JIA - Classification, patterns and Features

Definition And Core Diagnostic Criteria

Chronic inflammatory joint disease of unknown etiology.
Age of onset less than 16 years.
Arthritis duration exceeding 6 weeks.
Meticulous exclusion of other childhood arthritis etiologies mandatory.
Diagnosed by intraarticular swelling or minimum two clinical signs: limited range of motion, tenderness/pain on motion, warmth.

Standardized nomenclature categorizes JIA into seven mutually exclusive subtypes.
Classification determined by predominant clinical pattern during initial 6 months.
Subtypes require rigid adherence to specific exclusion criteria to ensure homogeneous diagnostic groups.

a. Psoriasis or familial psoriasis history in first-degree relative.
b. HLA-B27 positive male beginning arthritis after 6th birthday.
c. Ankylosing spondylitis, enthesitis-related arthritis, sacroiliitis with inflammatory bowel disease, Reiter syndrome, or acute anterior uveitis in first-degree relative.
d. Positive IgM Rheumatoid Factor (RF) on at least two occasions minimum 3 months apart.
e. Presence of systemic JIA in designated patient.

Accounts for 5-15% of JIA cases.
Lacks sex predilection, peak onset age 1-5 years.
Diagnosis requires arthritis accompanied by quotidian fever lasting $\geq$ 2 weeks (documented daily for 3 days).
Requires minimum one systemic feature: evanescent salmon-pink erythematous rash, generalized lymphadenopathy, hepatosplenomegaly, serositis (pericarditis/pleuritis).
Rash migratory, nonpruritic, transient (<1 hour duration), typically truncal.
Koebner phenomenon common; rash evoked by superficial trauma.
Marked inflammatory response: elevated Erythrocyte Sedimentation Rate (ESR), C-Reactive Protein (CRP), extreme hyperferritinemia, leukocytosis, thrombocytosis.

Most frequent subtype, comprising 40-70% of cases.
Predominantly affects young girls (peak 2-4 years).
Defined by involvement of 4 or fewer joints during initial 6 months.
Persistent Oligoarthritis: Disease restricted to $\leq$ 4 joints throughout entire course.
Extended Oligoarthritis: Disease extending to $\geq$ 5 joints after initial 6 months, bearing worse prognosis.
Asymmetric involvement of large lower limb joints (knees, ankles).
Carries highest risk for chronic asymptomatic anterior uveitis, strongly associated with Antinuclear Antibody (ANA) positivity.
Localized inflammation accelerates linear growth, causing leg-length discrepancy.

Involves 5 or more joints during first 6 months.
Rheumatoid Factor (RF) Negative:
- Accounts for 20-35% of cases.
- Bimodal peak (2-4 years and 10-14 years), female predominance.
- Symmetric or asymmetric large and small joint involvement.
- Temporomandibular joint and cervical spine frequently affected.
Rheumatoid Factor (RF) Positive:
- Constitutes <10% of cases, typically adolescent girls.
- Requires 2 positive RF tests at least 3 months apart.
- Severe, symmetric, additive, deforming arthritis of small joints (metacarpophalangeal, proximal interphalangeal).
- Rheumatoid nodules possible, indicating severe disease.

Affects older boys (mean age 12 years).
Asymmetric large joint arthritis of lower extremities alongside enthesitis.
Tarsitis (midfoot arthritis) highly specific clinical clue.
Diagnosis requires arthritis and enthesitis, OR one component plus two of: sacroiliac tenderness/inflammatory lumbosacral pain, HLA-B27 positivity, male >6 years, acute symptomatic anterior uveitis, first-degree relative with HLA-B27 disease.

Bimodal onset (preschool and adolescence).
Diagnosis mandates arthritis and psoriasis, OR arthritis plus two of: dactylitis, nail pitting/onycholysis, first-degree relative with psoriasis.
Asymmetric small and large joint involvement.
Distal interphalangeal (DIP) joint disease characteristic.

Fulfills criteria for no specific category OR meets criteria for more than one category.

Subtype	Peak Age & Sex	Articular Pattern	Extra-articular/Systemic Features	Laboratory Markers
Systemic JIA	1-5 years, Equal sex	Polyarticular (knees, wrists, ankles)	Quotidian fever, evanescent salmon rash, serositis, hepatosplenomegaly	Extreme leukocytosis, thrombocytosis, elevated ESR/CRP, hyperferritinemia
Oligoarticular	2-4 years, Girls > Boys	Asymmetric, $\leq$ 4 joints, large lower limb (knees)	Chronic asymptomatic anterior uveitis	ANA positive (60%), normal/mild ESR elevation
Polyarticular (RF-ve)	Bimodal, Girls > Boys	Symmetric/asymmetric, $\geq$ 5 joints, small & large	TMJ involvement, micrognathia, uveitis (10%)	ANA positive (40%), RF negative, elevated ESR
Polyarticular (RF+ve)	9-12 years, Girls > Boys	Symmetric, $\geq$ 5 small/large joints, aggressive	Rheumatoid nodules, low-grade fever	RF positive, elevated ESR, ANA positive
Psoriatic	Bimodal, Girls > Boys	Asymmetric small/medium, DIP joints	Dactylitis, nail pitting, psoriasis, uveitis	ANA positive (50%), mildly elevated ESR
Enthesitis-related	9-12 years, Boys > Girls	Asymmetric large lower limb, axial skeleton	Enthesitis, acute anterior uveitis	HLA-B27 positive (80%), RF/ANA negative

Macrophage Activation Syndrome (MAS): Life-threatening complication primarily in Systemic JIA. Clinical features include unremitting fever, sudden icterus, hepatosplenomegaly, bleeding tendency. Laboratory hallmarks: pancytopenia (leukopenia, thrombocytopenia), extreme hyperferritinemia (>684 ng/mL), hypofibrinogenemia (causing dramatic ESR drop), hypertriglyceridemia, elevated transaminases.
Chronic Anterior Uveitis: Predominantly early-onset ANA-positive Oligoarticular JIA. Asymptomatic progression leading to blinding complications: posterior synechiae, glaucoma, cataracts, band keratopathy. Mandates routine slit-lamp screening.
Skeletal Abnormalities: Premature epiphyseal closure leads to shortened bones. Persistent knee arthritis causes leg-length discrepancy. TMJ involvement causes micrognathia.

Disease Category	Specific Mimics	Distinguishing Clinical & Laboratory Features
Malignancy	Leukemia, Neuroblastoma, Bone Tumors	Disproportionate deep bone pain, severe night pain (awakening child), non-articular bony tenderness, leukopenia, thrombocytopenia, abnormal peripheral smear.
Infections	Septic Arthritis, Reactive Arthritis, Lyme Disease	Acute red hot joint, purulent synovial fluid (>50,000 WBCs/microL), marked toxicity, migratory/additive pattern, positive cultures.
Non-Inflammatory	Growing Pains, Hypermobility, Mechanical	Pain worsens with physical activity, exclusively nighttime occurrence (growing pains), absent joint swelling, normal inflammatory markers.
Connective Tissue	SLE, Juvenile Dermatomyositis (JDM)	Malar rash sparing nasolabial folds, Heliotrope rash, Gottron papules, symmetric proximal muscle weakness, specific autoantibodies.