Most common cause of intrinsic AKI; tubular epithelial injury from ischemia or nephrotoxins.
Also known as: ATN, acute tubular necrosis, ischemic ATN, nephrotoxic ATN
Overview
Intrinsic AKI characterized by injury and necrosis of renal tubular epithelial cells, most often from ischemic insult (prolonged hypoperfusion) or direct nephrotoxic exposure. The most common cause of hospital-acquired AKI.
Epidemiology
Accounts for ~50% of hospital-acquired AKI and ~75% of AKI in critically ill patients. Mortality 40-60% in ICU patients requiring RRT.
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Question 1RenalMedium
A 70-year-old man with hypertension is admitted for gastrointestinal bleeding. Lisinopril was started 2 weeks ago, and his baseline serum creatinine is 0.9 mg/dL. His systolic blood pressure stays between 75 and 85 mm Hg for 6 hours before transfusion restores it, and he undergoes contrast CT angiography to localize the bleeding. The next day he develops oliguria. Urinalysis shows muddy brown granular casts. Which of the following is the strongest risk factor for this condition?
ARecent intravenous contrast exposure
BProlonged renal ischemia from shock
CHypertensive nephrosclerosis
DBilateral renal artery stenosis
ENew angiotensin inhibitor therapy
Reveal answer & full explanation
Correct answer: B — Prolonged renal ischemia from shock
ARecent intravenous contrast exposure
BProlonged renal ischemia from shock✓
CHypertensive nephrosclerosis
DBilateral renal artery stenosis
ENew angiotensin inhibitor therapy
Why Prolonged renal ischemia from shock is correct
Muddy brown granular casts indicate acute tubular necrosis, and sustained hypotension from GI hemorrhage is a direct ischemic cause.
Prolonged hypoperfusion injures the metabolically active proximal tubular cells, which slough into the lumen and form the casts.
In this vignette the documented shock state is the strongest and most proximate precipitant of the tubular injury.
Why the others are wrong
New angiotensin inhibitor therapy — Prerenal-confusion trap: ACE inhibitors drop GFR hemodynamically but do not by themselves produce muddy brown casts of established tubular necrosis.
Bilateral renal artery stenosis — Anchoring trap: stenosis predisposes to ACE-inhibitor-induced prerenal failure, not the ischemic ATN driven here by hemorrhagic shock.
Recent intravenous contrast exposure — Right-disease-wrong-weight trap: he did receive IV contrast for the CT angiogram, but with normal baseline kidney function the risk of contrast-associated AKI is low, whereas 6 hours of systolic pressure at 75 to 85 mm Hg is a far stronger ischemic insult.
Hypertensive nephrosclerosis — Chronic-background trap: underlying hypertensive kidney disease does reduce renal reserve and raises the risk of tubular injury, but his baseline creatinine of 0.9 mg/dL suggests preserved kidney function, and 6 hours of systolic pressure at 75 to 85 mm Hg is the direct ischemic insult that produced the casts.
Question 2RenalMedium
A hospitalized patient develops acute kidney injury after prolonged hypotension. Urine microscopy shows muddy brown granular casts. Which of the following is the most appropriate management?
AUrgent initiation of hemodialysis
BSupportive care and treat the cause
CLow-dose renal dopamine infusion
DScheduled intravenous loop diuretic
EImmediate high-dose corticosteroids
Reveal answer & full explanation
Correct answer: B — Supportive care and treat the cause
AUrgent initiation of hemodialysis
BSupportive care and treat the cause✓
CLow-dose renal dopamine infusion
DScheduled intravenous loop diuretic
EImmediate high-dose corticosteroids
Why Supportive care and treat the cause is correct
Muddy brown granular casts after hypotension indicate ischemic acute tubular necrosis.
Management is supportive: restore perfusion, remove nephrotoxins, and correct volume and electrolytes while tubules recover.
Most ischemic ATN improves over days to weeks with these measures alone.
Why the others are wrong
Immediate high-dose corticosteroids — steroids treat allergic interstitial nephritis or certain glomerulonephritides, not ischemic tubular injury; this is the wrong-mechanism trap.
