Etiology
Obstructive uropathy has many causes but all cases involve interruption of the urine flow by a blockage or narrowing of some part of the urinary tract.[1] Common causes in adults include renal stones, benign prostatic hyperplasia, urethral stricture, neurogenic bladder, as well as advanced prostate and bladder cancer. Other less common causes include pelvic organ prolapse, trauma, iatrogenic injury during abdominopelvic surgery, and retroperitoneal malignancy. Congenital anomalies are the most common cause in the pediatric population, these include ureteropelvic junction (UPJ) or ureterovesical junction obstruction, and are often identified on prenatal ultrasound.
Unilateral obstructive uropathy: acute unilateral uropathy is most commonly due to renal stones. Trauma or iatrogenic injury to a ureter during abdominopelvic surgery, may cause obstructive uropathy if accidentally ligated, clamped, or cauterized.[8] Chronic unilateral obstruction may result from congenital or acquired UPJ obstruction, a neoplastic process causing extrinsic compression, as well as strictures or retroperitoneal fibrosis due to history of radiation therapy with a latency of several years.[8]
Bilateral obstructive uropathy: benign prostatic hyperplasia (BPH) can cause acute or chronic obstructive uropathy.[9][10] Although BPH is a common cause of obstructive uropathy, most patients with BPH do not go on to develop obstruction. In a study of over 3000 men, 18 (2.4%) of the 737 men in the placebo group went on to develop acute urinary retention with a mean follow-up of 4.5 years. No men in the trial went on to develop renal insufficiency due to BPH.[11] Urethral and meatal strictures can restrict urinary flow and cause obstructive uropathy. This may result from previous instrumentation. Rarer causes include retroperitoneal fibrosis, posterior urethral valves in newborn males, clot retention following hematuria, neurogenic bladder, pelvic organ prolapse, and hydronephrosis of pregnancy due to direct compression from the gravid uterus.
Bowel dysfunction has been described in young women in association with urinary retention, raising the possibility of a common etiology.[12] Constipation may be the triggering factor leading to acute retention in men when there is another underlying risk factor such as BPH. Drugs can also cause urinary retention, particularly anticholinergic agents, alpha receptor agonists and opioid analgesics.[13][14] Patients with injuries to the spinal cord, or neurologic disorders such as Parkinson disease and multiple sclerosis often have involvement of the bladder and, in many cases, the detrusor muscle fails to function properly, leading to urinary retention and obstructive uropathy.[13]
Pathophysiology
Obstructive uropathy, regardless of the specific cause, can cause back pressure on the kidney by obstructing urinary flow. This can result in decreased renal blood flow, decreased glomerular filtration rate, and up-regulation of the renin-angiotensin system.[15] This can in turn cause atrophy and apoptosis of the renal tubules and interstitial fibrosis with infiltration of the interstitial spaces by macrophages.[15] These changes may lead to decreased reabsorption of solutes and water, inability to concentrate the urine, and impaired excretion of hydrogen and potassium.[1]
Obstruction can ultimately cause tubulointerstitial fibrosis, tubular atrophy, and interstitial inflammation. If left untreated, obstructive nephropathy can cause irreversible renal damage.
Classification
Clinical classification
Obstructive uropathy can be classified as acute or chronic in onset and as affecting one or both sides:
Acute unilateral
Acute bilateral
Chronic unilateral
Chronic bilateral.
This classification helps to delineate the cause of the obstruction. Unilateral cases are more commonly caused by disorders in the kidney, ureter, retroperitoneum, or portions of the bladder. Bilateral cases are usually due to disorders of the bladder, prostate and urethra, or pelvic malignancy. Less commonly, bilateral cases are due to the simultaneous occurrence of unilateral causes of obstruction on both sides.
Clinical classification: extrinsic or intrinsic
This differentiates obstructive uropathy by whether it is extrinsic or intrinsic to the urinary tract. Extrinsic causes include retroperitoneal masses and cancer from adjacent organs. Intrinsic causes include renal calculi, tumors, and benign prostatic hyperplasia.
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