After reading this note you will be able to recognise the clinical picture of septic arthritis, list the common local risk factors, interpret the key investigations and outline the immediate and definitive management steps – remembering to verify drug choices and local protocols against the Kenya MoH guideline and your hospital formulary.
Definition
Septic arthritis (also called infectious arthritis) is an acute, usually bacterial, invasion of a synovial joint space that provokes intense inflammation, rapid cartilage destruction and potential systemic sepsis if untreated.
Anatomy and physiology you need
- Synovial joint structure – articular cartilage, synovial membrane, joint capsule, and a thin layer of synovial fluid that supplies nutrients to avascular cartilage.
- Blood supply – the synovium is highly vascularised, providing a route for hematogenous organisms to enter the joint.
- Lymphatic drainage – limited; infection can spread to adjacent bursae and bone.
- Joint defence – synovial fluid contains neutrophils and complement; however, the avascular cartilage lacks intrinsic immune cells, making it vulnerable once bacteria breach the synovium.
Causes and risk factors
| Category | Common agents / conditions | Why relevant in Kenya |
|---|---|---|
| Bacterial | Staphylococcus aureus (incl. MRSA), Streptococcus pyogenes, Neisseria gonorrhoeae | Most frequent; skin colonisation and STIs common |
| Gram‑negative rods | Klebsiella, E. coli, Pseudomonas | Urinary or gastrointestinal sources, especially in immunocompromised |
| Mycobacterial | Mycobacterium tuberculosis | High TB prevalence; often sub‑acute presentation |
| Fungal | Candida spp., Cryptococcus | HIV, prolonged steroids |
| Viral (rare) | Parvovirus B19, Hepatitis B | May mimic but not true septic arthritis |
| Trauma | Open fractures, intra‑articular injections | Direct inoculation |
| Joint disease | Rheumatoid arthritis, osteoarthritis | Damaged cartilage and synovium lower barrier |
| Systemic risk factors | HIV infection, sickle cell disease, diabetes, chronic kidney disease, malnutrition | Impaired immunity, sickle cell vaso‑occlusion predisposes to Salmonella |
| Tropical infections | Salmonella (especially in sickle cell), Burkholderia pseudomallei (melioidosis) | Environmental exposure in rural settings |
| Iatrogenic | Arthroscopy, joint aspiration, intra‑articular steroids | Breach of sterile barrier |
Pathophysiology
- Bacterial entry → hematogenous spread, direct inoculation, or contiguous spread from osteomyelitis.
- Synovial colonisation → bacteria adhere to synovial membrane via adhesins.
- Rapid proliferation → synovial fluid provides nutrients; bacterial load rises quickly.
- Innate immune activation → neutrophil influx, cytokine release (IL‑1, TNF‑α).
- Enzymatic damage → bacterial toxins and neutrophil proteases degrade cartilage matrix.
- Joint effusion & pressure → increased intra‑articular pressure impairs perfusion, causing ischemic cartilage death.
- Systemic spill‑over → cytokine storm may lead to sepsis, especially in immunocompromised hosts.
The cascade proceeds within hours; early interruption is critical to preserve joint integrity.
Clinical features
| History element | Why it matters |
|---|---|
| Onset – rapid (hours‑days) vs insidious | Septic arthritis classically has an acute, fulminant onset; a slower course suggests crystal arthropathy or sub‑acute TB arthritis. |
| Pain – severe, constant, worsens with movement | Infection inflames the synovium and joint capsule, producing intense nociceptive pain; mechanical irritation is less prominent in non‑infective arthritis. |
| Swelling – tense, warm, often unilateral | Synovial exudate and hyperemia raise intra‑articular pressure; warmth reflects increased blood flow from inflammatory mediators. |
| Fever – documented or subjective chills | Systemic response to bacterial toxins; absence does not exclude infection, especially in immunocompromised or elderly patients. |
| Joint involved – large joints (knee, hip, shoulder) most common; small joints possible in children or TB | Large joints have more synovial tissue and are more prone to hematogenous seeding; TB often affects weight‑bearing joints. |
| Recent trauma or intra‑articular procedure | Provides a portal for bacteria; even minor skin breaches can seed the joint via bloodstream. |
| Co‑morbidities – HIV, diabetes, sickle‑cell disease, chronic steroid use, recent malaria or TB treatment | Impair immune defenses, increase bacteremia risk, and may alter typical organism profile (e.g., Salmonella in sickle‑cell). |
| Drug history – recent antibiotics, immunosuppressants | May mask fever or alter culture yield; immunosuppression predisposes to atypical organisms. |
| Systemic symptoms – weight loss, night sweats, cough | Suggest alternative diagnoses such as TB or disseminated fungal infection. |
