To our knowledge, this is one of the first observational study assessing the clinical characteristics of potentially severe PsA [12]. Although there is no consensus on the definition of severity, this study may contribute to this intriguing topic, which is considered an unmet need in clinical practice. As PsA may show with various clinical manifestations, it could be important to define the severity of disease in each clinical domain. However, our study confirmed that the majority of patients fulfilling the definition had severe disease based on the peripheral joint involvement while, for instance, entheseal involvement was less frequently represented. Moreover, there is the clinical need to distinguish the concept of disease activity from disease severity and this study tried to address this intriguing point.

In our group, at baseline, severe patients with PsA tend to have higher disease duration (not statistically significant) and to have statistically significant higher values of PtGA, PtPvN, PGA, tender joint count (TJC), swollen joint count (SJC), C-reactive protein (CRP), and LEI. Furthermore, the rate of patients with dactylitis and the median PASI tend to be higher in the severe PsA but the difference was not statistically significant. Male/female ratio was also not statistically significant. However, at last follow-up, no females satisfied the criteria for severe PsA. This is quite interesting because generally, female patients with PsA tend to have higher disease activity, worse scores in the outcome measures, and reduced response and persistence to advanced biologic treatments in respect to male sex [26]. This latter result could be helpful in distinguishing the concept of severity from that of severe disease activity, which is higher in the female sex. The definition of severe PsA by mCPDAI is mainly based on the clinical evaluation of patients and on HAQ-DI. This may explain this apparent discrepancy since disease activity scores and outcome measures such as DAPSA and MDA contains “patient-driven” domains such as PtGA and pain that may influence disease activity, but the mCPDAI did not. Our results are in keeping with other studies in axial spondyloarthritis in which male sex is associated with a more severe disease course [27] and further corroborate the hypothesis that severity and disease activity are different concepts.

Generally, after a median of 2 years of follow-up and treatment, the rate of patients with severe disease dropped from 36 to 10%, which means that severity, assessed by using this instrument, could be influenced by disease activity, as well as some patients remained severe (10%) even at 2 years of advanced treatment. However, as suggested by the results summarized in Table 3, severity and disease activity are not interchangeable. In fact, the agreement between the definition of severity and the absence of MDA or low disease activity is slight. This, in turn, supports the concept that severity is not totally aligned with disease activity and that the two indices intercept patients with different characteristics. Indeed, our results showed that mCPDAI could be considered a “surrogate” of severity; in the meantime further research on this topic will be able to validate a specific instrument for severity.

We are aware that mCPDAI as a candidate index may be weak, but to our knowledge, no formal instrument to assess severity has been published. Furthermore, mCPDAI could identify reversible severity (in fact, in our study the rate of patients satisfying the proposed criteria dropped from 36 to 10%) and, hopefully, further study will identify irreversible severity, which could be linked to joint damage, radiographic progression, and to the development of disease-related complications.

A very recent study was published on the efficacy of the anti-IL-17A ixekizumab in patients with severe peripheral PsA. In this post hoc analysis of the SPIRIT-P1, the authors adopted the same definition of severe PsA using the mCPDAI and showed an improvement in joint and skin symptoms in patients with severe disease treated with ixekizumab. Looking at the population with severe PsA, generally, the disease activity indices (in terms of TJC, SJC, LEI, and skin involvement) were worse in respect to our findings, but it is not surprising, due to the intrinsic nature of the enrolment criteria in randomized controlled trials. However, similar findings in terms of HAQ-DI, perception of pain, and disease activity by patients are present [28].

In our study, predictors of severe PsA were male sex and the extension of skin involvement. Concerning male sex, different studies showed that men assembling more peripheral and axial joint impairment have higher scores of functional disabilities, leading to a more severe disease course [29]. This could explain our findings. However, although we perform a multivariable analysis, we have to say that the number of severe patients at last follow-up was low. This in turn might lead to a careful interpretation of our results.

Recently, Queiro et al. defined the presence of severe PsA as fulfilment of at least 1 of the following criteria: treatment with DMARDs, HAQ > 0.5, polyarthritis. Interestingly, they found that over 70% of their patients with PsA can be classified as severe. However, this definition is less stringent in respect to our definition. Moreover, in this study, several factors were associated with severe disease, including pain, localization of psoriasis, and clinical form at diagnosis, while no mention of male sex was reported [12].

Our study has some limitations. First, data on the radiographic progression were not assessed. This could be an important aspect to be addressed since more severe patients with PsA may have to show more radiographic damage and further studies are needed. Second, we stratified patients based on mCPDAI, a clinical index that was initially structured to evaluate disease activity, as stated above. Third, the study lacks data on the clinical form of the disease and information on the so-called malignant location of the disease such as a severe involvement of the hip joint, which should be taken into account when assessing the severity. Finally, overall, we reported low prevalence of enthesitis in our group (defined with the LEI, which explores only six sites) and this may have an impact on the results.

In conclusion, our study showed that severe patients with PsA had more disease activity, pain, and impact of disease than non-severe patients. They also tend to have higher disease activity, pain, and impact of disease during the course of follow-up. Furthermore, we demonstrated that severity and disease activity in PsA are not interchangeable concepts, and open the way for further studies in this field.