What is the differential diagnosis (ddx) for sudden onset bilateral posterior patellar knee pain in a 69-year-old patient that started after consuming wine and venison?

Differential Diagnosis for Sudden Onset Bilateral Posterior Patellar Knee Pain

The most likely diagnosis in this 69-year-old patient with sudden onset bilateral knee pain after consuming wine and venison is acute gout (crystal arthropathy), given the classic dietary triggers (alcohol and purine-rich meat), nocturnal onset, and bilateral presentation. 1

Primary Diagnostic Considerations

Crystal Arthropathy (Gout - Most Likely)

• Acute gout classically presents with sudden onset pain, often beginning at night, triggered by alcohol consumption and purine-rich foods like venison 2
• The bilateral presentation, while less common than monoarticular gout, can occur in polyarticular gout, particularly in older patients 2
• The posterior patellar (retropatellar) location suggests patellofemoral joint involvement, which can be affected by crystal deposition 2
• Joint aspiration for crystal analysis is the definitive diagnostic test and should be performed if effusion is present 2

Pseudogout (Calcium Pyrophosphate Deposition Disease)

• Can present similarly to gout with acute onset pain, particularly in older patients 2
• Bilateral knee involvement is more common in pseudogout than gout 1
• Chondrocalcinosis may be visible on radiographs, though absence does not exclude the diagnosis 2
• Requires synovial fluid analysis to differentiate from gout 2

Secondary Considerations

Patellofemoral Pain Syndrome

• While typically affecting younger patients (<40 years), it can occur in older adults 1, 3
• Characterized by retropatellar pain with activities like squatting, stair climbing, and prolonged sitting with knees flexed 3, 4
• The sudden nocturnal onset and bilateral presentation make this less likely than crystal arthropathy 1
• Pain during decline squat test would support this diagnosis if present 2

Patellar Tendinopathy

• Presents with pain localized to the inferior pole of the patella, not typically posterior patellar 4
• Load-related pain that increases with knee extensor demand 4
• The sudden nocturnal onset without preceding trauma or overuse makes this diagnosis unlikely 2, 5

Subchondral Insufficiency Fracture

• Most commonly affects the medial femoral condyle in middle-aged to elderly females, though can involve patellofemoral joint 2
• Radiographs are often initially normal; MRI is more sensitive for early detection 2
• Bilateral simultaneous presentation would be extremely rare 2

Septic Arthritis

• Must be excluded in any acute monoarticular or oligoarticular arthritis 2
• Bilateral simultaneous septic arthritis is rare but possible in immunocompromised patients 2
• Joint aspiration with synovial fluid analysis (cell count, Gram stain, culture) is mandatory if infection is suspected 2

Diagnostic Approach

Immediate Assessment

• Obtain standing AP, lateral, tunnel, and tangential patellar radiographs bilaterally to exclude fracture, loose bodies, and identify chondrocalcinosis 2
• Assess for joint effusion clinically and with ultrasound if available 2
• Perform arthrocentesis if effusion present for crystal analysis (polarized microscopy), cell count, Gram stain, and culture 2

Laboratory Testing

• Serum uric acid level (though may be normal during acute gout attack) 1
• Complete blood count, inflammatory markers (ESR, CRP) to assess for infection or inflammatory arthritis 2
• Basic metabolic panel to assess renal function before NSAID use 1

Advanced Imaging (If Initial Workup Non-Diagnostic)

• MRI without contrast is indicated if radiographs are normal but symptoms persist, to evaluate for subchondral insufficiency fracture, bone marrow lesions, or occult pathology 2
• MRI can detect early cartilage abnormalities and bone marrow edema patterns associated with crystal deposition 2

Critical Pitfalls to Avoid

• Do not attribute bilateral knee pain solely to age-related osteoarthritis without excluding acute inflammatory or crystal-induced arthropathy 1
• Do not delay joint aspiration if septic arthritis is in the differential, as this is a surgical emergency 2
• Recognize that serum uric acid may be normal during acute gout attack; diagnosis relies on synovial fluid analysis 1
• NSAIDs should be used cautiously in elderly patients and avoided if renal insufficiency is present 2, 1
• The dietary history (wine and venison) is highly suggestive but not diagnostic; crystal confirmation is required 1