Hydronephrosis is swelling of the kidney caused by a buildup of urine that can’t drain properly. It isn’t a disease on its own but rather a sign that something is blocking or slowing the flow of urine somewhere between the kidney and the bladder (or beyond). The condition can affect one or both kidneys, range from mild to severe, and occur at any age, including before birth.
Why Urine Backs Up Into the Kidney
Your kidneys constantly filter blood and send urine down two narrow tubes called ureters into the bladder. If anything narrows or blocks an ureter, or if urine flows backward from the bladder, pressure builds inside the kidney and stretches the tissue that collects urine (the renal pelvis and calyces). That stretching is hydronephrosis.
The most common causes depend on age and sex. In younger adults, kidney stones are the leading trigger. In older adults, an enlarged prostate or cancers in the pelvis or abdomen more often press on the ureter from outside. In women, gynecologic cancers and pregnancy are frequent causes. Other possibilities include blood clots, scar tissue from prior surgery, ureteral strictures (areas where the tube has narrowed), and neurogenic bladder, a condition where the nerves controlling the bladder don’t work correctly.
What It Feels Like
Symptoms vary dramatically depending on how fast the blockage develops. A kidney stone that suddenly lodges in the ureter can cause intense, wave-like flank pain (renal colic) that radiates down toward the groin, testicle, or labia. You might also notice blood in your urine, nausea, or fever if an infection is present.
Slow-growing blockages tell a different story. A tumor gradually compressing an ureter or a congenital narrowing at the top of the ureter may produce little or no pain for months. In these cases, hydronephrosis can develop silently, sometimes discovered only when routine imaging or blood work reveals a problem. Patients with severe bilateral obstruction (both kidneys affected) can drift into kidney failure without dramatic symptoms, and their urine tests may look surprisingly normal aside from a few stray blood or white cells.
Where you feel pain offers a clue about the blockage’s location. Obstructions high in the ureter or at the kidney itself cause flank pain or tenderness. Lower obstructions tend to send pain toward the groin. A palpably full bladder suggests the problem is at the bladder outlet, such as an enlarged prostate.
How Severity Is Graded
Doctors grade hydronephrosis on ultrasound using a scale from 0 to 4, originally developed for fetal and pediatric imaging but widely referenced in adults as well.
- Grade 0: The kidney’s central collecting area looks normal with no swelling.
- Grade 1: A slight split in the collecting area. Kidney tissue thickness is still normal.
- Grade 2: The collecting area is clearly widened but still contained within the kidney’s border. Tissue remains normal thickness.
- Grade 3: The renal pelvis bulges beyond the kidney border and the individual calyces (cup-shaped structures that funnel urine) are dilated. Tissue is still normal.
- Grade 4: Further dilation of the pelvis and calyces, and now the kidney tissue itself has thinned. This signals the most concern for lasting damage.
Grades 1 and 2 often resolve on their own once the underlying cause is treated. Grades 3 and 4 typically need more urgent intervention.
How It’s Diagnosed
Ultrasound is usually the first test because it’s fast, radiation-free, and reliably shows whether the kidney is swollen. CT scans provide more detail, especially for identifying kidney stones or tumors causing the blockage. When doctors need to determine whether the swelling is actually impairing kidney function, a specialized nuclear medicine scan called diuresis renography is the standard of care. This test measures how efficiently each kidney drains after you’re given a diuretic to increase urine flow.
In pregnant patients, where radiation should be avoided, ultrasound measurements of the renal pelvis help predict whether intervention is needed. A renal pelvis diameter above 21 mm on the right side or above 25 mm on the left has been shown to predict the need for treatment with high accuracy.
Treatment Options
Treatment targets the cause of the blockage, not just the swelling. A small kidney stone may pass on its own with fluids and pain management. Larger stones may require procedures to break them up or remove them.
When the blockage can’t be quickly resolved, doctors relieve pressure on the kidney in one of two ways. A ureteral stent is a thin, flexible tube threaded through the ureter to hold it open, allowing urine to flow past the obstruction. Stents are commonly used for kidney stones, scar tissue from endometriosis, tumors, and junction obstructions. They’re placed through the bladder and don’t require an external bag.
If scarring or other problems make it impossible to place a stent through the ureter, a nephrostomy tube is placed directly into the kidney through the skin of the back. This tube drains urine into an external collection bag, bypassing the ureter and bladder entirely. It’s a less common approach, typically reserved for cases where internal drainage isn’t feasible or when the patient is too ill for a longer procedure.
For structural problems like a congenital narrowing at the top of the ureter, surgery to widen or reconstruct the junction may be needed. Tumors or other masses compressing the ureter from outside require treatment of the underlying cancer or disease.
Risks of Delayed Treatment
The kidney can tolerate mild, temporary swelling without lasting harm. But prolonged or severe hydronephrosis damages the kidney tissue itself, and that damage can become permanent. Severe cases can lead to kidney failure requiring dialysis or transplant. The thinning of kidney tissue visible in Grade 4 hydronephrosis reflects this process already underway.
How quickly damage becomes irreversible depends on whether the obstruction is complete or partial, whether one or both kidneys are affected, and whether infection is present. Fever combined with an obstructed kidney is treated as an emergency because infected urine trapped under pressure can lead to sepsis rapidly. A history of diabetes adds further urgency to evaluation and treatment.
Hydronephrosis in Babies
Hydronephrosis is one of the most common findings on prenatal ultrasound, and the majority of cases are mild and resolve without intervention. Current guidelines from the American Academy of Pediatrics classify prenatal urinary tract dilation into risk tiers that determine how closely the baby needs to be monitored.
Low-risk dilation found before 32 weeks warrants a repeat ultrasound after 32 weeks. If the swelling has resolved by then, no further testing after birth is needed. If it persists, a postnatal kidney ultrasound is recommended between 48 hours and 6 weeks of life. For higher-risk dilation, ultrasound is repeated every 4 weeks during pregnancy, and the newborn gets an ultrasound during the birth hospitalization, ideally after 48 hours of life (earlier scans can be misleading because newborns are relatively dehydrated in the first two days).
After birth, low-risk babies get a follow-up ultrasound in 3 to 6 months and generally don’t need preventive antibiotics or additional imaging. Intermediate-risk babies are rescanned in 1 to 3 months, with antibiotics and further imaging considered on a case-by-case basis. High-risk babies are rescanned at 1 month, and preventive antibiotics, a voiding study to check for urine reflux, and functional kidney imaging are all recommended.
Preventive antibiotics, when used, are typically continued until 12 months of age. They may be stopped earlier if imaging shows improvement, unless the child has confirmed urine reflux or a dilated ureter measuring 7 mm or larger, both of which increase the risk of urinary tract infections. If a child with known dilation develops a febrile UTI, a voiding study is performed to check for reflux regardless of the original risk tier.