INFECTIOUS ARTHRITIS & JOINT DISORDERS Bone & Soft Tissue Pathology Dr. Lilian Bosire — GENERAL PRINCIPLES Microorganisms can reach joints by: - Haematogen
INFECTIOUS ARTHRITIS & JOINT DISORDERS Bone & Soft Tissue Pathology Dr. Lilian Bosire --- GENERAL PRINCIPLES Microorganisms can reach joints by: Haematogenous dissemination (most common) Direct inoculation (trauma, surgery) Contiguous spread from soft tissue abscess or osteomyelitis Key point: Cartilage has limited regenerative capacity unlike bone — rapid joint destruction from infection can cause permanent deformity if not treated promptly. --- 1. SUPPURATIVE (BACTERIAL) ARTHRITIS Acute bacterial joint infection — almost always haematogenous in adults. Causative Organisms by Age Age Group Organism --- --- Neonates Contiguous spread from epiphyseal osteomyelitis; S. aureus < 2 years Haemophilus influenzae Older children & adults Staphylococcus aureus (most common overall) Late adolescence / young adults Neisseria gonorrhoeae Sickle cell disease (any age) Salmonella spp. IV drug users Axial joint involvement; Pseudomonas , S. aureus Except gonococcal arthritis (more common in women), joint infections affect males and females equally . --- Predisposing Factors Complement deficiencies (C5–C9 / membrane attack complex) → especially susceptible to disseminated gonococcal infection Immunodeficiencies (congenital and acquired, e.g. HIV) Debilitating illness Joint trauma or pre-existing chronic arthritis Intravenous drug use --- Clinical Features Classic presentation: Sudden onset of acutely painful, swollen joint with restricted range of motion Fever, leukocytosis, elevated ESR/CRP Gonococcal arthritis: Often subacute; usually monoarticular — knee, hip, shoulder, elbow, wrist, or sternoclavicular joints; may be associated with skin pustules and tenosynovitis (classic triad) Diagnosis & Treatment Joint aspiration → purulent fluid + organism identification = diagnostic Gram stain and culture of synovial fluid; blood cultures Prompt antimicrobial therapy prevents permanent joint destruction Drainage of purulent joint may be required (surgical or repeated aspiration) --- 2. MYCOBACTERIAL ARTHRITIS (TB ARTHRITIS) Chronic, progressive monoarticular infection caused by Mycobacterium tuberculosis Occurs in adults; arises from adjacent osteomyelitis OR haematogenous spread from a pulmonary focus Onset is insidious — gradually increasing pain; systemic TB symptoms (fever, night sweats, weight loss) may or may not be present Joints Affected (in order of frequency) Hips → Knees → Ankles (weight-bearing joints) Spinal TB (Pott's disease) = vertebral involvement → can cause kyphosis and cord compression Morphology Mycobacterial seeding → confluent granulomas with caseous necrosis in synovium Synovium grows as a pannus over articular cartilage → bone erosion along joint margins Chronic disease → fibrous ankylosis and obliteration of joint space Diagnosis Synovial biopsy showing caseating granulomas is more reliable than culture alone AFB stain, Ziehl-Neelsen; culture on Lowenstein-Jensen medium (slow) PCR for rapid confirmation Mantoux/TST and IGRA (interferon-gamma release assay) helpful --- 3. VIRAL ARTHRITIS Common causative viruses: Alphaviruses (e.g., Chikungunya, Ross River virus) — direct joint infection Parvovirus B19 — especially in adults; symmetric small joint arthritis mimicking RA Rubella — direct synovial infection; also associated with rubella vaccination Epstein-Barr virus Hepatitis B & C — immune complex-mediated arthritis Mechanism: Either direct viral infection of the joint (rubella, some alphaviruses) OR autoimmune reaction triggered by the infection (molecular mimicry, immune complex deposition) Manifestations range from acute to subacute arthritis Usually self-limiting — resolves with clearance of the infection --- 4. LYME ARTHRITIS Aetiology Caused by the spirochete Borrelia burgdorferi Transmitted by deer ticks of the Ixodes ricinus complex (also I. scapularis in USA) Vector requires prolonged attachment ( 36–48 hours) for transmission Three Clinical Stages Stage Timing Manifestations --- --- --- Early localized Days after tick bite Erythema migrans (expanding "bull's eye" rash) Early disseminated Days to weeks later Multiple skin lesions, cranial nerve palsies (CN VII most common), cardiac block, meningitis Late disseminated Months after infection Lyme arthritis , encephalopathy Arthritis now occurs in <10% of cases because most are treated early If untreated: up to 80% develop migratory arthritis lasting weeks to months Joint Involvement Large joints: Knees shoulders elbows ankles Usually one or two joints at a time; migratory pattern Initial attacks last weeks to months; may recur at new sites Diagnosis Spirochetes identified in synovium in only 25% of arthritis cases Serologic detection of anti-Borrelia antibodies (ELISA confirmed by Western blot) is diagnostic Two-tier testing recommended Histology Chronic synovitis Synoviocyte hyperplasia Fibrin deposition Mononuclear cell infiltrate (especially CD4+ T cells ) Obliterative endarteritis (vessel wall inflammation) Severe cases: histopathology mimics