Low-dose renal dopamine infusion — so-called renal-dose dopamine may transiently raise urine output but does not prevent dialysis, speed recovery, or reduce mortality in ATN, and it can provoke arrhythmias; KDIGO recommends against it; this is the outdated-therapy trap.
Urgent initiation of hemodialysis — dialysis is reserved for refractory hyperkalemia, acidosis, volume overload, or uremia, none stated here; this is the overtreatment trap.
Scheduled intravenous loop diuretic — a loop diuretic can raise urine output in some patients with ATN, which makes it feel like treatment, but it does not speed tubular recovery, reduce the need for dialysis, or lower mortality, and it can worsen volume depletion; KDIGO advises against diuretics to treat AKI except to manage volume overload, which is not described here; this is the treat-the-urine-output trap.
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IV iodinated contrast: clinically meaningful risk mainly with eGFR <30 or existing AKI (ACR-NKF 2020); risk is low when baseline kidney function is normal
Hemodynamic drug combinations: NSAIDs plus ACEi/ARB with or without a diuretic (triple whammy), especially in elderly or volume-depleted patients
Pathophysiology
Ischemic or toxic injury depletes tubular ATP, disrupts apical brush borders, and causes epithelial cell death and desquamation. Sloughed cells obstruct the tubular lumen → backleak of filtrate, tubuloglomerular feedback, and reduced GFR. Three phases: initiation (hours-days), maintenance (days-weeks of established oliguric AKI), and recovery (polyuric phase as tubules regenerate).
Clinical presentation
Symptoms
Often asymptomatic AKI discovered on labs in ICU or postoperative setting
Oliguria or anuria common; nonoliguric ATN in 30-50% (better prognosis)
Symptoms of underlying cause: sepsis, shock, recent surgery, exposure to nephrotoxin
Uremic symptoms if severe: nausea, confusion, dyspnea
Signs / physical exam
Volume status variable — euvolemic, overloaded, or depleted depending on cause and management
Findings of underlying critical illness
Skin findings in atheroemboli (livedo, blue toes)
Bladder not distended; no obstruction
Classic findings
Muddy brown granular casts with renal tubular epithelial cells in the urine sediment are the characteristic, highly suggestive finding of ATN (not pathognomonic, and absent in some cases).
Differential diagnosis
Prerenal AKI — BUN:Cr >20:1, FENa <1%, bland sediment; responsive to volume — distinguish before fluid challenge
Postrenal obstruction — Hydronephrosis on US, distended bladder, anuria; rapid reversal with drainage
Renal atheroembolic disease — Days-weeks after vascular procedure; livedo reticularis, blue toes, eosinophilia, eosinophiluria
Contrast-induced nephropathy — Cr rise within 24-72 h of IV contrast; typically resolves by day 7-10
Diagnostic workup
Diagnostic criteria
Clinical diagnosis: AKI meeting KDIGO criteria with characteristic sediment (muddy brown casts), FENa >2%, recent ischemic or nephrotoxic event, and exclusion of prerenal/postrenal/other intrinsic causes. Biopsy rarely needed.
Labs
BMP — rise in creatinine and BUN, often with hyperkalemia, acidosis
Urinalysis with microscopy — muddy brown granular casts, renal tubular epithelial cells, FENa >2%
Diuretics do not change outcome but help volume management. 'Furosemide stress test' (no response to 1-1.5 mg/kg) predicts severe AKI.
Nonoliguric ATN has better prognosis than oliguric.
No medication has been shown to reverse or shorten ATN. Dopamine, fenoldopam, mannitol, and 'renal-dose' dopamine are not recommended.
References
KDIGO 2012 — KDIGO Clinical Practice Guideline for Acute Kidney Injury
ADQI — Acute Disease Quality Initiative consensus statements on AKI biomarkers and management
ACR-NKF 2020 — Davenport MS, et al. Use of Intravenous Iodinated Contrast Media in Patients With Kidney Disease: Consensus Statements from the American College of Radiology and the National Kidney Foundation. Radiology. 2020;294(3):660-668.
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