Excerpt from Acute Tubular Necrosis

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Background

The causes of acute renal failure (ARF) are conventionally and conveniently divided into 3 categories: prerenal, renal, and postrenal. Prerenal ARF involves an essentially normal kidney that is responding to hypoperfusion by decreasing the glomerular filtration rate (GFR). Renal or intrinsic ARF refers to a condition in which the pathology lies within the kidney itself. Postrenal failure is caused by an obstruction of the urinary tract. Acute tubular necrosis (ATN) is the most common cause of ARF in the renal category.

ATN is the second most common cause of all categories of ARF in hospitalized patients, with only prerenal azotemia occurring more frequently. The history, physical examination, and laboratory findings, especially the renal ultrasound and the urinalysis, are particularly helpful in identifying the cause of ARF. This article focuses on the pathophysiology, diagnosis, and management of ATN, the most common renal cause of ARF in hospitalized patients. Obstruction is the second most common cause of ARF (after prerenal azotemia) in outpatients. Glomerulonephritis and interstitial nephritis can also present as ARF.

Pathophysiology

Acute tubular necrosis

ATN usually occurs after an acute ischemic or toxic event, and it has a well-defined sequence of events. The initiation phase is characterized by an acute decrease in GFR to very low levels, with a sudden increase in serum creatinine and blood urea nitrogen (BUN) concentrations. The maintenance phase is characterized by a sustained severe reduction in GFR, and this phase continues for a variable length of time, most commonly 1-2 weeks. Because the filtration rate is so low during the maintenance phase, the creatinine and BUN continue to rise. The recovery phase, in which tubular function is restored, is characterized by an increase in urine volume (if oliguria was present during the maintenance phase) and by a gradual decrease in BUN and serum creatinine to their preinjury levels.

Ischemic acute tubular necrosis

Ischemic ATN is often described as a continuum of prerenal azotemia. Indeed, the causes of the 2 conditions are the same (see Causes). 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 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).

In addition, ischemia leads to decreased production of vasodilators (ie, nitric oxide, prostacyclin [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.

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.

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.

The recovery phase of ATN is characterized by regeneration of tubular epithelial cells. 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.

Nephrotoxic acute tubular necrosis

Most of the pathophysiologic features of ischemic ATN are shared by the nephrotoxic forms. Thus, the cellular events described above apply to nephrotoxic ATN as well. Nephrotoxic ATN has induction, maintenance, and recovery phases, and recovery can be associated with an abnormal diuresis as is described above in ischemic ATN.

Nephrotoxic injury to tubular cells occurs by multiple mechanisms. These include direct drug toxicity, intrarenal vasoconstriction, and intratubular obstruction.

Frequency

United States

The syndrome of ARF is observed in about 5% of all hospital admissions. In the ICU, it occurs in up to 30% of patients admitted. Prerenal causes are responsible for approximately half of all cases. The frequency of each type of intrinsic renal disease varies depending on the population studied, but ATN (other than prerenal azotemia) is the most common cause of ARF in hospitalized patients.

Mortality/Morbidity

As with other causes of ARF, complications associated with ATN are often life threatening. The in-hospital survival rate of patients with ATN is approximately 50%, with about 30% of patients surviving for 1 year. Factors that are associated with an increased mortality rate include poor nutritional status, male sex, the presence of oliguria, the need for mechanical ventilation, acute myocardial infarction, stroke, or seizures.

• Disturbances in fluid and electrolyte balance
  • Hyperkalemia can be associated with life-threatening cardiac arrhythmias (see Complications).
  • Salt and water retention often leads to hypertension, edema, and congestive heart failure (CHF).
  • Hyponatremia causes concern because of its effects on the central nervous system.
  • Other electrolyte disturbances include hyperphosphatemia, hypocalcemia, and hypermagnesemia.
  • Metabolic acidosis
• Uremia results from the accumulation of nitrogenous waste. It is a potentially life-threatening complication associated with ARF.
  • Neurologic impairment and pericarditis can occur.
  • Platelet dysfunction is common and can lead to life-threatening hemorrhage.
• Infections
  • For ARF, the mortality rate is 20-50% in patients with underlying medical illnesses, but 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 50-80%.
  • With dialysis intervention, the frequency of uremia, hyperkalemia, and volume overload as causes of death have decreased. The most common causes of death now are sepsis, cardiovascular and pulmonary dysfunction, and withdrawal of life support.

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