthritis, enthesitis, dactylitis), which are usually referred to as psoriatic arthritis (PsA). Depending on the definition used, the prevalence of axial disease varies from 25% to 70% of patients with PsA. There is an urgent need for an evidence-based definition and a unified and widely accepted nomenclature for axial involvement in PsA that would allow defining a homogeneous subgroup of patients in the heterogeneous PsA population.

Aim and study objectives. To develop a data-driven definition of axial involvement in PsA based on cross-sectional evaluation of patients with a diagnosis of PsA fulfilling the CASPAR criteria. The study objectives are: to determine the prevalence of axial involvement in a cohort of patients with PsA; to identify the frequency of imaging findings suggestive of inflammatory involvement of axial skeleton in PsA; to identify factors (clinical, lab, imaging) associated with the presence of axial involvement in PsA.

Study design and study population. This is a multinational multicenter cross-sectional study in patients with a definite diagnosis of PsA. The population of interest will consist of 400 adult patients diagnosed with PsA and fulfilling the classification criteria for PsA and not receiving biological or targeted synthetic disease-modifying antirheumatic drugs (b or tsDMARDs). Participating rheumatologists are encouraged to include consecutive PsA patients not treated with biologic or a targeted synthetic disease-modifying antirheumatic drug because of their potential impact on active inflammatory changes in the axial skeleton, which will be the focus of the current study. Patients will be recruited prospectively in selected study centres and will undergo study-related examinations including imaging of the axial skeleton (X-rays and magnetic resonance imaging). Collected data will serve as a basis for the judgement on the presence or absence of axial involvement by the local investigator and, independently, by the central study committee.

Analysis and study outcomes. The prevalence of axial involvement in a cohort of patients with PsA according to the local and central assessment will be calculated in PsA patients presenting with peripheral involvement (the primary analysis population) as well as in the entire group. The prevalence of axial involvement as assessed by the local investigator in PsA patients presenting with peripheral involvement will be the primary outcome of the study. The agreement between local and central judgements will be analyzed. A sub-analysis of a group presenting without peripheral involvement (i.e., pure axial disease) will be performed. The frequency of imaging findings (on MRI and X-rays) suggestive of inflammatory involvement of axial skeleton (sacroiliac joints and spine) in PsA according to the local and central assessment will be calculated. The agreement between local and central judgements will be analyzed. Parameters associated with the presence of axial involvement in PsA will be explored as well. The diagnostic / classification value of each parameter will be evaluated.