Inflammatory Arthritis in the Child and Adolescent

Introduction

Inflammatory arthritis (IA) in children and adolescents represents a collection of medical conditions that may ultimately affect the axial and appendicular joints of the musculoskeletal system of children and teenagers. These conditions are not uncommon and can have substantial impact on functional status and quality of life. Hip involvement in these conditions is frequently associated with worse outcomes. Contrasting features of different forms of IA are compared in Table 41.1 .

IA in children and adolescents is generally attributed to juvenile idiopathic arthritis (JIA). In the past, the term juvenile rheumatoid arthritis (JRA) was used to describe many of the children who would meet diagnostic criteria for JIA; JRA has largely been replaced by JIA. The definition of JIA is highlighted in Box 41.1 . Ankylosing spondylitis (AS) can also have peripheral joint involvement. Criteria for the diagnosis of AS are shown in Box 41.2 . About 10% of AS cases are diagnosed in childhood. Conditions such as psoriasis and inflammatory bowel disease (IBD) may also have associated appendicular and axial joint involvement in a minority of cases. For the purposes of this chapter, we will focus primarily on JIA and AS.

Epidemiology and Risk Factors

The incidence and prevalence of JIA varies with the population studied, but an incidence of about 1 new case per 10,000 children per year and a prevalence of about 1 in 2000 children was found in a recent US population-based cohort. In this cohort, about 63% were oligoarticular onset (≤ 4 joints); girls were more likely to develop JIA than boys. The overall incidence rate in this study from Olmsted County in Minnesota was stable over a 50-year period. Studies from other centers have a higher prevalence of polyarticular-onset JIA, likely a reflection of the referral nature of those centers studied.

Genetic associations for JIA have been described but do not presently have prominent clinical applications. For AS, however, the association with the class I HLA B-27 molecule is clinically important. The presence of B-27 is correlated with axial involvement in conditions such as psoriasis and IBD and may be found in almost 90% of teenagers with AS.

Pathophysiology

JIA and AS are chronic inflammatory conditions that affect the axial and appendicular musculoskeletal system. In general, these conditions are considered to be autoimmune, suggesting that established immune mechanisms somehow become misguided and initiate and propagate damage to these structures. This is an area of intense and evolving research. Components of an innate immune system, adaptive immune system, cytokines, autoantibodies, and molecular genetics are examples of aspects of the immune system being studied for these conditions.

Of these areas, the association of AS with HLA B-27 (as discussed earlier) and the development of serum autoantibodies clearly have the most clinical relevance. Autoantibodies are antibodies that are activated by the organism's own tissue and are a hallmark of autoimmune conditions. Autoantibodies add prognostic information about forms of JIA but are not required to diagnose IA.

While there are many types of autoantibodies, for children with IA, two are most useful for clinical management. The first of these is rheumatoid factor (RF). RF is simply an antibody that has a binding affinity to another antibody. Usually, serum RF is measured by an enzyme-linked immunosorbent assay (ELISA), a sensitive, inexpensive technique. The presence of RF in serum of children with polyarticular JIA increases the likelihood of damage and progressive disease, but most children with polyarticular JIA are RF negative.

The other important autoantibody in children with IA is antinuclear antibody (ANA). Like RF, most children with IA are ANA negative, but in children with oligoarticular JIA (oligo-JIA), the likelihood of uveitis (see later discussion) is increased if ANA is found in the serum. Since levels of these autoantibodies do not generally help in the longitudinal management of children who have IA, they are not generally serially assessed.

Clinical Features and Diagnosis

The key to diagnosing JIA and AS in children is recognizing the patterns of clinical manifestations of these conditions. The criteria for JIA require at least 6 weeks of symptoms. Similarly, AS is a chronic condition. While acute joint inflammation could be associated with the initial presentation of one of these conditions, strong consideration of other causes of joint inflammation, such as infection, must be considered in this setting. Septic arthritis is a medical emergency and cannot be overlooked. Clinical features of inflammation are highlighted in Box 41.3 .