Infectious Arthritis

Definition

Infectious arthritis, also called septic arthritis, is a medical emergency characterized by infection of a joint space caused by bacteria, viruses, or fungi, leading to rapid joint destruction if untreated.

Epidemiology

• Incidence: approximately 2–10 cases per 100,000 person-years in adults, higher in children
• More common in adults with comorbidities (diabetes, rheumatoid arthritis, immunosuppression)
• Bacterial causes more frequent than viral or fungal
• Knee and hip are the most commonly affected joints
• Increased risk in prosthetic joints or post-surgical cases

Etiology

• Bacterial infection, commonly Staphylococcus aureus (including MRSA), Streptococcus species, and Neisseria gonorrhoeae
• Gram-negative bacteria in elderly or immunocompromised patients
• Viral infections less commonly cause transient arthritis (parvovirus B19, hepatitis B/C)
• Fungal infections in immunocompromised individuals (Candida, Aspergillus)
• Hematogenous spread from distant infection
• Direct inoculation from trauma, surgery, or injections

Pathophysiology

• Pathogens enter the joint via hematogenous spread, direct inoculation, or contiguous spread
• Rapid proliferation of microorganisms within synovial fluid
• Intense inflammatory response with neutrophil infiltration
• Release of proteolytic enzymes leading to cartilage destruction
• Synovial membrane swelling causes pain, warmth, and restricted motion
• Chronic infection may cause joint deformity and ankylosis
• Systemic inflammatory response may result in sepsis in severe cases