| Functional limitation – inability to bear weight or use the limb | Reflects joint effusion and pain; severe limitation raises concern for rapid joint destruction. |
Examination findings and pathophysiologic basis
| Finding | Reason it occurs |
|---|---|
| Joint effusion – visible swelling, bulging capsule | Accumulation of purulent synovial fluid raises intra‑articular pressure. |
| Warmth & erythema – localized increase in temperature, redness | Vasodilation and hyperemia driven by cytokines (IL‑1, TNF‑α). |
| Tenderness – disproportionate to range of motion | Inflammatory mediators sensitize nociceptors in the synovium and capsule. |
| Reduced active & passive ROM – especially painful on passive movement | Joint capsule stretch stimulates pain fibers; effusion mechanically limits motion. |
| Joint instability or crepitus – may appear later | Cartilage destruction and ligamentous laxity from enzymatic degradation. |
| Systemic signs – tachycardia, fever, hypotension (if sepsis) | Reflect systemic inflammatory response; important for severity assessment. |
| Skin changes – punctate lesions, ulceration, sinus tract (especially in TB) | Direct extension of infection from adjacent skin or chronic draining sinuses. |
| Neurovascular status – intact distal pulses, sensation | Needed to rule out compartment syndrome or vascular compromise secondary to swelling. |
Differential diagnosis
| Condition | Points toward it | How to separate from septic arthritis |
|---|---|---|
| Crystal arthropathy (gout, pseudogout) | Sudden severe pain, often mono‑articular, may have fever | Synovial fluid shows negatively birefringent (gout) or positively birefringent (pseudogout) crystals; cultures negative |
| Reactive arthritis | Post‑infectious (GI or GU), asymmetric oligo‑arthritis, may have conjunctivitis or urethritis | No purulent joint fluid; sterile cultures; history of preceding infection |
| Tuberculous arthritis | Insidious onset, weight loss, night sweats, involvement of weight‑bearing joints | Chronic course, positive TB PCR or culture, granulomatous histology; often low‑grade inflammation |
| Septic arthritis secondary to fungal infection | Immunocompromised host, sub‑acute course, may have skin lesions | Fungal elements on microscopy, growth on specialized media; often refractory to antibacterial therapy |
| Osteomyelitis with secondary joint involvement | Persistent bone pain, overlying sinus, systemic signs | Imaging shows bone marrow edema; joint fluid may be secondary, cultures may grow same organism as bone |
| Hemarthrosis (trauma, hemophilia) | History of bleeding disorder or trauma, swollen joint without warmth | Joint aspirate is bloody, no leukocytosis, cultures negative |
| Rheumatoid arthritis flare | Symmetrical polyarthritis, chronic history, morning stiffness | Serology (RF, anti‑CCP) positive, synovial fluid low WBC, no organism growth |
Investigations
| Test | What it answers | Interpretation |
|---|---|---|
| Joint aspiration & Gram stain | Presence of organism, inflammatory cell count | >50 000 WBC/mm³ with neutrophil predominance strongly suggests infection; Gram‑positive or Gram‑negative organisms guide empiric therapy |
| Synovial fluid culture (aerobic & anaerobic) | Definitive pathogen identification | Growth confirms septic arthritis; negative culture does not exclude if antibiotics given |
| Blood cultures (2 sets) | Bacteremia source, systemic spread | Positive in up to 50 % of cases; same organism as joint fluid supports hematogenous seeding |
| CBC with differential | Systemic inflammatory response | Leukocytosis with neutrophilia common; anemia may indicate chronic disease |
| CRP & ESR | Inflammation severity, trend monitoring | Markedly elevated in infection; rapid fall with effective therapy |
| Plain radiograph of the joint | Baseline bony changes, effusion | May be normal early; later shows joint space narrowing, erosions |
| Ultrasound of the joint | Detects effusion, guides aspiration | Helpful for deep joints (hip) and to assess for synovial thickening |
| MRI (if diagnosis uncertain or to assess adjacent bone) | Detailed soft‑tissue and bone involvement | Shows marrow edema, abscess, or early cartilage loss; distinguishes TB or osteomyelitis |
| HIV test (if status unknown) | Underlying immunosuppression | Positive status raises suspicion for atypical organisms |
| Sickle‑cell screen (if relevant) | Predisposition to Salmonella septic arthritis | Positive test prompts coverage for Salmonella species |
Diagnosis and severity
Diagnostic criteria (clinical + laboratory)
- Acute mono‑articular arthritis with pain, swelling, warmth, and limited motion.
- Synovial fluid showing >50 000 WBC/mm³ (predominantly neutrophils) or positive Gram stain/culture.
- Systemic signs (fever, tachycardia) or elevated inflammatory markers (CRP, ESR).
Presence of any two of the above, plus a compatible clinical picture, confirms septic arthritis; isolation of a pathogen makes the diagnosis definitive.
