Recognizing SpA in children, particularly early in the course of disease, presents a unique set of challenges. The signs and symptoms at disease onset differ from those seen in adults, with inflammatory back pain being less prominent, reflecting the infrequent involvement of the sacroiliac and other vertebral joints in juvenile disease.20
By contrast, hip and peripheral arthritis, together with enthesitis, are common presenting features in children. As a consequence, juvenile SpA might be missed or confused with other forms of juvenile arthritis. In recognition of these differences, Rosenberg and Petty proposed criteria for a syndrome of seronegative enthesopathy and arthropathy (SEA), including the presence of enthesitis with arthralgia or arthritis, the absence of RF and anti-nuclear antibodies, and symptom onset before 17 years of age.21
SEA provided a category that recognized undifferentiated juvenile SpA in the absence of axial symptoms, and thus became a useful means to distinguish this syndrome from JRA as it was defined at the time (1980s) by ACR criteria.
In the mid-1990s, the ILAR classification criteria were proposed, using the umbrella term JIA.6,7
The primary intent of these criteria was “to delineate, for research purposes, relatively homogeneous, mutually exclusive categories of idiopathic childhood arthritis based on predominant clinical and laboratory features”.1
Six categories of JIA were proposed, with a seventh (originally called “undefined arthritis”, now referred to as “undifferentiated arthritis”) applied if the criteria for other categories were either not met or did not enable unambiguous classification. The practical application of these criteria, however, identified a large number of children as having ‘undefined arthritis’,22,23
prompting revision and further clarification of the categories ().1
ILAR classification for JIA1
Under the revised ILAR classification system published in 2004 (), juvenile SpA is largely categorized in one of three ways: as ERA, psoriatic arthritis (PsA), or undifferentiated arthritis (if features of both ERA and PsA are present). The definition of ERA is based on criteria similar to those of SEA, but applied differently and with some important distinctions. Coexisting arthritis and enthesitis are sufficient for the classification of ERA, provided other exclusions are absent. If the patient does not have both arthritis and enthesitis, then additional features suggestive of SpA must be present (). By contrast, enthesitis without arthritis would be sufficient for the classification of SEA. Other major differences include an upper age limit of 16 for ERA rather than 17 for SEA, and how psoriasis influences the classification. In the ILAR system, the presence of psoriasis in the patient or a first-degree relative is an exclusion criterion for ERA and all other subgroups of JIA except PsA. A classification of PsA is considered if the patient has both arthritis and psoriasis, or has arthritis plus at least two of dactylitis, characteristic nail changes and a first-degree relative with psoriasis ().
One could argue that PsA, as defined by the ILAR criteria, is not SpA. Recent studies suggest that ILAR-defined psoriatic and nonpsoriatic JIA can have similar clinical features, with the main differences being an increased frequency of dactylitis and nail pitting (in addition to the presence of psoriasis) in PsA.24,25
One study revealed small differences in outcomes, with juvenile PsA resulting in poorer physical health and more pain than oligoarticular or polyarticular JIA,25
and axial involvement was either absent25
or not specifically noted in psoriatic JIA.24
By contrast, application of the Vancouver criteria for juvenile PsA,26
which pre-date the ILAR classification and are more inclusive,1,7
reveals two distinct populations of patients: a young group (age at onset <5 years) with a disease that resembles JIA27
and is similar to ILAR-defined PsA, and an older group (age at onset ≥5 years) with a tendency to have enthesitis, axial arthritis and persistent oligoarthritis. Patients in the latter group are often excluded from a classification of ILAR-defined PsA (for example, because they are male and HLA-B27 positive, and/or have features of, or a family history of, SpA), and are instead classified as having undifferentiated arthritis ().28
The question of how to best define PsA in children, and in particular whether one should subdivide oligoarticular JIA and polyarticular JIA according to the presence or absence of psoriasis in the patient or their family, is an area of active debate.29
This issue extends to juvenile SpA, where the question is whether children with ERA who also have features of psoriasis or a family history of disease should be excluded from a classification of ERA.
The ERA classification criteria do not address reactive arthritis or coexisting IBD,30
which are considered features of SpA in adults. A diagnosis of IBD does not exclude a patient from having ERA, as long as the inclusion criteria are met. Nevertheless, one consequence of not recognizing IBD as a possible feature of SpA is that a 15-year-old male with IBD and arthritis, but not enthesitis, HLA-B27, or a family history of SpA, would not be considered to have SpA until he turned 16 years old, unless the criteria for undifferentiated SpA in adults (ESSG criteria13
or Amor criteria14
) were to be applied before that time. In addition, the criteria for ERA actually contradict those for JIA, since arthritis does not have to be present for a classification of ERA. The correct approach to the classification of ERA is uncertain, and this issue is clearly confusing to both pediatric and adult rheumatologists.
The ILAR classification system for JIA, and the ERA subgroup in particular, does not specifically address children who meet the criteria for AS. Approximately 10% to 20% of patients eventually diagnosed with AS begin to experience symptoms before the age of 16,31
and a proportion of these individuals would fulfill the modified New York criteria for AS at this early age. The criteria for AS do not specify a lower age limit, and when children fulfill these criteria it is logical to classify them as having AS, even if they also meet the criteria for ERA. The term ‘juvenile AS’, then, could be used to describe children who meet the criteria for AS before age 16, whereas ‘juvenile-onset AS’ would describe those whose symptoms began before age 16 but who do not fulfill the AS criteria until they are older.