From April 2020 to October 2022, all patients who came from the Department of Dermatology, West China Hospital, Sichuan University, and met the Classification Criteria for Psoriatic Arthritis (CASPAR) [11] with confirmed PsA diagnosis were enrolled in our PsA cohort, namely the Psoriatic Arthritis cohort of West China Hospital (PARWCH). We conducted this cross-sectional study using baseline data of these patients in the cohort.

The study was approved by the ethics committee of West China Hospital, Sichuan University (approval number 2022(1842)) and performed in accordance with the Helsinki Declaration of 1964, and its later amendments. Each participating patient signed an informed consent form. No identifying information of participants was included in the manuscript.

All patients in the cohort were interviewed and examined in detail, and a variety of data were collected, including demographics, medical histories, laboratory indexes, imaging findings, personal and family histories, comorbidities, extra-articular manifestations, and treatments.

Regarding the medical histories, we focused on the age at onset of skin lesions and arthritis and the disease duration of psoriasis and PsA. The disease duration of PsA means the interval between the onset of musculoskeletal disease and the time of the diagnosis. We calculated the percentage of patients with various onset orders. Co-occurrence of psoriatic lesions and arthritis means that the interval between the two is less than 1 year. Additionally, we divided the skin lesions of patients into plaque, erythrodermic, and pustular subtypes and used the Psoriasis Area and Severity Index (PASI) and Body Surface Area (BSA) to determine the severity of plaque psoriasis. For patients with nail involvement, we divided the nail damage into two groups: nail matrix lesions (pitting, leukonychia red spots in the lunula, and crumbling) and nail bed lesions (onycholysis, splinter hemorrhages, subungual hyperkeratosis, and oil drop) [12].

All patients with PsA in PARWCH were classified into three subtypes, namely peripheral type, axial type, and mixed type. We defined subtypes based on clinical symptoms, physical examination, and imaging manifestations. Patients with PsA who have only axial involvement present with inflammatory neck/back pain as well as limited mobility with radiographic sacroiliitis and typical structural changes in the spine (nonmarginal syndesmophytes, fusion of facet joints, bone marrow, soft tissue edema, etc.) which could be detected by CT and MRI [13]. Patients with PsA who have only peripheral involvement frequently exhibit swelling, pain, and stiffness in the peripheral joints, and some patients may show sausage digit (dactylitis), with abnormalities such as synovitis, enthesitis, and bone erosion detected by ultrasound [2]. Patients with mixed subtype have both peripheral and axial involvement. For patients with peripheral arthritis, we performed detailed physical examinations and recorded 66/68-swollen and tender joint counts (SJC66/TJC68). Furthermore, we also recorded the visual analog scale (VAS) scores of pain, patient’s and physician’s global assessment of disease activity, disability index of the health assessment questionnaire (HAQ), and acute phase reactant, including high-sensitivity C-reactive protein (hs-CRP) and erythrocyte sedimentation rate (ESR), which are necessary for the American College of Rheumatology (ACR) scoring system [14]. The Leeds enthesitis index (LEI) and Leeds dactylitis index (LDI) were used to evaluate the conditions of enthesitis and dactylitis in these patients, respectively. Moreover, to characterize the types of peripheral joint lesions, we performed ultrasound examinations for each patient in our cohort. The operations were conducted by three sonographers with more than 5 years of experience in musculoskeletal ultrasound imaging according to the guidelines for musculoskeletal ultrasound examination jointly developed by the American Society of Ultrasound in Medicine (AIUM) [15]. The sonographers examined all target joints, tendons, and bursae in grayscale mode and examined blood flow signals in PD mode, focusing on the changes in synovitis, osteophytes, joint effusion, enthesitis, tenosynovitis, bone erosions, bursitis, and dactylitis in these patients with PsA [16]. For patients with axial arthritis, variables associated with axial activity, including Bath Ankylosing Spondylitis Functional Index (BASFI), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and VAS scores of back pains, were assessed and recorded.

Regarding personal and family histories, we gathered information on previous smoking, drinking, surgeries, trauma, infections, and family history of psoriasis and PsA. Moreover, comorbidities and extra-articular manifestations, including hypertension, type 2 diabetes, cardiovascular disease, inflammatory bowel disease, uveitis, fatty liver disease, anxiety, and depression, were also recorded. With respect to treatment, we collected the patients’ previous treatment histories as well as current treatment options, focusing on methotrexate, nonsteroidal anti-inflammatory drugs (NSAIDs), traditional Chinese medicine (TCM), biologicals (interleukin (IL)-17 inhibitors; tumor necrosis factor alpha (TNFα) inhibitors, IL-23 inhibitors), and Janus kinase inhibitors (Jaki).

We performed descriptive statistical analyses of all data. Count (%) was used to express categorical data, and the mean (SD) was used to represent continuous variables. The analyses were performed using R (version 4.0.2), and the ggplot2 package in R was used for plotting.