Septic arthritis, also known as infectious arthritis, is a serious condition characterized by joint inflammation due to infection. While commonly associated with bacterial pathogens, it can also arise from fungal, mycobacterial, viral, or other infectious agents. Typically affecting a single joint, often a large one like the knee or hip, septic arthritis can also manifest in multiple or smaller joints. Recognizing and diagnosing this condition swiftly is crucial to prevent significant joint damage and ensure effective treatment.
Understanding the Causes of Infectious Arthritis
Infectious arthritis arises when pathogens invade the joint space, leading to inflammation. The causative agents vary depending on age and risk factors.
Common Pathogens in Children
Children are particularly susceptible to infectious arthritis, and the common culprits differ by age group:
- Staphylococcus aureus: This bacterium is the most frequent cause of septic arthritis across all age groups.
- Kingella kingae: Predominantly seen in children under 3, this gram-negative bacterium is a leading cause in this demographic.
- Neonatal Period: Group B Streptococcus, Staphylococcus aureus, Neisseria gonorrhoeae, and gram-negative bacilli are commonly observed in newborns.
- Neisseria gonorrhoeae: Sexually active adolescents are at risk of gonococcal arthritis.
- Salmonella species: Children with sickle cell disease are predisposed to Salmonella infections in the joints.
- Pseudomonas aeruginosa: Puncture wounds and injection drug use can introduce this bacterium, leading to joint infections.
In children, the hip joint is most frequently affected.
Common Pathogens in Adults
Similar to children, Staphylococcus aureus remains the primary cause of infectious arthritis in adults. However, other pathogens are also significant:
- Streptococcus pneumoniae: While less common than S. aureus, it’s still a notable cause in adults.
- Neisseria gonorrhoeae: In young, sexually active adults, especially those presenting with acute monoarthritis, N. gonorrhoeae should be considered. It’s important to culture from other sites like the oropharynx, vagina, cervix, urethra, or anus as synovial fluid cultures may be less reliable for this organism.
- Fungal and Mycobacterial Infections: These infections often have a slow onset and can be challenging to diagnose. Synovial biopsy is often more effective than acid-fast smears of synovial fluid in mycobacterial infections.
In adults, the knee is the most commonly affected joint, followed by the hip.
Polymicrobial infections, involving multiple types of bacteria, occur in about 5% of cases, often following trauma or abdominal infections. Intravenous drug users can develop infections in less common joints like the sternoclavicular and sacroiliac joints, frequently involving Serratia and Pseudomonas. Immunocompromised individuals, such as those with leukemia, are more vulnerable to Aeromonas infections. Pre-existing joint damage, particularly in rheumatoid arthritis patients, increases susceptibility to infection.
Epidemiology and Risk Factors for Infectious Arthritis
The incidence of septic arthritis ranges from 2 to 6 cases per 100,000 people, but this varies with risk factors. It is more prevalent in children than adults, peaking between 2 and 3 years of age with a male predominance.
Risk Factors:
- Children:
- Neonates
- Hemophiliacs with joint bleeds (hemarthroses)
- Immunocompromised children (e.g., sickle cell anemia, HIV)
- Chemotherapy recipients
- Adults:
- Age over 80
- Diabetes mellitus
- Rheumatoid arthritis
- Recent joint surgery
- Joint prosthesis
- Prior intra-articular injections
- Skin infections and ulcers
- HIV infection
- Osteoarthritis
- Sexual activity (for gonococcal arthritis)
- Other causes of sepsis
Pathophysiology: How Infectious Arthritis Develops
The synovium, the lining of the joint, is highly vascular and lacks a basement membrane, making it susceptible to infection. Infection can occur through:
- Hematogenous Spread: Bacteria travel through the bloodstream from a distant infection site to the joint. This is the most common route.
- Direct Inoculation: Trauma, puncture wounds, or intra-articular injections can directly introduce pathogens into the joint.
- Contiguous Spread: Infection can spread from nearby bone infections (osteomyelitis). The hip and shoulder joints are more prone to this type of spread.
Once pathogens enter the joint, they trigger an inflammatory response. Inflammatory cytokines and proteases contribute to joint destruction. Bacterial toxins and microbial surface components, like staphylococcal adhesins, further exacerbate joint damage by promoting bacterial adhesion to joint tissues.
Prosthetic Joint Infections: These are categorized by timing:
- Early Infections (within 3 months of implantation): Often caused by Staphylococcus species.
