In the present study, a low frequency of JPsA but a high frequency of MSK pain was observed among children and adolescents with Pso. MSK pain was associated with greater Pso severity, nail disease, and fatigue and negatively affected HRQoL.
Approximately 35% of the participants complained of recurring joint pain, a higher frequency than the 7% found among healthy children [15]. The number of TP (21% with more than 11) was higher than that observed among healthy children [16]. Although the practice of quantifying TP for the diagnosis of FM is being abandoned, it was useful to show that pediatric patients with severe Pso have reduced pain thresholds. Despite the presence of recurrent MSK pain and the association with fatigue, patients did not have the characteristics of widespread pain and somatic symptoms at a level high enough to warrant the diagnosis of FM, as recommended by the 2016 revision of the ACR criteria, which is valid for individual patient diagnosis [17, 18].
The issue of pain in patients with PsA is somewhat complex. It is known that 9,6–27.2% of adults with PsA have FM associated [19]. However, even without this comorbidity, adults with PsA and children with SpA-related JIA suffer from a high level of pain, whose presence often does not correlate with inflammation or radiographic measures of disease [20, 21]. The interleukin 17 (IL-17), important in PsA immunopathology, is elevated in plasma of patients with FM [22], and studies in mice show that IL-17 plays a role in pain hypersensitivity following neuropathic injury [23]. IL-17 blockade causes rapid and sustained pain relief in PsA [24]. Additional research investigating whether this is due to reduced inflammation or whether it is a direct effect of IL-17 on neuropathic pain is warranted. If children with Pso and MSK pain will progress to PsA, to FM or if the pain will disappear is something that will be verified with longitudinal follow-up of these cases. There is no data in the literature on the subject.
Enthesitis was defined by ILAR as the presence of tenderness at the insertion of a tendon, ligament, joint capsule, or fascia to bone [8]. This condition was observed in 23% of the participants in this study, a frequency higher than that in healthy children, at sites which are specific for patients with juvenile SpA [25]. Nevertheless, the detection of enthesitis via US was low, probably because it was not possible to perform the exam immediately after the initial clinical evaluation. However, physical examination and US conclusions can differ, both in adults and children. Weiss et al. evaluated the characteristics of enthesitis on US and the accuracy of physical examinations for its detection among 30 children with enthesitis-related arthritis (ERA); these authors observed that pain on palpation of entheses had a low positive and negative predictive value for enthesitis as confirmed by US [13].
Although participants in this study presented with complaints of low back pain and pain on palpation of the SIJ, they did not present sacroiliitis on MRI, the gold standard exam for detecting inflammatory sacroiliitis. Weiss et al. evaluated the presence of sacroiliitis among 40 children with early SpA (ERA and JPsA) using MRI as the standard reference [14]. They found that eight patients had active sacroiliitis; of these, only three (38%) reported histories of low back pain or pain on palpation of the joint. The authors concluded that neither the definition of sacroiliitis used by the ILAR (pain on palpation of the SIJ) nor the definition of the Assessment of SpondyloArthritis International Society (ASAS) for the presence of axial involvement in patients with SpA (which requires the presence of back pain) show adequate performance for application to juvenile SpA. The absence of MRI findings in the present study, reinforce this concept and suggests that pain on entheses and SIJ during physical examination is insufficient to define inflammation in these sites among pediatric patients with Pso. It is also possible that these patients present inflammation not detectable by the laboratorial and imaging methods used.
For the definition of the diagnosis of JPsA, complementary exams were considered to ensure greater specificity. It was also followed the concept of PsA in adults, which considers the disease as a spectrum of SpA, with potential axial and enthesitic involvement, since it has already been demonstrated that children and adolescents can have this type of disease presentation [26]. Zisman et al. showed that the use of the Classification criteria for Psoriatic Arthritis (CASPAR) increases the detection of pediatric patients with JPsA who are not being identified by the ILAR criteria [27]. The validation of CASPAR criteria in pediatric rheumatology could be a resource to favor the unification of the PsA classification across age spectrum, would better reflect the pathogenesis of the disease and facilitate its study, and could improve the continuity of care in the transition from childhood to adulthood [28]. Low frequency of JPsA was found in the present study, as expected, since the prevalence of PsA in patients with Pso is lower in children than in adults [4] and the sample size was small.
HRQoL impairment was significantly higher among participants with MSK impairment. The fact that the severity of Pso did not contribute (in the logistic regression model) to a worse HRQoL, only the MSK impairment, highlights the need for a new look at how psoriatic disease affects the life of the pediatric patient. Studies of adults have shown that PsA causes worse HRQoL scores than Pso alone [29]. Children with SpA-related JIA diseases (ERA and JPsA) experience more pain as well as poorer HRQoL than children with other forms of JIA [21]. The core measures recommended by the GRAPPA-OMERACT for PsA studies proved useful for assessing children and adolescents with Pso in a broad and multidimensional manner.
Fatigue was also significantly higher among patients with Pso and MSK pain and a higher NAPSI score predicted greater likelihood of MSK pain. These findings are unprecedented in children with Pso and reinforces the hypothesis that the inflammatory process of psoriatic disease acts systemically even before the development of PsA. Given that nail disease predicts joint disease and can occur years before the establishment of PsA [30], children and adolescents with Pso and nail involvement should be monitored for the development of JPsA, especially in cases with recurrent MSK pain.
The limitations of this study include its small sample size, that limits the extrapolation of the results, and the absence of a healthy control group, which was minimized by comparing patients with minimal cutaneous disease activity to those with moderate to severe disease. The cross-sectional design of this study prevents the observation of case progression, a longitudinal design would be useful for defining predictive factors of JPsA development.
