Pathogenesis and mechanisms of joint destruction
The disease is based on an uncontrolled immune response that leads to chronic inflammation. The main culprits of destruction are lymphocytes—they secrete an excessive amount of proteolytic enzymes that trigger the resorption of cartilage and underlying bone tissue.
A key role in damage is played by the pannus—an aggressive tissue that proliferates abundantly within the joint cavity. It consists of activated macrophages, fibroblasts, and actively dividing synovial cells (synoviocytes). Pannus proliferation leads to irreversible anatomical defects.
Classification of clinical variants of JIA (according to ILAR)
Modern classification is based on the criteria of the International League of Associations for Rheumatology (ILAR, 2001). Depending on the clinical picture in the first 6 months, 7 variants are distinguished:
- Systemic arthritis. Combines arthritis with fever lasting more than 2 weeks. The presence of at least two systemic signs is mandatory: evanescent erythematous rash, serositis, generalized lymphadenopathy, or hepatosplenomegaly.
- Polyarthritis (RF-negative). Involvement of 5 or more joints with a negative rheumatoid factor (RF) test. It is more common in girls, runs a symmetrical and relatively benign course.
- Polyarthritis (RF-positive). Symmetrical involvement of 5 or more joints with confirmed RF. It is the pediatric analogue of adult rheumatoid arthritis, is associated with HLA-DR4, and is characterized by rapid joint destruction.
- Oligoarthritis. Involvement of 1 to 4 joints. It is divided into persistent (no more than 4 joints throughout) and extended (5 or more joints 6 months after onset).
- Enthesitis-related arthritis. A combination of arthritis with enthesitis (inflammation of tendon insertion sites) and talalgia. It is typical for boys over 8 years old and is closely associated with HLA-B27.
- Psoriatic arthritis. Arthritis against the background of typical psoriatic rashes, or in combination with dactylitis, nail pitting, or a burdened family history.
- Other (undifferentiated) arthritis. A diagnosis of exclusion for overlap forms or cases that do not fit any category.
Characteristics of articular syndrome
The pathological process in JIA can affect any joints with a synovial membrane. The criterion for active inflammation is swelling, or a combination of pain with restricted movement, hyperemia, and hyperthermia.
- Topography: Large joints of the legs are most commonly affected (knees in 80–90% of cases, ankles), wrists, and hips.
- Specific locations: In the systemic variant in children under 3 years of age, the temporomandibular joint (TMJ) is often affected, leading to underdevelopment of the lower jaw and persistent facial deformity (prognathism). Involvement of the cervical spine is typical.
- Morning stiffness: An important marker of disease activity, presenting as stiffness after waking up (from 10–15 minutes to several hours).
- Local complications: In oligoarthritis, active local inflammation stimulates bone growth zones, which can lead to pathological elongation of the affected limb.
Extra-articular manifestations of the disease
Extra-articular manifestations largely depend on the form of JIA.
In the systemic form, they are most pronounced: visceral lesions develop, such as myopericarditis, pleuritis, serous peritonitis, as well as enlargement of lymph nodes, liver, and spleen.
Ophthalmological pathology (anterior iridocyclitis or uveitis) is the main factor of an unfavorable prognosis, especially in oligoarthritis. The acute course is accompanied by photophobia and red eyes. The chronic course is insidious because it proceeds sluggishly and asymptomatically, but consistently leads to band keratopathy, cataracts, and irreversible vision loss.
The diagnostic triad of JIA: "Under 16, more than 6, without cause" (age under 16, duration of inflammation over 6 weeks, diagnosis of exclusion).
Class and exam questions
What is the difference between persistent and extended oligoarthritis?
In the persistent variant, the number of inflamed joints does not exceed four throughout the entire course of the disease. In the extended subtype, 1–4 joints are affected in the first 6 months, but then 5 or more joints become involved.
What is the danger of the oligoarticular variant of JIA if there are no systemic manifestations?
The main threat in oligoarthritis (occurring in 50% of patients) is the development of chronic uveitis. It is asymptomatic, but can end in severe complications up to complete blindness.
Which genetic marker is associated with enthesitis-related arthritis?
The enthesitis-related variant of JIA, manifested by inflammation of tendon insertion sites and involvement of the sacroiliac joints, has a strong association with the carriage of the HLA-B27 antigen.
What else to study on this topic
- Pathogenesis of joint destruction: the role of proteolytic enzymes and pannus formation
- Differential diagnosis of RF-positive and RF-negative polyarthritis
- Ophthalmological complications of JIA: the course of anterior iridocyclitis and uveitis
- Systemic variant of JIA: features of visceral lesions
- Oligoarticular variant: immunological profile (ANA) and epidemiology
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