Epidemiology, Pathogenesis, and Imaging of Arthritis in Children

Juvenile idiopathic arthritis is a broad term used to describe a series of chronic arthritis occurring in children younger than 16 years of age. Even though the cause is not fully understood, several clues regarding the pathogenesis have been found. Diagnosis of the different types of juvenile idiopathic arthritis is made clinically, and imaging plays a role in answering specific questions pertaining to disease classification, staging, and outcome of treatment options.

Juvenile idiopathic arthritis (JIA) is a broad term that includes all forms of arthritis of unknown cause with onset before 16 years of age and that persist for at least 6 weeks in children and adolescents. A diagnosis of JIA is based mainly on clinical findings rather than on laboratory tests or imaging findings. Although the hallmark of all subtypes of JIA is synovial inflammation, the pathogenesis of each subtype differs. No imaging protocols have yet been established for JIA; however, a familiarity with imaging modalities and their indications proves helpful when attempting to answer specific questions to better define, classify, and treat patients with JIA.

Epidemiology

JIA is the most common chronic rheumatologic childhood disease, with an onset before 16 years of age and persisting for at least 6 weeks. It occurs in a worldwide distribution with regional variations thought to be due to differences in the distribution of HLA alleles and environmental factors. Among developed nations, JIA has a yearly incidence rate of 2 to 20 cases per 100,000 population and a prevalence of 16 to 150 cases per 100,000 population. In a comprehensive survey of data from 2002, the incidence of chronic arthritis of childhood ranged from 0.008 to 0.226 per 1000 children and the prevalence from 0.07 to 4.01 per 1000 children worldwide. The true incidence of JIA is believed to vary widely, however, in part because it is composed of a heterogeneous group of arthritic conditions that are clinically diagnosed. In a multiethnic cohort study by Saurenmann and colleagues, European ancestry was shown an important predisposing factor for developing all types of JIA, in particular the oligoarticular and psoriatic subtypes, except the rheumatoid factor (RF)-positive polyarticular type. In this study, native North American children were also found at a higher risk of developing polyarticular disease than children of European descent.

Classification

JIA is a broad term that was developed by the International League of Associations for Rheumatology (ILAR) to group chronic arthritis in patients younger than 16 years of age for research purposes. This task force created relatively homogeneous, mutually exclusive categories of JIA based on clinical findings, replacing the terms, juvenile chronic arthritis and juvenile rheumatoid arthritis . JIA encompasses a heterogeneous group of diseases classified into 7 subtypes according to age of onset, number of affected joints, presence of RF and HLA-B27, and clinical findings. These 7 subtypes are systemic arthritis, oligoarthritis, polyarthritis (RF positive), polyarthritis (RF negative), enthesis-related arthritis (ERA), psoriatic arthritis, and undifferentiated arthritis ( Box 1 ) .

Box 1