- Delayed Infections (3-24 months post-surgery): Frequently due to gram-negative bacteria and coagulase-negative staphylococci.
- Late Infections (after 24 months): Usually result from hematogenous spread from other infection sites in the body.
Recognizing Infectious Arthritis: History and Physical Examination
Infectious arthritis typically presents abruptly with intense pain in a single joint, accompanied by swelling, warmth, and limited movement. Patients often exhibit:
- Acute Onset Monoarticular Joint Pain
- Fever: Present in 40-60% of cases, but not always.
- Swelling, Warmth, and Redness around the affected joint.
- Reluctance or Refusal to Move the Joint: This is a key sign, especially in children, sometimes presenting as pseudoparalysis.
The lower extremities, particularly the knee, hip, and ankle, are most commonly affected. About 20% of cases may involve multiple joints (oligoarticular), especially in individuals with severe sepsis, compromised immune systems, rheumatoid arthritis, or multiple comorbidities. Injection drug users may present with involvement of the sacroiliac or sternoclavicular joints.
Gonococcal Arthritis: Suspect this in young, sexually active adults with acute monoarthritis. They may also have skin lesions (dermatitis), inflammation of tendon sheaths (tenosynovitis), and migratory arthritis.
Symptoms in Children:
- Local Symptoms: Pain, swelling, warmth, limited range of motion, limping, refusal to use the limb.
- Systemic Symptoms: Appearing unwell, fever, rapid heart rate (tachycardia), irritability, poor appetite.
Physical Examination Findings:
- Effusion: Fluid buildup in the joint is common.
- Limited Range of Motion
- Pain on Palpation
- Joint Distribution: Knee is most common, followed by hip, shoulder, and ankle. Neisseria infections often affect multiple joints, while Staphylococcus is usually monoarticular. Group B streptococci can involve the sternoclavicular and sacroiliac joints. Prosthetic joint infections may have draining sinuses.
Infectious Arthritis Diagnosis: Evaluation and Testing
Prompt and accurate diagnosis is crucial for effective management. Evaluation involves:
Laboratory Studies: Synovial Fluid Analysis
Arthrocentesis and Synovial Fluid Analysis is the cornerstone of diagnosis. Fluid is aspirated from the affected joint and analyzed for:
- White Blood Cell (WBC) Count with Differential: A WBC count exceeding 50,000 cells/µL with over 90% neutrophils strongly suggests bacterial infection.
- Gram Stain and Culture: Identifying bacteria in the fluid confirms the diagnosis.
- Crystal Analysis: To rule out crystal-induced arthritis (gout, pseudogout).
Other Laboratory Tests:
- Complete Blood Count (CBC): May show elevated WBC count with increased immature neutrophils (left shift), indicating infection.
- Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP): Elevated ESR and CRP levels support inflammation but are not specific for septic arthritis.
- Blood Cultures: Two sets should be obtained to check for bacteremia (bacteria in the blood).
- Cultures from Other Sites: If gonococcal arthritis is suspected, obtain cultures from cervix, rectum, and throat.
Important Note: A low synovial fluid WBC count can occur in early infections, disseminated gonococcal arthritis, in patients with peripheral leukopenia, and in prosthetic joint infections. In prosthetic joint infections, a synovial fluid WBC count of >1100 cells/µL with >64% neutrophils is suggestive of infection.
Imaging Studies
- Plain Radiographs (X-rays): May show widened joint space, soft tissue swelling, or late findings like subchondral bone changes. However, normal X-rays do not exclude septic arthritis.
- Ultrasonography: Useful for detecting and quantifying joint effusions and guiding joint aspiration.
- Magnetic Resonance Imaging (MRI): Highly sensitive for detecting early joint fluid and can reveal soft tissue and bone abnormalities, as well as cartilage involvement.
- Bone Scans: Not specific for septic arthritis but can help evaluate sacroiliac or hip joint infections when localized infection is suspected.
Treatment and Management Strategies
Treatment focuses on eradicating the infection and preserving joint function. It involves:
Antimicrobial Therapy
Prompt initiation of intravenous antibiotics is critical. Empiric therapy should begin immediately after joint aspiration and culture collection.
- Empiric Antibiotic Coverage: Should target Staphylococcus aureus as the most common pathogen. Options include nafcillin, oxacillin, or vancomycin. Vancomycin is often preferred due to increasing MRSA prevalence.
- Broadening Coverage: In immunocompromised patients, IV drug users, or when Gram stain is negative, add a third-generation cephalosporin (ceftriaxone, ceftazidime, or cefotaxime) for gram-negative coverage.
