Overview
Definition:
Septic arthritis is a joint infection caused by microbial invasion of the synovial space, leading to inflammation and potential joint destruction
It is a medical emergency requiring prompt diagnosis and treatment to prevent long-term sequelae.
Epidemiology:
Incidence varies by age and joint, with neonates and young children being particularly susceptible
Hematogenous spread is the most common route of infection, though direct inoculation from trauma or surgery can occur
Staphylococcus aureus is the most frequent pathogen in most age groups, followed by Streptococcus species and Haemophilus influenzae (less common with widespread vaccination).
Clinical Significance:
Untreated septic arthritis can lead to rapid cartilage destruction, subchondral bone erosion, and joint ankylosis, resulting in permanent functional disability
Early recognition and appropriate management are crucial for favorable outcomes.
Clinical Presentation
Symptoms:
Fever
Joint pain and swelling
Refusal to bear weight or use the affected limb
Irritability, especially in infants
Localized warmth and erythema over the joint.
Signs:
Profound joint effusion
Marked tenderness on palpation
Limited range of motion due to pain
Systemic signs of infection: fever (may be absent in neonates), tachycardia, lethargy
Common joints involved: hip, knee, ankle, elbow.
Diagnostic Criteria:
No universally established diagnostic criteria exist, but a combination of clinical suspicion, laboratory findings, synovial fluid analysis, and imaging is used
A high index of suspicion in a child with fever and joint symptoms is paramount.
Diagnostic Approach
History Taking:
Duration and onset of symptoms
Recent trauma or injury
History of skin infections, otitis media, or other potential sources of bacteremia
Immunization status (especially Hib)
Prior antibiotic use
Presence of other medical conditions.
Physical Examination:
Careful examination of the affected joint, assessing for swelling, erythema, warmth, and tenderness
Gentle assessment of range of motion, noting pain with passive movement
Examination of other joints and potential extra-articular sources of infection
Assessment of overall hydration and systemic stability.
Investigations:
Synovial fluid analysis: This is the cornerstone
Obtain a joint aspirate for cell count and differential (leukocytes >50,000/µL, predominantly neutrophils suggests infection), Gram stain (identifies bacteria in ~50-75% of cases), culture and sensitivity (definitive diagnosis, guides antibiotic therapy, crucial for guiding empiric therapy discontinuation), crystal analysis (to rule out crystal-induced arthritis, rare in pediatrics)
Blood tests: Complete blood count (leukocytosis with left shift), C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are elevated and helpful for monitoring response to treatment but are not diagnostic
Blood cultures should be drawn before antibiotics
Imaging: Radiographs may show soft tissue swelling or joint effusion, and later signs of bone erosion
Ultrasound is sensitive for detecting effusions and guiding aspiration
MRI is useful for evaluating deeper joints, osteomyelitis, and complex effusions.
Differential Diagnosis:
Transient synovitis (most common in younger children, usually hip)
Reactive arthritis
Juvenile idiopathic arthritis
Osteomyelitis
Trauma (fracture, sprain)
Transient tenosynovitis
Cellulitis.
Joint Aspiration Thresholds
Indications For Aspiration:
Any suspected septic arthritis requires joint aspiration
High clinical suspicion in a child with fever and joint symptoms is sufficient indication even if initial labs are non-diagnostic
Ultrasonography can aid in difficult aspirations.
Aspirate Analysis Interpretation:
Key parameters: WBC count (>50,000 cells/µL with >90% neutrophils strongly suggests septic arthritis)
Gram stain: Immediate clue to pathogen
Culture: Gold standard for definitive diagnosis and antibiotic guidance
Glucose: Lower than serum glucose
Protein: Elevated (>3 g/dL).
Joint Fluid Criteria:
Purulent synovial fluid: Cloudiness, poor viscosity, elevated WBC count, and positive Gram stain are highly suggestive
A low glucose to serum glucose ratio (<0.5) and high protein level (>3 g/dL) further support the diagnosis
WBC counts between 25,000-50,000/µL are concerning and warrant careful consideration and close monitoring.
