The assessment of the spondyloarthritis international society concept and criteria for the classification of axial spondyloarthritis and peripheral spondyloarthritis: A critical appraisal for the pediatric rheumatologist

The existence of two sets of criteria has academic and research implications, yet their validation in various populations and clinical scenarios are needed before they would be widely used in clinical practice;

The cost of MRI and HLA-B27 testing may limit the applicability of ASAS criteria in countries with budget restrictions or in segments of the population not covered by any health security system;

Despite the fact that the definition of some variables listed in both axial and peripheral SpA criteria differ from each other to avoid classification overlaps, the existence of different definitions may be confusing (Table 4);

Except for radiographic sacroiliitis, signs and symptoms in the ASAS criteria for SpA refer to active inflammatory and not structural damage; and

Regarding nomenclature, the term “peripheral SpA” may appear to be contradictory in itself and confusing

Is there a rationale for alternative criteria for children with ERA, PsA, and undifferentiated arthritis according to ILAR criteria? ERA and PsA (and perhaps some cases of undifferentiated arthritis) are the subgroups representing juvenile-onset SpA in the ILAR JIA classification criteria for JIA [29]. The concept of ERA and PsA in ILAR JIA classification does not correspond to the traditional concept of SpA because such ILAR criteria precludes the overlapping of inclusion criteria among different subgroups. Thus, the features that link ERA and PsA as SpA are the ones that may be thought to make them incompatible with each other. In contrast, the concept of SpA behind the ASAS axial and peripheral SpA criteria [11, 12] reflects the Moll and Wright’s [27] original idea of clinical overlaps between the different diseases. This author’s opinion is that we need common concepts and criteria if we want to keep juvenile and adult onset types of SpA as a disease continuum. Amor, et al. [37] and ESSG [38] criteria have been validated in children and at least the latter has been used to some extent (Table 5). Today, ASAS axial and peripheral SpA criteria might be a good substitute for Amor, et al. [37] and ESSG [38]criteria and perhaps a good alternative to ILAR criteria [29]. Regarding nomenclature, it seems more appropriate for pediatric rheumatologists to use of the ILAR terms ERA and PsA than the contradictory term “peripheral SpA” since most children and adolescents have peripheral disease and rarely axial symptoms.

If needed, which set of ASAS criteria is more appropriate for children, axial or peripheral? It seems clear that axial and peripheral SpA classifications have different purposes. While the former [11] is intended to identify the spinal and sacroiliac involvement in the early inflammatory stage of AS, the latter [12] relies on peripheral arthritis, enthesitis, and dactylitis as entry criteria (Table 3).Regarding axial involvement, children and adolescents may have both active sacroiliitis on MRI (Figure 3) and radiographic sacroiliitis grade 2 bilateral or grades 2 or 4 unilateral (Figure 4), but in most cases these events occur in association with peripheral arthritis and enthesitis (Figure 5).Axial symptoms, as isolated features, are unusual in youngsters. The ASAS axial SpA criteria suggest the need for a history of back pain for at least three months as entry criteria before performing MRI and/or radiographic studies of the sacroiliac joints. There seems to be no clear clinical rationale to perform MRI studies of the sacroiliac joints and the spine in children in the absence of back pain. Certainly, the logical criteria for children and adolescents is the ASAS peripheral SpA criteria since they include the most important signs and symptoms in patients with juvenile-onset SpA.Except for “good response to NSAIDs”, on that no specific reports in children exist, children and adolescents with juvenile-onset SpA could well fulfill all axial and peripheral ASAS SpA criteria (Table 6). The diagnostic properties of some of these criteria were determined during the validation of the Amor et al. [37] and ESSG [38] classification criteria of SpA [74] and in a comparative study of juvenile-onset AS and u-SpA with JRA [60]. As expected, the sensitivity of back pain in the validation study of SpA according ESSG [38] was very low, but its specificity very high (Table 5). In the latter study, sensitivity, specificity, and + LR of tarsitis and enthesopathy were very high suggesting that tarsitis should be considered an additional criterion in any classification criteria (Figure 6). The frequency of each criterion depends on the classification category. By definition, for example, IBP and radiographic sacroiliitis should be found in all patients with AS, whereas arthritis or enthesitis should be found in all patients with ERA. On the other hand, the definition of each criterion and its diagnostic value should be assessed in children. The question of whether ASAS criteria for axial and peripheral SpA [11, 12] have any role in the classification of children with SpA, ERA, PsA, and even undifferentiated arthritis remains to be determined. Ideally, all related clinical conditions in children and adults should be encompassed under the same criteria in order to facilitate scientific communication and patients transition from childhood to adulthood medical care. From the therapeutic point of view, there should be some advantages if the management of juvenile and adult onset forms could have the same opportunity to be treated in the early inflammatory stage of the disease.