Severity grading (used to guide urgency of intervention and level of monitoring)
| Severity | Clinical hallmarks | Management implications |
|---|---|---|
| Mild | Stable vitals, limited systemic response, joint <24 h symptom duration, no comorbid immunosuppression | Outpatient IV/IM antibiotics possible after aspiration; close follow‑up within 24 h |
| Moderate | Fever, tachycardia, CRP markedly raised, joint symptoms 1–3 days, single comorbidity (e.g., diabetes) | Admit for IV antibiotics, repeat aspiration, monitor labs daily |
| Severe | Hypotension or septic shock, multi‑organ dysfunction, >3 days symptoms, poly‑articular involvement, immunocompromised (HIV, sickle‑cell) | Immediate surgical drainage (arthroscopy or open), ICU care, broad‑spectrum IV antibiotics, serial joint assessments |
Key points for clinicians
- Do not delay joint aspiration; it is both diagnostic and therapeutic.
- Initiate empiric antibiotics after obtaining cultures, covering Staphylococcus aureus (including MRSA where prevalence is high), Streptococcus species, and Gram‑negative bacilli; add Salmonella coverage in sickle‑cell disease and consider fungal agents in profoundly immunocompromised patients.
- Early surgical drainage (within 6–12 h of diagnosis) reduces cartilage loss, especially for large joints (hip, shoulder) or when purulent material is thick.
- Monitor CRP daily; a >50 % fall by day 3 predicts good outcome.
- Re‑aspirate if clinical improvement is not evident within 48 h or if the effusion re‑accumulates.
Prompt recognition, timely joint drainage, and appropriate antimicrobial therapy are the cornerstones of preventing irreversible joint damage and systemic complications in septic arthritis.
Management
| Phase | Key actions | Rationale / safety notes |
|---|---|---|
| Immediate stabilisation | • Assess airway, breathing, circulation; give O₂ if hypoxic. <br>• Establish large‑bore IV access. <br>• Give empirical IV broad‑spectrum antibiotics (cover Staphylococcus aureus, Streptococci, Gram‑negatives, and Neisseria gonorrhoeae in sexually active adolescents). <br>• Obtain blood cultures before antibiotics if possible (≤ 1 hour). <br>• Analgesia with paracetamol ± opioid; avoid NSAIDs until infection control is confirmed. | Prevent septic shock, ensure adequate tissue perfusion, and start antimicrobial therapy within the “golden hour”. |
| Definitive treatment | 1. Joint aspiration under aseptic technique; send synovial fluid for Gram stain, culture, cell count, glucose, protein, crystal analysis. <br>2. Targeted antibiotics once organism identified; continue IV for 2‑4 weeks (adjust for TB, atypical mycobacteria, or fungal pathogens). <br>3. Surgical drainage – arthroscopic washout for most joints; open arthrotomy for hip, shoulder, or when arthroscopy unavailable. <br>4. Adjuncts – consider steroids only after adequate antimicrobial coverage and in selected immune‑mediated co‑pathology. | Synovial analysis confirms infection and guides therapy. Early drainage reduces bacterial load, prevents cartilage destruction, and improves outcomes. |
| Monitoring | • Vital signs q4‑6 h; watch for fever, hypotension, tachycardia. <br>• Serial CRP/ESR (falling trend indicates response). <br>• Repeat joint aspiration if clinical deterioration or no improvement after 48‑72 h. <br>• Renal and hepatic function tests if using nephro‑ or hepatotoxic agents. | Detect treatment failure, drug toxicity, and need for repeat drainage. |
| Follow‑up | • Transition to oral antibiotics after ≥ 2 weeks IV and clinical stability; total duration 4‑6 weeks (longer for TB or prosthetic joints). <br>• Physiotherapy to restore range of motion once inflammation subsides. <br>• Monitor for late complications (e.g., osteomyelitis) with repeat imaging if indicated. | Prevent stiffness, maintain joint function, and ensure eradication of infection. |
| Contra‑indications / safety | • NSAIDs before drainage may mask fever and worsen renal perfusion. <br>• Intra‑articular steroids without antibiotics can exacerbate infection. <br>• Arthrocentesis in a joint with overlying cellulitis should be delayed until skin infection controlled, unless septic shock mandates immediate drainage. | Avoid iatrogenic worsening and ensure safe procedural environment. |
Complications and prognosis
- Joint destruction – rapid cartilage loss; irreversible if drainage delayed > 48 h.
- Chronic osteomyelitis – especially with Staphylococcus aureus or TB; may need long‑term antibiotics and debridement.
- Septic emboli – to lungs (particularly with Staph bacteremia) or other organs.
- Systemic sepsis / multi‑organ failure – high mortality in immunocompromised (HIV, sickle‑cell) or delayed treatment.
