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Acute Kidney Injury
Acute kidney injury (also called acute kidney failure) is a rapid (days to weeks) decline in the kidneys’ ability to filter metabolic waste products from the blood.
Causes include disorders that decrease blood flow to the kidneys, that damage the kidneys themselves, or that block drainage of urine from the kidneys.
Symptoms may include swelling, nausea, fatigue, itching, difficulty breathing, and symptoms of the disorder that caused the acute kidney injury.
Serious complications include heart failure and high levels of potassium in the blood.
Diagnosis is with blood and urine tests and usually imaging studies.
Treatment involves correcting the cause of acute kidney injury and sometimes doing dialysis.
Acute kidney injury can result from any condition that decreases the blood supply to the kidneys, any disease affecting the kidneys themselves, or any condition that obstructs urine flow anywhere along the urinary tract. In many people, no cause of acute kidney injury can be identified. If both kidneys function normally, damage to one kidney (for example due to blockage by a kidney stone) does not usually cause major problems because the remaining good kidney can compensate and usually maintain laboratory measures of kidney function near normal. Thus, acute kidney injury may be hard to detect. For acute kidney injury to cause significant problems, usually both kidneys must be damaged or function abnormally.
Major Causes of Acute Kidney Injury
Symptoms depend on the severity of kidney function decline, its rate of progression, and its cause.
In some people, the first symptom of acute kidney injury is water retention, causing weight gain and swelling of the feet and ankles or puffiness of the face and hands. People may pass cola-colored urine, which may indicate a number of kidney diseases. The amount of urine (which for most healthy adults is between 3 cups [about 750 milliliters] and 2 quarts [about 2 liters] per day) often decreases to less than 2 cups (about 500 milliliters) per day or stops completely. Very little urine production is called oliguria, and no urine production is called anuria. However, some people with acute kidney injury continue to produce normal amounts of urine.
As acute kidney injury persists and metabolic waste products accumulate in the body, people may experience fatigue, a decreased ability to concentrate on mental tasks, loss of appetite, nausea, and overall itchiness (pruritus). People with acute kidney injury may develop chest pain, muscle twitching, or even seizures. If fluid accumulates in the lungs, people may become short of breath.
If the cause is a blockage (an obstruction), the backup of urine within the kidneys causes the drainage system to stretch (a condition called hydronephrosis). Urinary obstruction often causes a constant dull ache under the lower ribs but may cause crampy pain—ranging from mild to excruciating—usually along the sides (flanks) of the body. Some people with hydronephrosis have blood in their urine. If the obstruction is located below the bladder, the bladder will enlarge. If the bladder enlarges rapidly, people are likely to feel severe pain in the pelvis, just above the pubic bone. If the bladder enlarges slowly, pain may be minimal, but the lower part of the abdomen may swell because of the markedly distended bladder.
If acute kidney injury develops during hospitalization, the condition often relates to some recent injury, surgery, drug, or illness such as infection. The symptoms of the cause of the acute kidney injury may predominate. For example, high fever, life-threatening low blood pressure (shock), and symptoms of heart failure or liver failure may occur before symptoms of kidney failure and be more obvious and urgent.
Some of the conditions that cause acute kidney injury also affect other parts of the body. For example, Goodpasture syndrome or granulomatosis with polyangiitis (formerly, Wegener granulomatosis), which damages blood vessels in the kidneys, may also damage blood vessels in the lungs, causing a person to cough up blood. Rashes are typical of some causes of acute kidney injury, including polyarteritis nodosa, systemic lupus erythematosus (lupus), and some toxic drug exposures.
Blood tests that measure levels of creatinine and urea nitrogen in the blood are needed to confirm the diagnosis. A progressive daily rise in creatinine indicates acute kidney injury. The level of creatinine is also the best indicator of the degree or severity of kidney function decline. The higher the level, the more severe the decline in kidney function is likely to be. Other blood tests detect metabolic imbalances that occur if the decline in kidney function is severe, such as an increase in blood acidity (acidosis, which causes a low bicarbonate level), a high potassium level (hyperkalemia), a low sodium level (hyponatremia), and a high phosphorus level (hyperphosphatemia).
