The kidneys filter waste products and excess fluids from the blood to produce urine. The urine continually drains from the kidneys through the ureters and into the bladder---the balloon-like muscle that stores urine. Defects or diseases in the urinary tract system that interfere with the normal flow of urine from the kidneys can cause hydronephrosis---a condition characterized by the swelling within the kidney due to the buildup of urine, in adults.
Obstructive Uropathy
Obstructive uropathy describes a condition in which the kidneys continue to produce urine, but the flow of urine from the kidneys becomes blocked. This condition may occur due to structural abnormalities or functional problems and may be classified as acute, meaning sudden onset, or chronic, occurring over time. Bilateral obstructive uropathy affects both the right and left kidney, while unilateral obstructive uropathy affects only one kidney.
The Merck Manual reports the most common cause of obstructive uropathy in young adults is the formation of a kidney stone that travels into the ureter. As men age, their risk of developing obstructive uropathy increases due to the incidence of an enlarged prostate, a condition known as benign prostatic hyperplasia. Other causes of obstructive uropathy that can lead to hydroneprhosis include the presence of tumor in the pelvic region, the formation of a blood clot in the ureter, or the buildup of scar tissue due to surgery or radiation therapy.
Bladder Outlet Obstruction
In addition to conditions that interfere with the flow of urine out of the kidneys, conditions that cause a backflow of urine from the bladder to the kidneys can also result in hydronephrosis. A bladder outlet obstruction describes a condition in which a blockage at the base of the bladder interferes with the flow of urine from the bladder to the urethra. Because the urine cannot flow out of the body, pressure builds in the bladder causing a backflow of urine back up the ureters.
A bladder outlet obstruction can occur due to an enlarged prostate, a tumor in the bladder or a tumor in the pelvic region. The buildup of scar tissue around the urethra and the presence of bladder stones can also cause a bladder outlet obstruction.
Neurogenic Bladder
As the bladder becomes full of urine, nerve cells in the bladder wall send signals to the brain. The brain sends signals back instructing the muscle of the bladder wall to contract while the sphincter muscle separating the bladder and the urethra relaxes, therefore, allowing urine to flow out of the bladder. Conditions that interfere with the ability of the nerves to send signals, such as spinal cord trauma, a stroke, or a tumor affecting the central nervous system, cause a neurogenic bladder. A neurogenic bladder can result in a spastic or overactive bladder that contracts too frequently and too strongly, or a flaccid bladder that fails to contract at all, as described by the Cleveland Clinic. These conditions can cause the bladder to overfill, resulting in a backflow of urine that can lead to hydronephrosis.
