Epidemiology & Risk Factors

Most common chronic rheumatic disease of childhood.
Age of onset: < 16 years old; symptoms persist for ≥ 6 weeks.
Peak incidence: 1–3 yo (oligoarticular) and early adolescence (polyarticular).
Female predominance overall (except enthesitis-related JIA: males > females).
Subtypes:
- Oligoarticular (most common, ~50%): ≤ 4 joints involved within first 6 months.
- Polyarticular (~30%): ≥ 5 joints involved (RF (+) or RF (-)).
- Systemic JIA (sJIA / Still Disease) (~10-15%): Auto-inflammatory; equal male:female ratio.
- Enthesitis-related (HLA-B27 associated) and Psoriatic JIA.

Clinical Features

General: Joint pain, swelling, warmth, morning stiffness that improves with activity, and limp without refusal to bear weight.
Oligoarticular JIA:
- Asymmetric involvement of large joints (knee > ankle > wrist > elbow).
- Typically painless or mildly painful; child may present with a limp.
- High risk of asymptomatic anterior uveitis (insidious, leads to blindness if missed).
Polyarticular JIA:
- Symmetric involvement of small and large joints (hands, feet, knees, cervical spine).
- Resembles adult RA; may have low-grade fever and mild anemia.
Systemic JIA (sJIA):
- Quotidian fever: Daily, high-spiking fever (> 38.5°C) typically occurring once/twice daily in the late afternoon/evening, returning to baseline. c
- Evanescent salmon-pink macular rash: Coincides with fever spikes; non-pruritic, transient, mostly on trunk/extremities.
- Hepatosplenomegaly, generalized lymphadenopathy, and serositis (pericardial/pleural effusions).
- Polyarthritis (often develops weeks to months after systemic features).

Clinical diagnosis of exclusion (age < 16 yo, persistent joint swelling/effusion ≥ 6 weeks, other causes ruled out).
Key Labs:
- Inflammatory markers: ↑ ESR, ↑ CRP, thrombocytosis, leukocytosis, microcytic anemia of chronic disease.
- sJIA Labs: Extreme leukocytosis (WBC > 20,000-30,000/mm³), marked thrombocytosis, and dramatically elevated serum ferritin.
- Autoantibodies:
  - ANA: Positive in ~60-80% of oligoarticular JIA (strongly associated with risk of anterior uveitis; does not correlate with disease activity).
  - RF / Anti-CCP: Checked in polyarticular JIA (determines prognosis; RF (+) carries worse prognosis). Negative in sJIA.
Screening:
- Regular Slit-Lamp Examination: Mandatory at diagnosis and scheduled every 3–6 months for all oligoarticular/polyarticular pts (especially ANA(+)) to screen for silent anterior uveitis.
Imaging:
- Plain X-ray (Initial): Soft tissue swelling, osteopenia, accelerated epiphyseal growth (early); joint space narrowing and erosions (late).
- Ultrasound / MRI: Detects subclinical synovitis, tenosynovitis, and cartilage loss.
Arthrocentesis (if monoarticular): Performed to rule out septic arthritis (JIA synovial fluid: WBC 5,000–50,000/mm³ with moderate PMN predominance; cultures negative).

Septic Arthritis:
- Diff by: Acute onset, single joint, high fever, severe pain with refusal to bear weight, synovial WBC > 50,000/mm³ with > 75% PMNs, (+) Gram stain/culture.
Transient Synovitis:
- Diff by: Acute hip pain following viral URI in a child aged 3–8 yo, self-limiting within 1–2 weeks, normal to mildly elevated ESR/CRP.
Acute Lymphoblastic Leukemia (ALL):
- Diff by: Severe bone/joint pain (worse at night, waking from sleep), out of proportion to physical exam findings, cytopenias (anemia, thrombocytopenia, leukopenia), peripheral blasts.
Acute Rheumatic Fever (ARF):
- Diff by: Migratory polyarthritis (moves from joint to joint), preceding group A Streptococcus pharyngitis, carditis, Sydenham chorea, erythema marginatum, elevated ASO/anti-DNase B titers.
Lyme Arthritis:
- Diff by: History of tick bite/travel to endemic areas, prominent knee effusion with minimal pain, positive Borrelia burgdorferi serology (ELISA followed by Western blot).

First-line / Mild Disease:
- Oligoarticular: Intra-articular corticosteroid injections (e.g., triamcinolone hexacetonide) or oral NSAIDs (e.g., naproxen, ibuprofen).
- Polyarticular or Refractory Oligoarticular: Non-biologic DMARDs (First-line: Methotrexate; requires baseline CBC, LFTs, and folate supplementation).
- Systemic JIA (sJIA): IL-1 inhibitors (Anakinra, Canakinumab) or IL-6 inhibitors (Tocilizumab) ± systemic corticosteroids for acute flare control.
Second-line / Refractory Disease:
- Biologic DMARDs: TNF-α inhibitors (e.g., Etanercept, Adalimumab, Infliximab) added to Methotrexate for polyarticular/extended oligoarticular disease.
- Note: Rule out latent TB (PPD/IGRA) and Hepatitis B/C prior to starting biologic DMARDs.
Supportive & Monitoring:
- Physical and occupational therapy to maintain joint mobility and muscle strength.
- Routine ophthalmology slit-lamp examinations.

Macrophage Activation Syndrome (MAS):
- Life-threatening complication of sJIA (form of secondary HLH).
- Features: Persistent non-remitting fever, hepatosplenomegaly, bleeding/purpura, mental status changes.
- Labs: Extreme hyperferritinemia (> 10,000 ng/mL), pancytopenia, elevated AST/ALT, hypofibrinogenemia, elevated triglycerides, elevated D-dimer.
- Rx: High-dose pulse IV methylprednisolone, Cyclosporine, or high-dose Anakinra.
Anterior Uveitis: Untreated leads to posterior synechiae, band keratopathy, cataracts, glaucoma, and irreversible visual loss/blindness.
Growth Abnormalities: Localized limb-length discrepancies (hyperemia causes accelerated epiphyseal growth early; premature closure late) and generalized growth failure from chronic inflammation/steroid use.
Joint Destruction / Ankylosis: Chronic erosive arthritis causing joint contractures and fusion (e.g., micrognathia from TMJ involvement, cervical spine fusion).

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