Empiric Antibiotic Coverage
Initial Antibiotic Choice:
Empiric antibiotics should be started immediately after aspiration and obtaining blood cultures, covering the most likely pathogens based on age and local epidemiology
For infants <3 months: Cefotaxime or Ceftriaxone (covering Gram-negatives including Listeria)
For children >3 months: Vancomycin (covering MRSA) plus Ceftriaxone or Cefotaxime (covering S
aureus, Strep spp., and Gram-negatives).
Pathogen Specific Coverage Considerations:
Staphylococcus aureus is the most common pathogen in all age groups
Consider MRSA coverage (Vancomycin) if local prevalence is high or if there are risk factors
For neonates, cover Listeria monocytogenes with Ampicillin in addition to a third-generation cephalosporin
For adolescents, Neisseria gonorrhoeae should be considered, particularly for sexually active individuals, and treated with Ceftriaxone.
Duration And Deescalation Of Therapy:
Intravenous antibiotics are typically continued for at least 1-2 weeks, followed by oral therapy
The total duration is usually 3-6 weeks
De-escalation of therapy is guided by culture and sensitivity results, local resistance patterns, and clinical response
If the Gram stain is negative and cultures remain negative after 48-72 hours, antibiotics may be discontinued if suspicion for infection is low
If cultures are positive, switch to a targeted antibiotic based on sensitivities
Monitor clinical signs, inflammatory markers (CRP, ESR), and joint aspiration results.
Management
Initial Management:
Immediate joint aspiration for diagnosis and synovial fluid analysis
Administration of empiric intravenous antibiotics after obtaining blood cultures
Pain management with analgesics
Immobilization of the affected joint initially to reduce pain.
Surgical Management:
Arthrotomy and thorough joint lavage are indicated for large effusions, purulent fluid, failure to improve with antibiotics alone, or involvement of difficult-to-drain joints like the hip
Repeated needle aspiration may be sufficient for smaller effusions in responsive patients
Surgical intervention is crucial to remove infected debris and purulent material to prevent further cartilage damage.
Supportive Care:
Aggressive pain control
Physical therapy to maintain range of motion and prevent stiffness once infection is controlled
Close monitoring for signs of treatment failure or complications
Nutritional support.
Complications
Early Complications:
Joint destruction (cartilage loss, osteomyelitis)
Sepsis and septic shock
Spread of infection to adjacent structures (e.g., osteomyelitis, bursitis).
Late Complications:
Chronic arthritis
Joint stiffness and limited range of motion
Growth disturbances (in children)
Chronic pain
Recurrent infections.
Prevention Strategies:
Prompt diagnosis and treatment
Judicious use of antibiotics in susceptible individuals
Strict aseptic techniques during invasive procedures
Prompt management of underlying conditions that predispose to infection.
Prognosis
Factors Affecting Prognosis:
Timeliness of diagnosis and treatment is the most critical factor
Pathogen virulence
Age of the patient (neonates have poorer outcomes)
Presence of co-morbidities
Joint involved (hip is often associated with worse outcomes)
Adequacy of surgical intervention.
Outcomes:
With prompt and appropriate treatment, most children can achieve good functional recovery
However, residual joint stiffness, pain, and even chronic arthritis can occur
Poor outcomes are associated with delayed treatment and aggressive pathogens.
Follow Up:
Regular follow-up appointments are essential for monitoring recovery, assessing joint function, and detecting late complications
This typically involves clinical examinations and serial imaging
Physical therapy may be required for several months to regain full function.
Key Points
Exam Focus:
Always suspect septic arthritis in a febrile child with a painful joint, especially if they refuse to use the limb
Joint aspiration is the cornerstone of diagnosis
Empiric coverage should be age-appropriate and broad-spectrum, covering S
aureus and common Gram-negatives.
Clinical Pearls:
In neonates, consider Listeria and cover with Ampicillin
In sexually active adolescents, consider gonococcal arthritis
Ultrasound is excellent for guiding joint aspirations
CRP is a useful marker for monitoring treatment response but less so for diagnosis.
Common Mistakes:
Delaying joint aspiration due to fear of complications or equivocal lab results
Inadequate empiric antibiotic coverage
Failure to cover MRSA in endemic areas
Discharging patients too early without adequate follow-up.