- Tailoring Therapy: Adjust antibiotics based on Gram stain results, patient risk factors (e.g., third-generation cephalosporin for suspected Salmonella or N. gonorrhoeae), and culture sensitivities.
- Duration of Treatment: Nongonococcal septic arthritis typically requires 2 weeks of IV antibiotics followed by 1-2 weeks of oral antibiotics (total 3-4 weeks). Pseudomonas infections may need longer treatment (4-6 weeks). Gonococcal arthritis responds well to IV ceftriaxone, transitioning to oral therapy after clinical improvement.
Joint Drainage
Essential to remove infected fluid and debris. Methods include:
- Arthrotomy: Surgical opening of the joint.
- Arthroscopy: Minimally invasive surgical drainage using a camera and instruments.
- Daily Needle Aspiration: Repeatedly drawing fluid out with a needle.
The orthopedic surgeon determines the best drainage method based on individual factors. Early orthopedic consultation is crucial.
Supportive Care
- Pain Management
- Physical Therapy: Initiated after the acute phase to restore joint function and prevent muscle weakness. Immobilization is generally avoided after the first few days.
Prosthetic Joint Infections Treatment
Often requires aggressive surgical debridement and/or removal of the prosthesis, followed by replacement with antibiotic-impregnated cement.
Differential Diagnosis: Ruling Out Other Conditions
Several conditions can mimic infectious arthritis, especially acute monoarticular arthritis. Differential diagnoses include:
- Other Infections: Viral, fungal, spirochetal, mycoplasmal arthritis.
- Crystal-Induced Arthropathies: Gout, pseudogout, calcium oxalate, cholesterol, hydroxyapatite crystal deposition.
- Osteoarthritis
- Intra-articular Injuries: Fracture, meniscal tear, osteonecrosis, foreign body, plant thorn synovitis.
- Inflammatory Arthritides: Rheumatoid arthritis, Behcet’s syndrome, seronegative spondyloarthropathies (ankylosing spondylitis, psoriatic arthritis, reactive arthritis, IBD-related arthritis), sarcoidosis, systemic lupus erythematosus, Still’s disease.
- Systemic Infections: Bacterial endocarditis, HIV, Lyme arthritis.
- Tumors: Metastasis, pigmented villonodular synovitis.
- Other: Hemarthrosis, clotting disorders, neuropathic arthropathy, dialysis-related amyloidosis, avascular necrosis.
Prognosis and Potential Complications
Despite antibiotics, septic arthritis carries a mortality rate of 7-15% in hospitalized patients. Morbidity occurs in about one-third of cases. Prognosis worsens with older age, comorbidities, pre-existing joint disease, and prosthetic joints. Early diagnosis and treatment are essential to improve outcomes. Neisseria infections have a better prognosis than Staphylococcus infections, which can have mortality rates exceeding 50%.
Complications of Septic Arthritis:
- Osteomyelitis (bone infection)
- Chronic pain
- Osteonecrosis (bone death)
- Leg length discrepancies (in children)
- Sepsis (blood infection)
- Death
Consultations and Interprofessional Team Approach
Early consultation with an orthopedic surgeon is vital for joint drainage. Interventional radiology may be needed for drainage of axial joints (sacroiliac, sternoclavicular). Managing septic arthritis effectively requires an interprofessional team, including:
- Orthopedic Surgeons: For joint drainage.
- Infectious Disease Specialists: To guide antibiotic therapy.
- Primary Care Providers: To manage comorbidities and coordinate care.
- Nurses: Inpatient care, wound care.
- Pharmacists: Antibiotic monitoring.
- Physical and Occupational Therapists: Rehabilitation.
- Social Workers: Discharge planning and transition to outpatient care.
Effective communication and collaboration among team members are crucial for optimal patient outcomes.
Deterrence and Patient Education
Educating patients and caregivers about the seriousness of infectious arthritis and the potential for long-term complications is important, even with successful treatment.
Conclusion: Key Takeaways for Infectious Arthritis Diagnosis
Infectious arthritis is an orthopedic emergency requiring a high index of suspicion, especially in patients with risk factors. Prompt arthrocentesis and synovial fluid analysis are essential for diagnosis. Early initiation of antibiotics and joint drainage are crucial to minimize joint damage and improve patient outcomes. An interprofessional team approach is vital for comprehensive management and to address the complex needs of these patients. Recognizing the subtle signs and acting decisively are paramount in managing this potentially devastating condition.
