Common Causes of Hydronephrosis: Understanding the Underlying Factors
Hydronephrosis, a condition characterized by the swelling of the kidney due to the accumulation of urine, arises from a variety of underlying causes. While the pathology can be complex, the majority of cases are linked to congenital abnormalities. However, acquired conditions can also contribute significantly. This article explores the most common etiologies of hydronephrosis, offering insight into how urinary flow obstruction or reflux leads to kidney dilation.
Obstruction at the Ureteropelvic Junction (UPJ)
The ureteropelvic junction (UPJ) obstruction is the leading cause of hydronephrosis, particularly in infants and young children. This occurs when the connection between the renal pelvis and the ureter is narrowed, impeding the normal drainage of urine from the kidney. The blockage may result from intrinsic factors such as a congenital narrowing, a flap-like valve (valve or membrane), polyps, or abnormal tissue folding. In some cases, extrinsic compression from crossing blood vessels—such as a lower polar artery—or nearby tumors can also compress the UPJ, disrupting urine flow and causing progressive kidney swelling.
Blockage at the Ureterovesical Junction (UVJ)
Another frequent cause is obstruction at the ureterovesical junction (UVJ), where the ureter enters the bladder. When this passage is narrowed or dysfunctional, urine cannot efficiently drain into the bladder. This type of obstruction often stems from congenital stricture or functional abnormalities in the muscle layer at the junction. Over time, the backed-up urine increases pressure within the ureter and kidney, leading to dilatation and potential loss of renal function if left untreated.
Distal Ureteral Cysts
Cysts located in the distal (lower) portion of the ureter can obstruct urine flow into the bladder. These fluid-filled sacs, known as ureterocele, typically develop during fetal development and are more common in females. A ureterocele may block not only its own ureter but also interfere with the contralateral ureter's entry into the bladder, especially if it protrudes into the bladder cavity. This dual obstruction can lead to bilateral hydronephrosis and recurrent urinary tract infections.
Ectopic Ureter: An Abnormal Opening
An ectopic ureter refers to a ureter that does not terminate at the usual location within the bladder. Instead, it may open elsewhere along the urinary tract—such as in the urethra—or even outside the system, like in the genital area (e.g., vestibule or vagina in females). Because ectopic ureters often bypass the bladder's sphincter mechanism, they can cause continuous urinary leakage and incomplete bladder emptying. This anatomical anomaly commonly affects one side but can involve both ureters, contributing to unilateral or bilateral hydronephrosis.
Neurogenic Bladder Dysfunction
A neurogenic bladder results from impaired nerve signals between the brain and the bladder, often due to spinal cord injuries, spina bifida, multiple sclerosis, or other neurological disorders. When nerves fail to regulate bladder contraction and relaxation properly, the bladder may become overactive or underactive. In either case, poor coordination can lead to urinary retention or high-pressure voiding, both of which increase the risk of urine backing up into the kidneys and causing hydronephrosis over time.
Primary Vesicoureteral Reflux (VUR)
Primary vesicoureteral reflux is a congenital condition in which urine flows backward from the bladder into the ureters—and sometimes up to the kidneys—during urination. This happens due to an inadequate valve mechanism at the ureter-bladder junction. Children with VUR are prone to recurrent urinary tract infections (UTIs), which, if frequent and untreated, can lead to scarring of the kidneys and chronic hydronephrosis. Early diagnosis through imaging studies like voiding cystourethrography (VCUG) is crucial for preventing long-term damage.
Posterior Urethral Valves (PUV)
One of the most serious causes of hydronephrosis in males is posterior urethral valves (PUV), a rare congenital disorder in which abnormal flaps of tissue form in the male urethra. These valves obstruct urine flow from the bladder, increasing pressure throughout the entire urinary tract. As a result, both kidneys may become swollen, leading to bilateral hydronephrosis. PUV can also impair bladder development and reduce kidney function prenatally or shortly after birth, making timely intervention essential to preserve renal health.
In summary, hydronephrosis is not a disease itself but rather a sign of an underlying issue affecting urinary drainage. Whether caused by structural anomalies, nerve dysfunction, or reflux mechanisms, identifying the root cause early allows for effective management and helps prevent irreversible kidney damage. Anyone experiencing symptoms such as flank pain, urinary difficulties, or recurrent infections should seek medical evaluation to determine if hydronephrosis is present.