Postrenal ARF is caused by an acute obstruction that affects the normal flow of urine out of both kidneys. The blockage causes pressure to build in all of the renal nephrons (tubular filtering units that produce urine). The excessive fluid pressure ultimately causes the nephrons to shut down. The degree of renal failure corresponds directly with the degree of obstruction. Postrenal ARF is seen most often in elderly men with enlarged prostate glands that obstruct the normal flow of urine.
Some of the most notable causes of postrenal ARF include the following:
- Bladder outlet obstruction due to an enlarged prostate gland or bladder stone
- Kidney stones in both ureters (tubes that pass urine from each kidney to the bladder) or in patients with one kidney
- Neurogenic bladder (overdistended bladder caused by inability of the bladder to empty)
- Tubule obstruction (end channels of the renal nephrons)
- Renal injury (usually sustained in an automobile accident or while playing a sport)
- Retroperitoneal fibrosis (formation of fibrous tissue behind the peritoneum – the membrane that lines the abdominal cavity)
Pain varies in severity and location according to the type of obstruction and often suggests the cause of postrenal ARF. Some of the most common signs and symptoms of obstruction-related postrenal ARF follow:
- Difficult urination
- Distended bladder
- Edema (fluid retention and swelling)
- Hypertension (high blood pressure)
- Pain in the lower back, lower abdomen, groin, genitalia
- Severe hematuria (blood in urine)
Postrenal ARF is diagnosed after a complete physical examination and medical history. Often, the catheterization of a bladder holding a large amount of urine (2-3 liters) helps make the diagnosis. The physician passes a tube (catheter) through the urethra into the bladder to drain urine into a bag outside the body. The volume of urine is measured and bladder swelling is resolved.
Ultrasound of the kidneys, ureters, and bladder is the test of choice to detect obstruction. If the kidneys show signs of hydronephrosis-that is, if they are stretched, or dilated, beyond normal dimensions because of fluid buildup-the patient usually has obstructive ARF.
On rare occasions, computed tomography (CT or CAT scan) of the kidneys can provide additional information.
Obstruction relief is the goal of treatment of postrenal ARF. If the problem is bladder outlet obstruction due to an enlarged prostate (benign prostate hyperplasia; BPH), the placement of a catheter into the bladder (through the urethra) will alleviate the obstruction temporarily. The prostate should be examined and treated properly.
If there are kidney stones in both ureters, the stones must be removed. If the physician is unable to remove the stones, the patient may need to be fitted with tubes that drain urine from the kidneys through an opening in the skin (called percutaneous nephrostomy tubes).
The treatment of urinary obstruction is associated with a variety of complications.
Gross hematuria (a large amount of bloody urine) can occur when the catheter is placed in patients who have bladder outlet obstruction. This happens because the sudden decrease in pressure causes the bladder veins to bleed. Unfortunately, slow decompression of the bladder does not prevent hematuria.
Reflex hypotension (low blood pressure) is a rare complication that can occur if a patient experiences sudden stimulation of the vagus nerve during catheter insertion.
Postobstructive diuresis is high urine output that may, initially, exceed 500 to 1000 milliliters per hour. This frequently occurs after an obstruction is removed. The renal tubules typically cannot reabsorb water and electrolytes in a normal manner after having been obstructed for a period of time.
Rarely, a person suffers severe dehydration and requires large amounts of intravenous fluids.
The rate of recovery is largely determined by the duration and severity of obstructive disease. In general, the extent of recovery is determined within 7 to 14 days after the obstruction has been removed. Some patients may require short-term treatment with dialysis, until their renal function recovers. Hemodialysis involves removal of waste products from the blood via mechanical filtration outside of the body.
Along with the loss of renal function, some people with postrenal ARF develop irreversible tubular defects, which may produce the following symptoms:
- Hyperkalemia (excess amount of potassium in the blood)
- Metabolic acidosis (excess amount of chloride in the blood)
- Polyuria (large daily output of urine)