The primary aim of this study was to compare the prevalence of clinical and particularly ultrasonographic signs of enthesitis in patients with psoriatic arthritis (PsA), fibromyalgia (FM), or both. The secondary aim was to assess the impact of FM on disease activity and clinimetric scores.

This single-centre, observational cross-sectional study involved 101 consenting patients: 39 with PsA (CASPAR criteria), 23 with FM (2016 criteria), and 39 with both. Standard PsA and FM clinical, laboratory and clinimetric data were recorded, and entheses were assessed using the Leeds Enthesitis Index (LEI) and the Maastricht Ankylosing Spondylitis Enthesitis Score (MASES). All of the patients underwent B mode (grey scale) and Power Doppler (PD) ultrasonography bilaterally at the insertions of the quadriceps tendons, the proximal and distal patellar tendons, the Achilles tendons, and the plantar fascia insertions of the calcaneus, to evaluate the thickness of entheses, the hypoechogenicity, the presence of bony erosions, the enthesophytes, and the bursitis. The US findings were scored using the Glasgow Ultrasound Enthesitis Scoring System (GUESS). The data were statistically analysed using univariate and multivariate analyses, and receiver-operating characteristic (ROC) curves, concentrating on the shared clinical features of the two condition.

The mean age of the patients as a whole was 53.6±9.47 years. Females accounted for 64.1% of the PsA patients (disease duration 9.13 years), 95.6% of the FM patients (disease duration 5.09 years), and 92.3% of the patients with PsA-FM (disease duration 7.9 years). There were no between-group differences in the patients' body mass index (BMI). In accordance with the study inclusion criteria, none of the FM subjects had PsA or reported any personal or family history of psoriasis. The mean Psoriasis Area and Severity Index was 2.3±3.1 in the PsA group, and 1.2±2.45 in the PsA-FM group. Clinical evidence of enthesopathy was found in 43% of the patients with PsA, 51.3% of those with PsA-FM, and 50.8% of those with FM, while US entheseal abnormalities were detected in respectively 77%, 74% and 35%. The median Bath Ankylosing Spondylitis Disease Activity Index was significantly higher in the patients with PsA-FM than in those with PsA (7.7 [IQR 2.1] vs. 5.0 [IQR 3.8]; p<0.001), as was the median ESR-assessed Ankylosing Spondylitis Disease Activity Score (3.69 [IQR 1.00] vs. 2.82 [IQR 1.55; p=0.004), or CRP- assessed (median 3.27 [IQR 1.07] vs. 2.66 [IQR 1.26]; p=0.006). There was a correlation between GUESS scores and disease duration in the patients with PsA (rho=0.37; p=0.019, 95% CI 0.10-0.61) or PsA-FM (rho=0.38; p=0.016, 95% CI 0.10-0.61), but not in the FM group, and GUESS scores correlated with BMI (rho=0.2; p=0.05, 95% CI 0.00-0.37) and dyslipidemia (rho=0.34; p=0.006, 95% CI 0.11-0.58) in all three groups.

The use of a clinical examination and clinimetric scores alone may overestimate active enthesitis in FM patients. As US was more frequently positive in patients with PsA and PsA-FM than in those with FM, it may be useful in differentiating pain due to enthesitis from entheseal pain due to FM.