A 40-year-old man presents to the clinic with a 2-day history of severe pain, swelling, and redness in his right knee. He reports a fever and difficulty walking due to the pain. His medical history includes diabetes and a recent skin infection near the knee. Examination reveals a hot, swollen, and tender knee joint with a limited range of motion. Synovial fluid aspiration shows turbid fluid with a high white blood cell (WBC) count and Gram-positive cocci on microscopy.
Which of the following statements about septic arthritis is true?
A. Is usually self-limiting
B. Mostly affects a single joint
C. Joint should not be aspirated as it would introduce more infection
D. Blood cultures are mostly negative
E. Leukopenia is common
Correct Answer: B. Mostly affects a single joint
Detailed Explanation:
Septic arthritis most commonly presents as monoarthritis, affecting a single joint. The knee is the most frequently involved joint, though others such as the hip, shoulder, or ankle may also be affected. In this case, the patient’s fever, joint swelling, and findings on synovial fluid analysis point toward septic arthritis.
Key Characteristics:
- Single-joint involvement: Over 90% of cases present with monoarthritis, making it a hallmark feature.
- Systemic symptoms: Fever, chills, and malaise often accompany joint findings.
- Diagnosis: Requires joint aspiration and synovial fluid analysis, which typically reveals:
- High white blood cell count (>50,000/µL, predominantly neutrophils).
- Gram stain and culture to identify the causative organism.
- Causative organisms:
- Staphylococcus aureus (most common).
- Streptococcus species.
- Gram-negative bacteria, particularly in immunocompromised individuals.
Why Other Options Are Incorrect:
A. Is usually self-limiting
Septic arthritis is NOT self-limiting. If untreated, it can lead to permanent joint damage, sepsis, and death. Immediate medical intervention is essential.
C. Joint should not be aspirated as it would introduce more infection
Aspiration is critical for both diagnosis and treatment. It provides fluid for analysis and helps reduce pressure in the joint. When performed aseptically, aspiration does not increase the risk of infection.
D. Blood cultures are mostly negative
Blood cultures are positive in up to 50% of cases, particularly in hematogenous spread. They provide valuable information about the causative organism.
E. Leukopenia is common
Leukocytosis (elevated white blood cell count) is a typical feature of septic arthritis due to the body’s systemic inflammatory response. Leukopenia is rare and typically occurs in severely immunosuppressed individuals or in cases of overwhelming sepsis.
Treatment:
- Empirical antibiotics: Based on the most likely organism and adjusted according to culture results.
- Vancomycin: For Gram-positive coverage (S. aureus).
- Ceftriaxone: For Gram-negative organisms.
- Joint drainage: Essential to reduce bacterial load and prevent further damage.
- Supportive care: Pain management and physical therapy after the infection is controlled.