⚠️ Acute monoarthritis requires urgent investigation and treatment.
🔎 The key investigation is synovial fluid aspiration for Gram stain, culture, and crystal analysis (to diagnose gout or pseudogout).
⭐ Always consult orthopaedics before aspirating a prosthetic joint.

ℹ️ About Monoarticular Arthritis

Monoarticular arthritis = inflammation in a single joint.
🚨 Common urgent causes: Gout, Pseudogout, Septic Arthritis.
⚡ Pathophysiology: Crystals or bacteria trigger neutrophil recruitment → cytokine storm → intense synovitis.
🧩 Think “Don’t Miss”: Septic arthritis = limb- and life-threatening until proven otherwise.

🧬 Aetiology

💎 Gout: Monosodium urate crystals from hyperuricaemia.
📐 Pseudogout: Calcium pyrophosphate (CPPD) crystals → “chondrocalcinosis.”
🦠 Infection: Bacteria (esp. Staph. aureus) or Neisseria gonorrhoeae in young adults.
🦴 Others: Trauma/haemarthrosis, autoimmune flares (RA, SLE, vasculitis).

🩺 Clinical Features

⏱️ Sudden painful swollen joint, warm and erythematous.
Gout: Classically 1st MTP (podagra), red-hot swollen big toe.
Pseudogout: Knees & wrists most affected, often older patients.
Septic arthritis: Rapid onset pain, fever, systemic unwell ➝ orthopaedic emergency.
Limited range of movement due to pain ⛔.

🔬 Investigations

💉 Synovial fluid aspiration (gold standard):
– Gout ➝ needle-shaped, negatively birefringent crystals.
– Pseudogout ➝ rhomboid, positively birefringent crystals.
– Septic ➝ WCC >50,000, ↓ glucose, positive Gram stain/culture.
🧪 Bloods: FBC, CRP, uric acid, cultures.
📸 Imaging: X-ray may show joint damage, erosions, or chondrocalcinosis in pseudogout.
🧠 Rule out mimics: trauma, haemarthrosis (esp. anticoagulated patients).

🧪 Pathology

Gout: Chronic flares ➝ tophi deposits (chalky white nodules), joint erosion on XR.
Pseudogout: Chronic CPPD ➝ cartilage calcification + OA-like degeneration.

💊 Management

Acute attacks:
- NSAIDs ✅ (first-line if no contraindications).
- Colchicine 🌿 for gout (esp. if renal impairment limits NSAIDs).
- Steroids 💉 (oral or intra-articular) if NSAIDs/colchicine unsuitable.
Septic arthritis: 🚨 Urgent aspiration, IV antibiotics, surgical washout.
Chronic Gout:
- Allopurinol or Febuxostat to lower uric acid 🎯 (only after acute flare resolves).
- Lifestyle: ↓ purines (red meat, seafood), ↓ alcohol, hydration, weight control.
Pseudogout: Symptomatic control with NSAIDs, colchicine, steroids. No urate-lowering equivalent available.

📚 References

🗂️ Differential Diagnosis Table

🔎 Cause	💡 Key Clues
🔥 Gout	Podagra (1st MTP), red-hot swollen joint, urate crystals (– birefringent).
💎 Pseudogout	Knee/wrist pain, chondrocalcinosis, CPPD crystals (+ birefringent).
🦠 Septic Arthritis	Rapid pain + fever. WCC >50,000, urgent IV abx + washout.
🧬 Gonococcal Arthritis	Young, sexually active. Polyarthritis, tenosynovitis, skin lesions.
↔️ Reactive Arthritis	Follows STI/GI infection. “Can't see, can't pee, can't climb a tree.”
💥 Trauma / Haemarthrosis	Post-injury, anticoagulated, haemophilia.
🧩 RA Flare	Sometimes monoarticular. Synovitis, not red-hot.
🦾 Prosthetic Joint	NEVER aspirate without ortho advice. High infection risk.
🌍 Other	Lyme, viral arthritis (parvovirus, hep B/C), SLE.

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Acute Monoarthritis