Learning Objectives
Differentiate the common bacterial causes of Septic Arthritis and recognize the classic synovial fluid profile. Distinguish between localized purulent arthritis and the disseminated gonococcal triad. Identify the critical management steps required to prevent permanent joint destruction.
1. Pathogenesis & Microbiology
Septic arthritis is a medical emergency involving the bacterial invasion of the joint space. It is typically monoarticular (affecting a single large joint, like the knee).
- Common Pathogens:
- S. aureus (most common overall).
- Streptococcus species.
- Neisseria gonorrhoeae (common in sexually active young adults).
- Synovial Fluid Analysis: The “Gold Standard” for diagnosis. The fluid is purulent with a WBC count > 50,000/mm3.
2. Disseminated Gonococcal Infection (DGI)
This is a systemic manifestation of a sexually transmitted infection (STI) that can present in two distinct ways:
| Presentation Type | Clinical Features |
|---|---|
| Purulent Arthritis | Localized, highly inflammatory infection (e.g., knee). |
| DGI Triad | 1. Polyarthralgia (migratory joint pain). 2. Tenosynovitis (inflammation of tendon sheaths, often in the hands/wrists). 3. Dermatitis (painless pustules or maculopapular rash). |
3. Complications & Treatment
Delayed treatment leads to rapid destruction of articular cartilage due to bacterial enzymes and the host’s inflammatory response.
- Complications: Permanent joint damage, Osteomyelitis, chronic pain, and systemic Sepsis.
- Management:
- Drainage: Immediate aspiration or surgical debridement to clear the “pus” from the joint.
- Antibiotics: Long-term IV or oral therapy tailored to culture results.
Activity: Synovial Fluid Differential Diagnosis (Septic vs. RA vs. OA)
High-Yield Mnemonics:
- The DGI Triad: Pain (Polyarthralgia), Pustules (Dermatitis), and Pulled tendons (Tenosynovitis).
- Pus in the joint: “If it’s over 50,000 (WBC), it’s a Septic joint.”