A 58-year-old man with a 10-year history of osteoarthritis of both knees presents with severe knee pain. He has been taking ibuprofen 400 mg thrice daily for the past 6 months. On examination, his blood pressure is 148/92 mmHg (previously 130/80 mmHg). Serum creatinine has risen from 0.9 mg/dL to 1.4 mg/dL over 3 months. Urinalysis shows microscopic hematuria and proteinuria (1+ on dipstick). He denies any history of diabetes, autoimmune disease, or recurrent infections. What is the most likely mechanism of NSAID-induced renal injury in this patient?

Explanation

## NSAID-Induced Renal Injury Mechanism **Key Point:** NSAIDs inhibit cyclooxygenase (COX-1 and COX-2), reducing prostaglandin (PGE₂ and PGI₂) synthesis in the kidney. These prostaglandins are crucial for maintaining renal hemodynamics, particularly in states of volume depletion or renal hypoperfusion. ### Pathophysiology in This Case In this patient, chronic NSAID use has led to: 1. **Decreased renal blood flow** — Prostaglandins normally cause afferent arteriolar vasodilation; their inhibition causes vasoconstriction 2. **Reduced glomerular filtration pressure** — Results in declining GFR (evidenced by rising creatinine from 0.9 to 1.4 mg/dL) 3. **Functional acute kidney injury** — Reversible if the NSAID is withdrawn early 4. **Secondary hypertension** — NSAIDs impair sodium excretion and promote fluid retention, explaining the BP rise from 130/80 to 148/92 mmHg ### Why Hematuria and Proteinuria Occur The reduced glomerular filtration pressure and altered intraglomerular hemodynamics lead to: - Glomerular hypoperfusion causing ischemic injury - Increased glomerular permeability (proteinuria) - Microscopic hematuria from endothelial damage **Clinical Pearl:** This is **functional renal insufficiency** — the kidney architecture is intact, but perfusion is compromised. Discontinuing the NSAID often restores renal function if caught early. **High-Yield:** Risk factors for NSAID-induced renal injury include: - Age > 65 years - Pre-existing chronic kidney disease - Diabetes mellitus - Congestive heart failure - Volume depletion - Concurrent use of ACE inhibitors or ARBs (triple whammy when combined with NSAIDs) ### Differentiation from Other Mechanisms | Mechanism | Clinical Clue | Typical Presentation | |-----------|---------------|---------------------| | **Prostaglandin inhibition (correct)** | Reversible, dose-dependent, hemodynamic | Gradual rise in Cr, preserved urine Na, hyperkalemia | | Direct tubular toxicity | Acute onset, high-dose exposure | Acute tubular necrosis pattern, muddy brown casts | | Immune-complex (NSAID-induced lupus) | Systemic symptoms, ANA+, low complement | Rash, arthritis, serology positive | | Crystal obstruction | Massive overdose, dehydration | Acute anuria, crystal casts on UA | [cite:KD Tripathi 8e Ch 12] ## Management Implications **Tip:** In this patient, the next step is to: 1. Discontinue ibuprofen immediately 2. Monitor renal function weekly 3. Switch to acetaminophen or topical NSAIDs if analgesia is needed 4. Consider non-pharmacological measures (physiotherapy, weight loss) 5. If renal function does not improve in 1–2 weeks, investigate for other causes (glomerulonephritis, drug-induced interstitial nephritis)