Description
Juvenile Idiopathic Arthritis (JIA) refers to a group of conditions characterised by chronic joint inflammation in children below 16 years of age, persisting for more than six weeks. The inflammation primarily affects the joints but can also affect other body systems. JIA is a heterogenous condition encompassing multiple subtypes each with distinct clinical and radiological features.
Pathogenesis
The pathogenesis of JIA is a complex interplay of genetic predisposition and environmental factors that trigger an immune response leading to inflammation and destruction of joint tissues. While the precise pathophysiology remains elusive, it’s postulated that there may be an aberrant response to an environmental trigger such as an infection in a genetically susceptible individual.
Subtype
- Oligoarticular JIA: Involves fewer than five joints during the first six months. Most common subtype.
- Polyarticular JIA (RF-negative): Involves five or more joints. Rheumatoid factor is absent.
- Polyarticular JIA (RF-positive): Similar to RF-negative but presents with rheumatoid factor.
- Systemic JIA: Presents with systemic symptoms like fever and rash.
- Psoriatic JIA: Accompanied by skin psoriasis or a family history thereof.
- Enthesitis-related arthritis (ERA): Involves inflammation of entheses, the sites where tendons or ligaments insert into the bones.
Epidemiology, Risk Factors & Associations
- Prevalence: 1 in 1,000 children (approximately).
- Gender: Females are more affected than males, except in the ERA subtype.
- Genetic associations: Certain HLA genotypes.
Clinical Features
- Chronic joint inflammation: Pain, swelling, and stiffness, typically affecting the knees (approximately 90% of cases), ankles and wrists.
- Morning stiffness or stiffness after periods of inactivity.
- In ERA, axial involvement (spine and sacroiliac joints) is common, often leading to back pain and stiffness.
- Systemic symptoms: In systemic JIA, high-grade fever and evanescent (transient) rash can occur.
Complications
- Growth disturbances and asymmetry.
- Chronic uveitis.
- Joint deformities and disabilities if left untreated.
- Secondary osteoporosis.
Pathological Features
Histopathology
- Macroscopic: Thickened, hypertrophic synovium.
- Microscopic: Synovial lining layer hyperplasia, infiltration by inflammatory cells, predominantly lymphocytes.
Serology
- ANA-positive in many cases of oligoarticular JIA.
- RF and anti-CCP positive in polyarticular RF-positive subtype.
Biochemistry
- Elevated inflammatory markers (ESR, CRP).
Radiological Features
General Features
- Joint effusion and synovial thickening.
- Juxta-articular osteopenia.
- In late stages, erosions and joint space narrowing.
CT
- Helps in visualising bony erosions.
MRI
- T1WI: Low signal in bone marrow due to oedema.
- T2WI/STIR: High signal in bone marrow oedema, synovitis, enthesitis.
- T1 C+: Enhancement of synovium indicative of active synovitis.
Grading and Staging
There is no universally accepted grading or staging system for JIA. The disease is usually classified based on the number of joints involved and the presence of specific symptoms.
Diagnosis
The diagnosis of JIA is mainly clinical, based on history and physical examination. The presence of arthritis in one or more joints for six weeks or longer in a child under 16 years of age, with other conditions excluded, suggests JIA.
Differential Diagnosis
- Infectious Arthritis.
- Lyme Disease.
- Reactive Arthritis.
- Leukaemia.
Management
Management of JIA primarily aims to control inflammation, relieve symptoms, prevent damage, and maximise function and quality of life. It involves a combination of medication (NSAIDs, DMARDs, corticosteroids, and biologics) and physical and occupational therapy.