Acute Tubular Necrosis (ATN)

Acute tubular necrosis (ATN) follows a well-defined three-part sequence of initiation, maintenance, and recovery (see below). The tubule cell damage and cell death that characterize ATN usually result from an acute ischemic or toxic event. Most of the pathophysiologic features of ischemic ATN, as described below, are shared by the nephrotoxic forms.

Initiation phase

Ischemic ATN is often described as a continuum of prerenal azotemia. Indeed, the causes of the two conditions are the same. Ischemic ATN results when hypoperfusion overwhelms the kidney’s autoregulatory defenses. Under these conditions, hypoperfusion initiates cell injury that often, but not always, leads to cell death.

Injury of tubular cells is most prominent in the straight portion of the proximal tubules and in the thick ascending limb of the loop of Henle, especially as it dips into the relatively hypoxic medulla. The reduction in the glomerular filtration rate (GFR) that occurs from ischemic injury is a result not only of reduced filtration due to hypoperfusion but also of casts and debris obstructing the tubule lumen, causing back-leak of filtrate through the damaged epithelium (ie, ineffective filtration).

The earliest changes in the proximal tubular cells are apical blebs and loss of the brush border membrane followed by a loss of polarity and integrity of the tight junctions. This loss of epithelial cell barrier can result in the above-mentioned back-leak of filtrate.

Another change is relocation of Na⁺/K⁺-ATPase pumps and integrins to the apical membrane. Cell death occurs by both necrosis and apoptosis. Sloughing of live and dead cells occurs, leading to cast formation and obstruction of the tubular lumen (see the image below). Activation of the renal immune system—with damage to tubular cells stimulating local secretion of proinflammatory cytokines—in turn induces further necrosis. ^[3]

Acute tubular necrosis. Photomicrograph of a kidney biopsy specimen shows renal medulla, which is composed mainly of renal tubules. Features suggesting acute tubular necrosis are the patchy or diffuse denudation of the renal tubular cells with loss of brush border (blue arrows); flattening of the renal tubular cells due to tubular dilation (orange arrows); intratubular cast formation (yellow arrows); and sloughing of cells, which is responsible for the formation of granular casts (red arrow). Finally, intratubular obstruction due to the denuded epithelium and cellular debris is evident (green arrow); note that the denuded tubular epithelial cells clump together because of rearrangement of intercellular adhesion molecules.

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In addition, ischemia leads to decreased production of vasodilators (ie, nitric oxide, prostacyclin [prostaglandin I₂, or PGI₂]) by the tubular epithelial cells, leading to further vasoconstriction and hypoperfusion. 

On a cellular level, ischemia causes depletion of adenosine triphosphate (ATP), an increase in cytosolic calcium, free radical formation, metabolism of membrane phospholipids, and abnormalities in cell volume regulation. The decrease or depletion of ATP leads to many problems with cellular function, not the least of which is active membrane transport.

With ineffective membrane transport, cell volume and electrolyte regulation are disrupted, leading to cell swelling and intracellular accumulation of sodium and calcium. Typically, phospholipid metabolism is altered, and membrane lipids undergo peroxidation. In addition, free radical formation is increased, producing toxic effects. Damage inflicted by free radicals apparently is most severe during reperfusion.

Maintenance phase

The maintenance phase of ATN is characterized by a stabilization of GFR at a very low level, and it typically lasts 1-2 weeks. Complications (eg, uremic and others; see Complications) typically develop during this phase.

The mechanisms of injury described above may contribute to continued nephron dysfunction, but tubuloglomerular feedback also plays a role. Tubuloglomerular feedback in this setting leads to constriction of afferent arterioles by the macula densa cells, which detect an increased salt load in the distal tubules.

Recovery phase

The recovery phase of ATN is characterized by regeneration of tubular epithelial cells. ^[4] During recovery, an abnormal diuresis sometimes occurs, causing salt and water loss and volume depletion. The mechanism of the diuresis is not completely understood, but it may in part be due to the delayed recovery of tubular cell function in the setting of increased glomerular filtration. In addition, continued use of diuretics (often administered during initiation and maintenance phases) may also add to the problem.

