Acute kidney injury

Acute kidney injury, previously called acute renal failure, is a sudden decrease in kidney function that develops within seven days, as shown by an increase in serum creatinine or a decrease in urine output, or both.
Causes of AKI are classified as either prerenal, intrinsic renal, or postrenal. Prerenal causes of AKI include sepsis, dehydration, excessive blood loss, cardiogenic shock, heart failure, cirrhosis, and certain medications like ACE inhibitors or NSAIDs. Intrinsic renal causes of AKI include glomerulonephritis, lupus nephritis, acute tubular necrosis, certain antibiotics, and chemotherapeutic agents. Postrenal causes of AKI include kidney stones, bladder cancer, neurogenic bladder, enlargement of the prostate, narrowing of the urethra, and certain medications like anticholinergics.
The diagnosis of AKI is made based on a person's signs and symptoms, along with lab tests for serum creatinine and measurement of urine output. Other tests include urine microscopy and urine electrolytes. Renal ultrasound can be obtained when a postrenal cause is suspected. A kidney biopsy may be obtained when intrinsic renal AKI is suspected and the cause is unclear.
AKI is seen in 10–15% of people admitted to the hospital and in more than 50% of people admitted to the intensive care unit. AKI may lead to a number of complications, including metabolic acidosis, high potassium levels, uremia, changes in body fluid balance, effects on other organ systems, and death. People who have experienced AKI are at increased risk of developing chronic kidney disease in the future. Management includes treatment of the underlying cause and supportive care, such as renal replacement therapy.

Signs and symptoms

The clinical presentation is often dominated by the underlying cause. The various symptoms of acute kidney injury result from the various disturbances of kidney function that are associated with the disease. Accumulation of urea and other nitrogen-containing substances in the bloodstream lead to a number of symptoms, such as fatigue, loss of appetite, headache, nausea, and vomiting. Marked increases in the potassium level can lead to abnormal heart rhythms, which can be severe and life-threatening. Fluid balance is frequently affected, though blood pressure can be high, low, or normal.
Pain in the flanks may be encountered in some conditions. This is the result of stretching of the fibrous tissue capsule surrounding the kidney. If the kidney injury is the result of dehydration, there may be thirst as well as evidence of fluid depletion on physical examination. Physical examination may also provide other clues as to the underlying cause of the kidney problem, such as a rash in interstitial nephritis and a palpable bladder in obstructive nephropathy.

Causes

Prerenal

Prerenal causes of AKI are those that decrease effective blood flow to the kidney and cause a decrease in the glomerular filtration rate. Both kidneys need to be affected as one kidney is still more than adequate for normal kidney function. Notable causes of prerenal AKI include low blood volume, low blood pressure, heart failure, hepatorenal syndrome in the context of liver cirrhosis, and local changes to the blood vessels supplying the kidney. The latter include renal artery stenosis, or the narrowing of the renal artery which supplies the kidney with blood, and renal vein thrombosis, which is the formation of a blood clot in the renal vein that drains blood from the kidney.

Intrinsic or intrarenal

Intrinsic AKI refers to disease processes which directly damage the kidney itself. Intrinsic AKI can be due to one or more of the kidney's structures including the glomeruli, kidney tubules, or the interstitium. Common causes of each are glomerulonephritis, acute tubular necrosis, and acute interstitial nephritis, respectively. Other causes of intrinsic AKI are rhabdomyolysis and tumor lysis syndrome. Certain medication classes such as antibiotics and calcineurin inhibitors can also directly damage the tubular cells of the kidney and result in a form of intrinsic AKI.

Postrenal

Postrenal AKI refers to acute kidney injury caused by disease states downstream of the kidney and most often occurs as a consequence of urinary tract obstruction. This may be related to:

Benign prostatic hyperplasia
Kidney stones
Obstructed urinary catheter
Bladder stones
Cancer of the bladder, ureters, or prostate
Damage to the nerves controlling urine flow and bladder function.
Diagnosis

Definition

Introduced by the KDIGO in 2012, specific criteria exist for the diagnosis of AKI.
AKI can be diagnosed if any one of the following is present:

Increase in SCr by ≥0.3 mg/dl within 48 hours; or
Increase in SCr to ≥1.5 times baseline, which has occurred within the prior 7 days; or
Urine volume < 0.5 mL/kg/h for 6 hours.
Staging

The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative group, aid in assessment of the severity of a person's acute kidney injury. The acronym RIFLE is used to define the spectrum of progressive kidney injury seen in AKI:

