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Oliguria is a condition where too little urine is produced. A low production of urine may be the result of insufficient fluid being provided to the kidneys, damaged kidneys which are unable to make urine, or an obstruction that prevents urine from leaving the body. The most common causes of oliguria are those, such as dehydration, which involve an inadequate supply of fluid reaching the kidneys, and these are known as prerenal causes. Treatment of oliguria generally involves correcting any of the underlying causes, if possible. If there is a risk of kidney failure, dialysis may be required, where an artificial filter is used instead of the kidneys to remove waste products from the blood.
Where decreased urine production results from prerenal causes, there is not a large enough volume of fluid reaching the kidneys. This can occur as a result of dehydration, heart disorders, such as heart failure, where blood is not being efficiently pumped round the body, or collapse of the circulatory system, known as hypovolemic shock, due to problems such as hemorrhage. A low volume of fluid can cause the kidneys to fail, resulting in a lower output from the urinary system.
In the initial stages of renal failure, also known as acute kidney injury, the kidneys respond to their reduced fluid supply by conserving extra salt and water, and if the situation is reversed the urine output may return to normal. If the situation persists, some kidney cells die off in a condition called acute tubular necrosis. Where the damage is too extensive, the kidneys may be unable to produce a normal urine output even if the fluid load subsequently increases.
Acute tubular necrosis is known as a renal cause of oliguria, where there is damage to the kidneys themselves. Diseases which cause kidney damage, certain drugs which are toxic to the kidneys, and infection of the blood, can all give rise to acute tubular necrosis. Some of the causes lead to more serious renal damage than others, and oliguria is only associated with the more severe forms. For both prerenal and renal causes of oliguria, the underlying problems are identified and treated to give the kidneys the best chance of recovery. Any fluid loss is restored, and dialysis is used if necessary.
Postrenal causes of oliguria, where there is urinary obstruction, commonly involve problems such as a catheter being blocked. Catheters are flexible pipes used to drain urine from the bladder. In the case of a blockage, urinary retention occurs, where the person typically feels an urge to pass urine but it is not possible, and the onset of oliguria is relatively sudden. A blockage may be caused by the urine collecting bag being positioned higher than the bladder, or by a kink in the tube, in which case simple repositioning can correct the problem. If the inside of the tube is plugged by sediment or a blood clot the catheter may need to be replaced.