Hydronephrosis

Overview

Hydronephrosis refers to swelling of the kidney caused by urine that cannot drain properly into the bladder. The term derives from Greek words meaning “water in the kidney.” This condition is not a disease itself but rather a sign that something is blocking normal urine flow somewhere between the kidney and the urethra.

Under normal circumstances, the kidneys filter blood and produce urine, which collects in the renal pelvis before flowing through the ureter to the bladder. When an obstruction occurs anywhere along this pathway, urine accumulates behind the blockage, pressure builds within the kidney, and the organ begins to swell. If the obstruction persists, the increased pressure can damage kidney tissue and impair function permanently.

Hydronephrosis may affect one kidney (unilateral) or both kidneys (bilateral). Unilateral hydronephrosis typically results from obstruction between the kidney and bladder, most commonly from kidney stones or ureteropelvic junction obstruction. When only one kidney is affected, the other kidney can usually compensate and maintain adequate overall function. Bilateral hydronephrosis indicates obstruction below the bladder level and poses greater concern for overall kidney function. Common causes of bilateral involvement include benign prostatic hyperplasia, urethral stricture, and neurogenic bladder.

Physicians classify hydronephrosis severity using the Society for Fetal Urology grading system. Grade 1 involves mild dilation of the renal pelvis only, with no damage to kidney tissue. Grade 2 shows dilation extending to some of the calyces without thinning of the kidney parenchyma. Grade 3 demonstrates dilation of all calyces with early tissue thinning. Grade 4 represents severe dilation with obvious thinning of kidney tissue and risk of permanent damage.

Symptoms

Many people with chronic, slowly developing hydronephrosis experience no symptoms at all. The condition may be discovered incidentally during imaging performed for other reasons, particularly when only one kidney is affected and the other maintains normal function.

Acute obstruction typically produces more dramatic symptoms. Flank pain below the ribs on the affected side is the most common presentation, often described as severe and colicky, coming in waves of intensity. The pain may radiate to the groin and is frequently accompanied by nausea and vomiting. People with acute obstruction often appear restless, unable to find a comfortable position. Additional symptoms include decreased urine output and blood in the urine.

Chronic obstruction may cause a dull, persistent ache in the flank or back. Symptoms often fluctuate in intensity and may include recurrent urinary tract infections. Some individuals experience intermittent discomfort that worsens with increased fluid intake.

When an obstructed kidney becomes infected, a condition called pyonephrosis, symptoms escalate dramatically. Fever, chills, severe flank pain, and general malaise indicate a medical emergency requiring immediate treatment. An infected, obstructed kidney can progress rapidly to sepsis if not promptly drained.

Causes

Kidney stones represent the most common cause of hydronephrosis in younger adults. When a stone lodges in the ureter, it blocks urine flow and typically causes sudden, severe flank pain. Stones complicated by infection require urgent intervention to prevent serious complications.

Ureteropelvic junction obstruction occurs where the kidney meets the ureter. This blockage may be present from birth or develop later in life and often causes intermittent symptoms that fluctuate over time. Surgical correction through pyeloplasty achieves high success rates in restoring normal drainage.

In older men, benign prostatic hyperplasia frequently causes bilateral hydronephrosis by obstructing the bladder outlet. The enlarged prostate prevents complete bladder emptying, and the resulting back-pressure affects both kidneys over time.

Pregnancy commonly causes mild right-sided hydronephrosis due to hormonal relaxation of the ureters combined with compression from the enlarging uterus. This physiological change typically resolves after delivery and rarely requires intervention unless complicated by symptoms or infection.

Various tumors can obstruct urine flow at different levels. Bladder cancer may involve the ureteric openings, while cervical, prostate, or retroperitoneal cancers can compress or invade the ureters externally. Enlarged lymph nodes from metastatic disease may also cause obstruction.

Other causes include blood clots from kidney trauma or tumors, ureteric strictures from previous surgery or radiation, retroperitoneal fibrosis, endometriosis, ureterocele, and neurogenic bladder from conditions affecting nerve function.