Normotensive ischemic acute tubular necrosis

This is a condition that develops in patients without an overt severe hypotensive episode. These patients have low-normal blood pressure but still have severe ATN. The most common reason for this condition is renal susceptibility to the lower blood pressure because of impairment of autoregulatory function of the kidney. Normally, the afferent arteriole dilates (via prostaglandins) and efferent arteriole constricts (via angiotensin-II)  to maintain the glomerular capillary pressure. Factors that impair this autoregulatory mechanisms include the following ^[5] :

• Advanced age
• Atherosclerosis, hypertension, and chronic kidney disease
• Malignant hypertension
• Medications impairing the autoregulatory mechanism (eg, nonsteroidal anti-inflammatory drugs [NSAIDs])
• Afferent glomerular vasoconstriction  (eg, from sepsis, hypercalcemia, hepatorenal syndrome, cyclosporine/tacrolimus)
• Drugs blocking efferent arteriolar vasoconstriction - Angiotensin-converting enzyme (ACE) inhibitors/angiotensin receptor blockers (ARBs)

Endotoxemia (sepsis)-related ATN

Sepsis is a recognized cause of ATN. However, the hypothesis that ATN develops in these cases when sepsis-related hypotension leads to a reduction in renal blood flow has been challenged by several animal and human studies. Those studies have indicated that in fact, renal blood flow may increase in that setting, due to a mechanism leading to efferent arteriolar vasodilatation. ^[6]

Other suspected contributors to ATN in sepsis include the following:

• Intra-renal vasoconstriction and redistribution of renal blood flow to the cortex
• Activation of vasoactive intrarenal hormones (endothelin, renin-angiotensin- aldosterone)
• Nitric oxide synthase induction and release of several cytokines

ATN is generally caused by an acute event, either ischemic or toxic.

Causes of ischemic acute tubular necrosis

Ischemic ATN may be considered part of the spectrum of prerenal azotemia, and indeed, ischemic ATN and prerenal azotemia have the same causes and risk factors. Specifically, these include the following:

• Hypovolemic states: Hemorrhage, volume depletion from gastrointestinal (GI) or renal losses, burns, fluid sequestration
• Low cardiac output states: Heart failure and other diseases of myocardium, valvulopathy, arrhythmia, pericardial diseases, tamponade
• Systemic vasodilation: Sepsis, anaphylaxis
• Disseminated intravascular coagulation
• Surgical procedures in which renal blood flow is compromised (eg, those that involve clamping of the renal artery, such as coronary artery bypass grafting [CABG], aortic dissection repair, renal cell carcinoma resection)

Causes of nephrotoxic acute tubular necrosis

The kidney is a particularly vulnerable target for toxins, both exogenous and endogenous. Not only does it have a rich blood supply, receiving 25% of cardiac output, but it also helps in the excretion of these toxins by glomerular filtration and tubular secretion.

Exogenous nephrotoxins that cause ATN

Aminoglycoside-related toxicity occurs in 10-30% of patients receiving aminoglycosides, even when blood levels are in apparently therapeutic ranges. Risk factors for ATN in these patients include the following:

• Preexisting liver or kidney disease
• Concomitant use of other nephrotoxins (eg, amphotericin B, radiocontrast media, cisplatin)
• Shock
• Advanced age
• Female sex
• Higher aminoglycoside level 1 hour after dose (a high trough level has not been shown to be an independent risk factor)

Amphotericin B nephrotoxicity risk factors include the following:

• Male sex
• High maximum daily dose (nephrotoxicity is more likely to occur if >3 g is administered)
• Longer duration of therapy
• Hospitalization in the critical care unit at the initiation of therapy
• Concomitant use of cyclosporine

Radiographic contrast media can cause contrast-induced nephropathy (CIN, radiocontrast nephropathy); this commonly occurs in patients with several risk factors, such as elevated baseline serum creatinine, preexisting kidney insufficiency, underlying diabetic nephropathy, chronic heart failure [CHF], or high or repetitive doses of contrast media, as well as volume depletion and concomitant use of diuretics, ACE inhibitors, or ARBs. The 2011 UKRA guidelines recommend that patients at risk of CIN should have a careful evaluation of volume status and receive volume expansion with 0.9% sodium chloride or isotonic sodium bicarbonate before the procedure. ^[7]

Other exogenous nephrotoxins that can cause ATN include the following:

• Cyclosporine and tacrolimus (calcineurin inhibitors)
• Cisplatin
• Ifosfamide
• Foscarnet
• Pentamidine, which is used to treat Pneumocystis jiroveci infection in immunocompromised individuals (risk factors for nephrotoxicity include volume depletion and concomitant use of other nephrotoxic antibiotic agents, such as aminoglycosides, which is common practice in the immunosuppressed)
• Sulfa drugs
• Acyclovir and indinavir
• Tenofovir
• Mammalian (mechanistic) target of rapamycin (mTOR) inhibitors (eg, everolimus, temsirolimus) ^[8]

Endogenous nephrotoxins that cause ATN

In myoglobinuria, rhabdomyolysis is the most common cause of heme pigment–associated acute kidney injury (AKI) and can result from traumatic or nontraumatic injuries. Most cases of rhabdomyolysis are nontraumatic, such as those related to alcohol abuse or drug-induced muscle toxicity (eg, statins alone or in combination with fibrates).