Risk: 1.5-fold increase in the serum creatinine, or glomerular filtration rate decrease by 25 percent, or urine output <0.5 mL/kg per hour for six hours.
Injury: Two-fold increase in the serum creatinine, or GFR decrease by 50 percent, or urine output <0.5 mL/kg per hour for 12 hours.
Failure: Three-fold increase in the serum creatinine, or GFR decrease by 75 percent, or urine output of <0.3 mL/kg per hour for 24 hours, or no urine output for 12 hours.
Loss: Complete loss of kidney function for more than four weeks.
End-stage kidney disease: Complete loss of kidney function for more than three months.
Evaluation

The deterioration of kidney function may be signaled by a measurable decrease in urine output. Often, it is diagnosed on the basis of blood tests for substances normally eliminated by the kidney: urea and creatinine. Additionally, the ratio of BUN to creatinine is used to evaluate kidney injury. Both tests have their disadvantages. For instance, it takes about 24 hours for the creatinine level to rise, even if both kidneys have ceased to function. A number of alternative markers have been proposed, but none of them are established enough as of 2018 to replace creatinine as a marker of kidney function.
These may include urine sediment analysis, renal ultrasound and/or kidney biopsy. Indications for kidney biopsy in the setting of AKI include the following:

Unexplained AKI, in a patient with two non-obstructed normal sized kidneys.
AKI in the presence of the nephritic syndrome.
Systemic disease associated with AKI.
Kidney transplant dysfunction.

In medical imaging, the acute changes in the kidney are often examined with renal ultrasonography as the first-line modality, where CT scan and magnetic resonance imaging are used for the follow-up examinations and when US fails to demonstrate abnormalities. In evaluation of the acute changes in the kidney, the echogenicity of the renal structures, the delineation of the kidney, the renal vascularity, kidney size and focal abnormalities are observed. CT is preferred in renal traumas, but US is used for follow-up, especially in the patients suspected for the formation of urinomas. A CT scan of the abdomen will also demonstrate bladder distension or hydronephrosis.

Classification

Acute kidney injury is diagnosed on the basis of clinical history and laboratory data. A diagnosis is made when there is a rapid reduction in kidney function, as measured by serum creatinine, or based on a rapid reduction in urine output, termed oliguria.

Type	U_Osm	U_Na	Fe_Na	BUN/Cr
Prerenal	>500	<10	<1%	>20
Intrinsic	<350	>20	>2%	<10-15
Postrenal	<350	>40	>4%	>20

AKI can be caused by systemic disease, crush injury, contrast agents, some antibiotics, and more. AKI often occurs due to multiple processes.
The causes of acute kidney injury are commonly categorized into prerenal, intrinsic, and postrenal.
Acute kidney injury occurs in up to 30% of patients following cardiac surgery. Mortality increases by 60-80% in post-cardiopulmonary bypass patients who go on to require renal replacement therapy. Preoperative creatinine greater than 1.2 mg/dL, combined valve and bypass procedures, emergency surgery, and preoperative intra-aortic balloon pump are risk factors most strongly correlated with post-cardiopulmonary bypass acute kidney injury. Other well-known minor risk factors include female gender, congestive heart failure, chronic obstructive pulmonary disease, insulin-requiring diabetes, and depressed left ventricular ejection fraction. Volatile anesthetic agents have been shown to increase renal sympathetic nerve activity, which causes retention of salts and water, diminished renal blood flow and an increase in serum renin levels, but not in antidiuretic hormone.
Postoperative acute kidney injury is defined as an AKI based on the KDIGO criteria occurring within 7 days of an operative intervention.
Pediatric AKI is defined using the KDIGO definition and the modified neonatal KDIGO criteria for neonates.

Treatment

The management of AKI hinges on identification and treatment of the underlying cause. The main objectives of initial management are to prevent cardiovascular collapse and death and to call for specialist advice from a nephrologist. In addition to treatment of the underlying disorder, management of AKI routinely includes the avoidance of substances that are toxic to the kidneys, called nephrotoxins. These include NSAIDs such as ibuprofen or naproxen, iodinated contrasts such as those used for CT scans, many antibiotics such as gentamicin, and a range of other substances.
Monitoring of kidney function, by serial serum creatinine measurements and monitoring of urine output, is routinely performed. In the hospital, insertion of a urinary catheter helps monitor urine output and relieves possible bladder outlet obstruction, such as with an enlarged prostate.