How should I evaluate and manage an 80-year-old man with type 2 diabetes, hypertension, on furosemide (Lasix), dapagliflozin, and simvastatin (Zocor) who presents with sudden severe left‑knee pain and stiffness?

Acute Monoarticular Knee Pain in an Elderly Diabetic Patient

This patient requires immediate arthrocentesis to rule out septic arthritis or crystal arthropathy (gout or pseudogout), as sudden severe monoarticular pain with stiffness in an elderly diabetic on diuretics represents a high-risk presentation that demands urgent synovial fluid analysis before any other intervention.

Immediate Diagnostic Evaluation

Synovial Fluid Analysis (Arthrocentesis)

• Perform urgent arthrocentesis with synovial fluid sent for cell count with differential, Gram stain, bacterial culture, and crystal analysis under polarized microscopy 1
• Septic arthritis is a medical emergency requiring same-day diagnosis, particularly in diabetic patients who have increased infection risk 2
• Crystal arthropathy (gout or pseudogout) is highly likely given the patient's diuretic use (furosemide), which increases uric acid levels and precipitates gout 2
• White blood cell count >50,000/μL with >90% neutrophils suggests septic arthritis, while 2,000-50,000/μL may indicate crystal disease 1

Laboratory Studies

• Obtain serum uric acid, complete blood count, inflammatory markers (ESR, CRP), and basic metabolic panel to assess for hyperuricemia, systemic infection, and renal function 1
• Check serum creatinine and electrolytes, as both dapagliflozin and furosemide can affect kidney function and volume status 2, 3
• Measure blood glucose, as SGLT2 inhibitors like dapagliflozin rarely cause euglycemic diabetic ketoacidosis, which can present with nonspecific symptoms 4

Imaging

• Order plain radiographs of the left knee (AP, lateral, and sunrise views) to evaluate for chondrocalcinosis (suggesting pseudogout), joint effusion, or underlying osteoarthritis 1
• Radiographs showing chondrocalcinosis support pseudogout diagnosis, while normal films do not exclude septic arthritis or gout 1
• MRI is not indicated acutely unless septic arthritis is excluded and mechanical pathology is suspected after initial workup 1

Critical Differential Diagnoses

Septic Arthritis (Highest Priority)

• Must be excluded first due to potential for rapid joint destruction and systemic sepsis, especially in diabetic patients 2
• Risk factors present: diabetes, advanced age (80 years), potential immunosuppression from chronic disease 2
• If suspected based on clinical presentation or synovial fluid analysis, initiate empiric IV antibiotics immediately after cultures obtained 1

Crystal Arthropathy

• Gout is highly probable given furosemide use, which causes hyperuricemia through increased proximal tubular uric acid reabsorption 2
• Loop diuretics like furosemide are well-established precipitants of acute gout attacks 2
• Pseudogout (calcium pyrophosphate deposition disease) is also common in elderly patients and can present identically 1
• Diagnosis confirmed by identifying negatively birefringent needle-shaped crystals (gout) or positively birefringent rhomboid crystals (pseudogout) in synovial fluid 1

Statin-Related Myopathy (Lower Priority but Consider)

• Evaluate for simvastatin-induced muscle symptoms, though typically bilateral and affecting larger muscle groups rather than isolated joint pain 2
• The ACC/AHA guidelines recommend evaluating muscle symptoms (pain, tenderness, stiffness) in statin-treated patients by checking creatine kinase (CK) and creatinine if severe symptoms develop 2
• However, acute monoarticular knee pain is atypical for statin myopathy, which usually presents as proximal muscle weakness or diffuse myalgias 2

Medication-Related Considerations

Diuretic Effects

• Furosemide significantly increases gout risk through volume depletion and enhanced uric acid reabsorption 2
• Consider whether recent dose adjustments or dehydration episodes preceded symptom onset 2

SGLT2 Inhibitor Effects

• Dapagliflozin causes mild osmotic diuresis with volume contraction, which could theoretically precipitate gout through hemoconcentration 5, 4
• Monitor for volume depletion signs (orthostatic hypotension, elevated hematocrit) 5, 4
• Rare cases of acute kidney injury with SGLT2 inhibitors have been reported, though typically not presenting as isolated joint pain 6

Initial Management Algorithm

If Septic Arthritis Suspected or Confirmed

1. Initiate empiric IV antibiotics (vancomycin plus ceftriaxone or cefepime) immediately after blood and synovial fluid cultures obtained 1
2. Consult orthopedic surgery for consideration of surgical drainage if purulent fluid or loculations present 1
3. Hold dapagliflozin temporarily due to infection stress and potential volume depletion 4

If Crystal Arthropathy Confirmed

1. For acute gout: Initiate colchicine 1.2 mg followed by 0.6 mg one hour later, then 0.6 mg daily, OR indomethacin 50 mg three times daily (if renal function adequate), OR prednisone 40 mg daily for 5 days 1
2. Avoid NSAIDs if possible given age 80, diabetes, hypertension, and potential renal impairment; consider intra-articular corticosteroid injection as alternative 1
3. Do not start urate-lowering therapy (allopurinol) during acute attack; wait until inflammation resolves 1
4. Consider switching furosemide to alternative antihypertensive if blood pressure control permits, as loop diuretics perpetuate hyperuricemia 2

If Osteoarthritis Flare

1. Initiate structured physical therapy focusing on quadriceps strengthening 1
2. Consider intra-articular corticosteroid injection, which provides approximately 2 months of relief 1
3. Acetaminophen up to 3 grams daily in divided doses as adjunctive therapy 1

Critical Pitfalls to Avoid

• Never delay arthrocentesis in acute monoarticular arthritis—septic arthritis can destroy a joint within 24-48 hours 1
• Do not assume osteoarthritis based on age alone without excluding infection and crystal disease 1
• Avoid empiric NSAID therapy before establishing diagnosis, as it may mask septic arthritis and cause acute kidney injury in elderly diabetic patients on diuretics 2, 1
• Do not discontinue simvastatin based solely on knee pain without evidence of myopathy (elevated CK, diffuse muscle symptoms), as cardiovascular benefits outweigh risks in this high-risk patient 2
• Monitor renal function closely given combination of furosemide, dapagliflozin, and potential NSAID use 2, 6