Clinical Analysis: 60F, Right Ankle Pain & Swelling, 10 Days, No Trauma

Problem Summary

• Acute/subacute monoarthritis of the right ankle
• 60-year-old female
• No trauma, no RA, no SLE, no thyroid disease, no DM, no neuropathy
• Ankle effusion + medial joint line tenderness
• ROM pain-free (important - suggests intra-articular not periarticular)
• No bony tenderness over malleoli
• Normal: CBC, serology, RBS, uric acid, RA factor
• X-ray: mild arthritic changes (lateral view)
• Not responding to analgesics + immobilisation

Key Diagnostic Clues

Differential Diagnosis (Most to Least Likely)

1. CPPD / Pseudogout - TOP DIFFERENTIAL

• Most common crystal arthropathy at this age in females, after gout is excluded
• Presents as acute monoarthritis mimicking gout - "pseudogout"
• Uric acid is normal - consistent with CPPD
• Ankle is a recognised site (though knee and wrist are more common)
• X-ray may show chondrocalcinosis (cartilage calcification) - check carefully on the lateral view
• Diagnosis: synovial fluid aspiration - CPPD crystals are weakly positive birefringent, rhomboid-shaped under polarised microscopy
• As noted in Grainger & Allison's Diagnostic Radiology: "Pseudogout: an acute clinical condition produced by CPPD crystals within joints, resulting in acute attacks of gout-like symptoms. The diagnosis is clinical and cannot be made radiologically."

2. Gout (Acute Attack with Normal Uric Acid)

• Serum uric acid drops during an acute attack - a normal level does NOT exclude gout
• However, ankle gout is less common than first MTP involvement at this age
• Synovial fluid aspiration: monosodium urate crystals - needle-shaped, strongly negatively birefringent

3. Osteochondral Lesion of the Talus (OLT)

• Can present as ankle pain and swelling with no trauma history (up to 25% are atraumatic)
• Medial talar dome is the most common site - explains medial joint line tenderness
• X-ray often misses early lesions - MRI is required
• ROM is often preserved in early lesions
• Does NOT respond to analgesics or immobilisation if the fragment is unstable

4. Septic Arthritis (Low probability but must exclude)

• Normal CBC and serology make this less likely
• However, early septic arthritis can have a normal or mildly elevated WBC
• Synovial fluid analysis is the gold standard - WBC >50,000/mm³ with >90% PMNs + positive culture
• As per Firestein & Kelley's Rheumatology: "In acute monoarthritis, synovial fluid analysis is the most valuable laboratory test"
• Must not be missed - permanent joint destruction if untreated

5. Pigmented Villonodular Synovitis (PVNS)

• Rare, but can present as monoarticular swelling in ankle (as referenced in Campbell's Operative Orthopaedics)
• Insidious onset, recurrent bloody effusions, marked synovial thickening
• MRI is diagnostic: diffuse synovial thickening with hemosiderin deposits giving "blooming" on gradient echo sequences

6. Reactive Arthritis / Early Spondyloarthropathy

• Seronegative, asymmetric, lower limb predominant
• History of prior GI/urogenital infection? Psoriasis? Back pain?
• Consider HLA-B27 testing

7. Occult Stress Fracture / Bone Contusion

• No trauma, but insufficiency fractures can occur in older postmenopausal women (osteoporosis)
• X-ray negative early; MRI shows marrow oedema pattern
• No bony tenderness over malleoli - but midfoot/subtalar pathology possible

Immediate Next Steps

A. Synovial Fluid Aspiration (MOST IMPORTANT)

• From Tietz Textbook of Laboratory Medicine: Synovial fluid WBC >50×10⁹/L gives a likelihood ratio of 3.59 for infection; >100×10⁹/L gives +LR of 13.2

B. MRI Ankle (RIGHT)

• Detects: osteochondral lesion of talus, PVNS, occult fractures, ligamentous pathology, early bone marrow oedema
• Should be done even if aspiration gives a diagnosis, to look for any underlying structural lesion

C. Additional Blood Tests

• Serum uric acid (repeat, ideally 4-6 weeks after acute attack to catch baseline hyperuricemia)
• ESR, CRP (baseline inflammatory markers)
• HLA-B27 (if reactive/spondyloarthropathy suspected)
• PTH, calcium, phosphate, ferritin, Mg (screen for metabolic causes of CPPD - hyperparathyroidism, haemochromatosis, hypomagnesaemia)
• DEXA scan (postmenopausal female - osteoporosis risk for insufficiency fractures)
• Blood cultures if septic arthritis is still in the running

D. Repeat X-ray Review

• Look carefully for chondrocalcinosis (linear calcification in cartilage) in the ankle joint - may be subtle
• Weight-bearing AP and mortise views if not already done

Likely Diagnosis

Management Once Diagnosis Confirmed

References

• Firestein & Kelley's Textbook of Rheumatology (10th Ed.) - Chapter on Early Arthritis
• Grainger & Allison's Diagnostic Radiology - CPPD Deposition Disease
• Tietz Textbook of Laboratory Medicine (7th Ed.) - Synovial Fluid Analysis
• Campbell's Operative Orthopaedics (15th Ed.) - PVNS of the Ankle
• Swisher et al., Acute Monoarthritis: Diagnosis in Adults, American Family Physician, 2025