- Functional impairment – stiffness, reduced range of motion, need for joint replacement later.
Prognosis is excellent when diagnosis and drainage occur within 24‑48 h and appropriate antibiotics are started; otherwise, morbidity rises sharply.
High‑yield exam points
- Look‑alike conditions: crystal arthropathy (gout, pseudogout), reactive arthritis, early rheumatoid arthritis, osteomyelitis, transient synovitis of the hip.
- Commonest wrong first step: prescribing NSAIDs or oral antibiotics without joint aspiration; examiners expect immediate arthrocentesis.
- Bedside examiner questions:
- “What fluid findings make septic arthritis most likely?” (WBC > 50 000 µL, predominantly neutrophils, low glucose).
- “Why is joint aspiration before antibiotics important?” (to obtain culture, avoid false‑negative results).
- “How would you differentiate septic hip from transient synovitis?” (fever, inability to bear weight, elevated CRP/ESR, ultrasound effusion size).
Clinical case
Case: A 7‑year‑old boy from a rural Kenyan clinic presents with a 2‑day history of left knee pain, swelling, fever 38.9 °C, and refusal to bear weight. He is HIV‑negative, has no trauma history, and lives in a malaria‑endemic area. Examination shows a warm, erythematous knee with effusion; passive flexion limited to 30°. No skin ulcer overlying the joint.
Questions
- What is the next immediate step?
Answer: Perform urgent sterile arthrocentesis for synovial fluid analysis and send for Gram stain, culture, cell count, and crystal examination before starting antibiotics.
- If Gram stain shows Gram‑positive cocci in clusters, what is the most appropriate empirical antibiotic regimen?
Answer: Start IV anti‑staphylococcal therapy (e.g., a first‑generation cephalosporin or a penicillinase‑resistant penicillin) covering methicillin‑sensitive S. aureus; adjust if MRSA risk is high.
- When should surgical drainage be considered in this child?
Answer: If the joint is the knee, arthroscopic washout is preferred within 24 h of presentation; open drainage is reserved for failure of arthroscopy or if the joint is difficult to access (e.g., hip).
Practice questions
Question 1
A 55‑year‑old man with poorly controlled diabetes presents with a painful, swollen right shoulder, fever 39 °C, and limited active motion. Synovial fluid WBC is 70 000 µL, neutrophil‑predominant, Gram stain negative. What is the most appropriate next step?
A. Start oral NSAIDs and arrange physiotherapy
B. Immediate arthroscopic washout of the shoulder
C. Obtain MRI before any intervention
D. Begin empirical IV antibiotics only
Answer: B. immediate arthroscopic washout of the shoulder
Explanation: High‑grade effusion with systemic signs mandates prompt surgical drainage; antibiotics alone are insufficient.
Question 2
A 30‑year‑old woman with recent unprotected intercourse presents with a hot, swollen left knee, fever, and a history of vaginal discharge. Synovial fluid shows Gram‑negative diplococci. Which organism is most likely?
A. Staphylococcus aureus
B. Neisseria gonorrhoeae
C. Streptococcus pyogenes
D. Escherichia coli
Answer: B. Neisseria gonorrhoeae
Explanation: Gram‑negative diplococci in a sexually active adult point to gonococcal septic arthritis.
Question 3
A child with sickle‑cell disease develops a painful hip with limited motion but no fever. Ultrasound shows a small effusion. Which condition is the most likely differential?
A. Septic arthritis
B. Transient synovitis
C. Osteomyelitis
D. Avascular necrosis
Answer: B. Transient synovitis
Explanation: In sickle‑cell disease, painless or low‑grade fevers with small effusions suggest transient synovitis; septic arthritis usually presents with high fever and larger effusion.
Question 4
During treatment of septic knee arthritis, the patient’s CRP falls from 150 mg/L to 30 mg/L after 5 days, but the knee remains swollen and painful. What is the best next action?
A. Discontinue antibiotics and start oral NSAIDs
B. Perform repeat joint aspiration for culture
C. Switch to oral antibiotics immediately
D. Schedule physiotherapy and observe
Answer: B. Perform repeat joint aspiration for culture
Explanation: Persistent clinical signs despite falling inflammatory markers warrant repeat aspiration to rule out ongoing infection or resistant organism.
Question 5
Which of the following is a contraindication to intra‑articular steroid injection in a patient with suspected septic arthritis?
A. Prior use of systemic antibiotics
B. Presence of a small effusion on ultrasound
C. Positive Gram stain from synovial fluid
D. Controlled HIV infection
Answer: C. Positive Gram stain from synovial fluid
Explanation: Steroids can exacerbate infection; they are contraindicated when bacterial presence is confirmed.
Drafted with AI assistance from standard references and reviewed for structure. Check details against your lecture notes and the Kenya MoH guideline.