In children, hydronephrosis detected prenatally during routine ultrasound represents the most common fetal urological abnormality. Many cases resolve spontaneously without intervention. Congenital causes include ureteropelvic junction obstruction, vesicoureteral reflux, posterior urethral valves in boys, ureterocele, and megaureter.

Diagnosis

Ultrasound serves as the first-line imaging test for suspected hydronephrosis. This non-invasive examination requires no radiation exposure and reliably demonstrates kidney swelling and dilation. Ultrasound is safe during pregnancy and can assess kidney size, but may not always identify the specific cause of obstruction.

CT scanning provides the most detailed anatomical information. A non-contrast CT (CT KUB) offers the best imaging for detecting stones, while a CT urogram with intravenous contrast visualizes the entire urinary tract and helps identify the obstruction site and cause. The radiation exposure from CT limits its use in pregnancy and requires consideration in young patients.

MRI urography provides an alternative to CT without radiation exposure, making it suitable for pregnant women and children when detailed imaging is necessary. Nuclear medicine renal scans using MAG3 or other tracers assess kidney function and drainage, helping determine whether surgical intervention will benefit kidneys with prolonged obstruction.

Blood tests measure kidney function through creatinine and estimated glomerular filtration rate. A complete blood count and inflammatory markers help identify infection. Urinalysis detects blood, protein, and signs of urinary tract infection, while urine culture identifies specific bacterial pathogens if infection is present.

Treatment

Treatment principles focus on relieving the obstruction, addressing the underlying cause, and protecting kidney function. The urgency of intervention depends on the presence of infection, severity of symptoms, and whether kidney function is compromised.

When urgent drainage is required for infection, impaired kidney function, or severe symptoms, two main approaches exist. A ureteric stent is an internal plastic tube placed through the bladder via cystoscopy that bypasses the obstruction and allows urine to drain. A nephrostomy tube provides an external drain placed through the back directly into the kidney under ultrasound or X-ray guidance, with urine collecting in an external bag. Both serve as temporary measures until definitive treatment can be performed.

Treatment varies according to the underlying cause. Kidney stones may pass spontaneously if small, or may require medical expulsive therapy, extracorporeal shock wave lithotripsy, ureteroscopy with laser fragmentation, or percutaneous nephrolithotomy for larger stones. Ureteropelvic junction obstruction is treated with pyeloplasty, a surgical reconstruction now commonly performed laparoscopically or robotically with excellent success rates. Benign prostatic hyperplasia is managed with medications such as alpha-blockers or 5-alpha reductase inhibitors, or surgical procedures including transurethral resection or holmium laser enucleation. Pregnancy-related hydronephrosis usually resolves after delivery, though stenting may be needed if symptoms or infection develop. Tumor-related obstruction requires treatment of the underlying cancer, with some patients needing long-term stents or permanent nephrostomy drainage.

Mild hydronephrosis often requires only monitoring with repeat imaging and kidney function testing. Many mild cases, particularly in pregnancy and children, resolve without intervention.

When to See a Doctor

Seek emergency care for severe flank pain accompanied by fever, inability to urinate, markedly decreased urine output, or fever with a known kidney problem. Severe pain with vomiting and inability to keep fluids down also warrants immediate evaluation. Anyone with a known single kidney who develops signs of obstruction should seek urgent assessment.

If you have a ureteric stent, some discomfort and frequent urination are normal. Blood in the urine may occur, particularly with physical activity. Take prescribed medications as directed and keep all follow-up appointments. Stents require removal or replacement, typically within three to six months, to prevent encrustation and complications.

Long-term monitoring for hydronephrosis involves regular imaging to confirm resolution, periodic kidney function tests, and awareness of symptoms that might indicate recurrent obstruction. Contact your doctor promptly for new or worsening pain, fever, decreased urine output, or any concerns about your condition.