In hemoglobinuria, AKI is a rare complication of hemolysis and hemoglobinuria, and most often is associated with transfusion reactions (in contrast to myoglobin, hemoglobin has no apparent direct tubular toxicity, and AKI in this setting is probably related to hypotension and decreased renal perfusion). ^[8]

Acute crystal-induced nephropathy occurs when crystals are generated endogenously due to high cellular turnover (ie, uric acid, calcium phosphate), as observed in certain malignancies or the treatment of malignancies. However, this condition is also associated with ingestion of certain toxic substances (eg, ethylene glycol) or nontoxic substances (eg, vitamin C). Choudhry et al reported a case of AKI caused by ingestion of excessive quantities of calcium-containing antacids. ^[9]

In multiple myeloma, renal impairment results from the accumulation and precipitation of light chains, which form casts in the distal tubules that cause renal obstruction. In addition, myeloma light chains have a direct toxic effect on proximal renal tubules. ^[10]

COVID-19

AKI occurs in about 30% of hospitalized patients with COVID-19. ^[11] While COVID-19–related AKI is due to multiple factors, ATN is usually present. ^[12]  Direct entry of the virus into the renal cells has been confirmed; however, the main mechanisms leading to ATN and AKI in COVID-19 patients are thought to be hypovolemia, collapsing glomerulopathy, and cytokine storm. ^[13]

For patients with ATN, the in-hospital survival rate is approximately 50%, with about 30% of patients surviving for 1 year. Factors associated with an increased mortality rate include the following:

• Poor nutritional status
• Male sex
• Oliguria
• Need for mechanical ventilation
• Acute myocardial infarction
• Stroke
• Seizures

The mortality rate in patients with ATN is probably related more to the severity of the underlying disease than to ATN itself. For example, the mortality rate in patients with ATN after sepsis or severe trauma is much higher (about 60%) than the mortality rate in patients with ATN that is nephrotoxin related (about 30%). The mortality rate is as high as 60-70% with patients in a surgical setting. If multiorgan failure is present, especially severe hypotension or acute respiratory distress syndrome, the mortality rate ranges from 50 to 80%.

Patients with oliguric ATN have a worse prognosis than patients with nonoliguric ATN. This probably is related to more severe necrosis and more significant disturbances in electrolyte balance. In addition, a rapid increase in serum creatinine (ie, >3 mg/dL) probably also indicates a poorer prognosis. Again, this probably reflects a more serious underlying disease.

Of the survivors of ATN, approximately 50% have some impairment of renal function. Some (about 5%) continue to undergo a decline in renal function. About 5% never recover kidney function and require dialysis.

A review of United States Renal Data System data (n = 1,070,490) for 2001 through 2010 found that although the incidence of end-stage renal disease (ESRD) attributed to ATN increased during that period, the prospects for renal recovery and survival also increased. Recovery of renal function was more likely in patients with ATN than in matched controls (cumulative incidence 23% vs. 2% at 12 weeks, 34% vs. 4% at 1 year), as was death (cumulative incidence 38% vs. 27% at 1 year). Hazards ratios for death declined in stepwise fashion to 0.83 in 2009-2010. ^[14]

For post AKI hospitalization outcomes and monitoring see Treatment/Long-Term Monitoring

The landmark PICARD (Program to Improve Care in Acute Renal Disease) study was an observational study of a cohort of 618 patients with acute kidney injury in the intensive care units of 5 academic centers in the United States. Ischemic ATN was the presumed etiology for 50% of all patients with renal failure, an additional ~12% due to unresolved pre-renal factors, and ~25% from nephrotoxic ATN. ^[15]  These data were similar to those from the Madrid Acute Renal Failure Study Group, which assessed 748 cases of acute injury from that region of Spain and estimated that the incidence of ATN was 88 cases per million population. ^[16]