The physical examination findings may help doctors identify the cause of the acute kidney injury. For example, enlarged or tender kidneys may indicate obstruction with hydronephrosis. Urine tests, such as a urinalysis and measurement of certain electrolytes, may enable doctors to determine whether the cause of kidney injury is insufficient blood flow to the kidneys, damage to the kidneys, or urinary obstruction.
Imaging of the kidneys using ultrasonography or computed tomography (CT) is helpful, sometimes by identifying hydronephrosis or an enlarged bladder. Imaging can also reveal the size of the kidneys. X-rays of the arteries or veins that lead to and from the kidneys (angiography) may be done if obstruction of blood vessels is the suspected cause. However, angiography is done only when other tests do not provide enough information, because angiography uses radiopaque dye (contrast agent) that contains iodine, which carries a risk of additional kidney damage. Magnetic resonance angiography (MRA) can provide the same type of information. However, MRA has traditionally used gadolinium, a substance that rarely causes a disorder that triggers production of scar tissue in the body (nephrogenic fibrosing dermopathy). Thus, MRA is now less likely to be used. If other tests do not reveal the cause of kidney injury, a biopsy may be necessary to determine the diagnosis and the prognosis.
Acute kidney injury and its immediate complications, such as water retention, high acid and potassium levels in the blood, and increased urea nitrogen in the blood, can often be treated successfully. The overall survival rate is about 50%. Survival is less than 50% for people who have several organs failing at the same time. Yet, survival is about 90% for people whose acute kidney injury is due to decreased blood flow because body fluids have been lost through bleeding, vomiting, or diarrhea—conditions that are reversible with treatment. About 10% of people who survive acute kidney injury require dialysis or kidney transplantation—half right away and the others because their kidney function slowly deteriorates.
Some complications of acute kidney injury are serious and may even be life-threatening. People may need to be treated in a critical care unit.
Any treatable cause of acute kidney injury is treated as soon as possible. For example, if obstruction is the cause, a catheter (a tube placed into the bladder), endoscopy, or surgery may be needed to relieve the obstruction.
Often, the kidneys can heal themselves, especially if the kidney injury has existed for less than 5 days and there are no complicating problems such as infection. During this time, measures are taken to prevent the decreased kidney function from causing serious problems. Such measures may include the following:
Doctors strictly limit the intake of all substances that are eliminated through the kidneys, including a large number of drugs. Salt (sodium) and potassium intake is usually restricted. Fluid intake is restricted to replacing the amount lost from the body, unless fluid is needed because there is too little blood flowing to the kidneys. Weight is measured every day because a change in weight is a good indicator of whether there is too much or too little water in the body.
A healthy diet is provided to people whose condition allows them to eat. Moderate amounts of protein are acceptable, typically 0.8 to 1 grams per kilogram of body weight (0.4 to 0.5 grams per pound). Restricting the intake of foods that are high in phosphorus, such as dairy products, liver, legumes, nuts, and most soft drinks, lowers the phosphate concentration in the blood.
Sodium polystyrene sulfonate is sometimes given by mouth or rectally to treat a high level of potassium in the blood. Calcium salts (calcium carbonate or calcium acetate) or sevelamer may be given by mouth to prevent or treat a high level of phosphorus in the blood.
Fluids are not restricted in people who are recovering from acute kidney injury caused by obstruction. During the recovery period, the kidneys are unable to reabsorb sodium and water normally, and a large amount of urine may be produced for a period of time after the obstruction is relieved. During recovery, people may also need replacement of fluids and electrolytes, such as sodium, potassium, and magnesium.
Acute kidney injury may be prolonged, necessitating removal of waste products and excess water. Waste removal can be done through dialysis, usually hemodialysis (see Hemodialysis). If loss of kidney function is predicted to be prolonged, dialysis is started as soon as possible after diagnosis. Dialysis may be needed only temporarily, until the kidneys recover their function, usually in several days to several weeks.
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