To the best of our knowledge, this was the first systematic analysis of a single-center cohort of axial and peripheral ERA studies in a Chinese population. The study described and compared the clinical characteristics and treatments of patients with axial and those with peripheral ERA. Patients with axial ERA tended to be older, have a longer delay in diagnosis, and exhibit significantly higher levels of inflammatory markers. There were no clear differences in active peripheral joint distribution, except for increased hip arthritis in the axial ERA group. Additionally, patients with axial ERA required more frequent and longer treatment with biologics. These findings highlight the importance of early recognition and identification of axial involvement in ERA.
All of our patients with enthesitis-related, systemic, oligo-, poly-, psoriatic, and undifferentiated JIA were reclassified according to the corresponding ILAR category [1]. Similarly to the previously reported distribution of JIA category in China, ERA was the most common type of JIA in our study cohort, with a prevalence of 34.88% [13]. However, studies from North America or Europe have reported that the most prevalent category of JIA is oligoarthritis, whereas ERA accounts for only 10–16% of all JIA cases [14–16]. The fact that the epidemiology of JIA in a multiethnic cohort has also shown significant differences in the distribution of JIA subtypes among ethnic groups could explain the different range of values that have been reported [17].
ERA is one of the seven JIA subtypes classified by the ILAR with unique characteristics, including male predominance with later onset, association with HLA-B27, enthesitis, and axial skeleton involvement, in addition to peripheral joint involvement [1]. One pitfall of the ERA category is that it cannot distinguish axial from peripheral phenotypes [8]. The ineffectiveness of DMARDs in axial disease contributes to the overall poor prognosis for patients with axial ERA [18, 19]. The efficacy of conventional DMARDs on axial disease appears to be less than that on peripheral disease. Increased utilization of MRI has allowed early detection of axial disease, which is sometimes asymptomatic [20–23]. Anti-TNF agents have been reported to be successful in clinically improving the signs and symptoms of the disease [23, 24]. Biologic therapy has variable effects on spinal radiographic progression in patients with axial disease [25]. To achieve a therapeutic response and prevent progressive damage, the axial phenotype of ERA may require more aggressive treatments than the peripheral phenotype.
The most commonly used anti-TNF agents for ERA are etanercept and adalimumab [26, 27]. In our study, more anti-TNF drugs were used in the axial ERA group than in peripheral ERA group from disease onset to the 24th month of follow-up. TNF inhibitors have been an important treatment choice for patients with active axial ERA [25]. A review recently published on this topic determined that no significant difference in spinal radiographic progression was apparent between patients receiving and not receiving TNF inhibitors over the first 2 years; however, after 2 years, a potential protective effect of TNF inhibitors treatment was observed [28]. Although initiated at an early stage in axial disease, the application of biologics still increased gradually over an 18-month period and remained clinically high within the 2-year follow-up. The proportion of biologics used in the peripheral ERA group peaked at 6 months and began to decline thereafter. Furthermore, recently, the American College of Rheumatology has recommended not to use MTX as a monotherapy for children with sacroiliitis [29]. Therefore, treatment for patients with axial ERA should be better tailored, including careful follow-up for disease progression [22].
In the past, patients with ERA were considered to develop axial involvement only after long periods of inflammation [8]. However, in our study, axial involvement occurred much earlier. According to a recent study, the Assessment in Spondyloarthritis International Society classification criteria for peripheral spondyloarthritis were the most sensitive criteria for classifying patients with ERA and for axial spondyloarthritis criteria, which may aid early detection of axial involvement [30]. During the early stages of the disease, more than half of our patients had axial joint involvement. The reason for this high proportion of patients with axial ERA is that more than one-third of patients with asymptomatic axial ERA were identified and confirmed by available imaging methods [31]. Our study confirmed a high rate of positive MRI findings in such patients at disease onset. Inflammatory and/or erosive lesions of the thoracolumbar spine along with sacroiliitis may exist in the early stage, regardless of the presence of symptoms. Clinical assessment of axial involvement, unlike that of peripheral involvement, can be challenging in the absence of symptoms. Application of MRI for suspected axial involvement may be worth considering for ERA, as this may play an important role in preventing the under-determination of axial involvement in patients [20–22].
In the present study, there was an average of 10 months of diagnostic delay for patients with axial ERA, which was longer than that for patients with peripheral ERA. Diagnostic delay always contributes to poor radiographic and functional outcome. Moreover, in our study, the axial ERA group was associated with higher ESR levels and more hip joint involvement than was the peripheral ERA group. Elevated ESR is a marker of disease severity and can be used to predict the development of sacroiliac arthritis [7]. Several epidemiological studies of ankylosing spondylitis have reported extra-articular manifestations as the consequences of persistently high levels of inflammation [32].
Hip involvement is a poor prognostic factor for ERA, which increases the risk of sacroiliac arthritis [33–35]. These various factors could explain why nearly half of our patients with axial ERA were prescribed biological therapy at disease onset. A longer diagnosis delay was correlated with a poor radiographic finding and a higher chance of biological therapy, underscoring the “window of opportunity” concept for JIA treatment. Furthermore, several studies have reported a lack of efficacy of DMARDs in the treatment of axial disease [18–19]. The axial subtype of ERA is associated with a high mutilation rate and may require an aggressive management approach to achieve a therapeutic response. The lack of recognition of early axial inflammation may lead to delays in provision of appropriate treatment.
Uveitis is a common extra-articular manifestation in patients with ERA. Although the early-onset inflammation is limited to the anterior portion of the eye, the progression of chronic active inflammation may eventually cause significant damage to the posterior pole [36]. Acute anterior uveitis has been reported to be associated with HLA-B27 and ANA [37]. We found a significant increase in the risk for development of uveitis in our patients with ERA who were ANA-positive and female. Interestingly, despite not finding differences in HLA-B27 positivity, ANA positivity, or uveitis incidence between the groups, all five patients with uveitis in this study were both males and HLA-B27-positive. Meanwhile, none of the 18 ANA-positive patients with ERA developed uveitis. This might reflect that ANA has little correlation with uveitis in ERA. The disease characteristics of ERA-associated uveitis are assumed to be different from those of oligoarthritis-associated uveitis [37]. The predominance of male sex and HLA-B27 positivity among patients with uveitis are distinguishing features of ERA, in contrast to those of patients with oligo-related uveitis, which is seen more frequently in female patients, in which ANA positivity predominates and HLA-B27 is not an independent risk factor.