Bilateral hydronephrosis is swelling of both kidneys caused by a buildup of urine that cannot drain properly into the bladder. The “bilateral” part is what distinguishes it from the more common unilateral form, which affects only one kidney. Because both kidneys are involved, bilateral hydronephrosis poses a greater risk to overall kidney function and typically signals a blockage located downstream, at a point where it affects urine flow from both sides simultaneously.
How the Blockage Works
Your kidneys continuously filter blood and collect the resulting urine in a funnel-shaped area called the renal pelvis. From there, urine travels down two narrow tubes (ureters) into the bladder, then exits the body through the urethra. When something blocks this pathway at or below the bladder, urine backs up into both kidneys at once, stretching the renal pelvis and the surrounding tissue.
This is different from unilateral hydronephrosis, where a kidney stone or a kink in one ureter blocks just one side. In bilateral cases, the obstruction is almost always a single blockage at a shared point: the bladder outlet or the urethra. Less commonly, both ureters can be compressed or blocked separately by something like a large tumor in the pelvis.
Common Causes in Adults
The most frequent cause in older men is an enlarged prostate, which surrounds the urethra and can gradually squeeze it shut. As the prostate grows, urine has more and more trouble leaving the bladder, and the backup eventually reaches both kidneys. Bladder outlet obstruction from other causes, including bladder stones or tumors at the base of the bladder, produces the same effect.
Pelvic cancers are another important cause. Tumors of the cervix, bladder, prostate, or rectum can grow large enough to compress both ureters where they pass through the pelvis. Retroperitoneal fibrosis, a rare condition where scar-like tissue forms behind the abdominal organs, can also encase and narrow both ureters. In some cases, severe constipation or large abdominal masses create enough pressure to obstruct drainage from both kidneys.
Causes in Newborns and Children
Bilateral hydronephrosis is sometimes detected before birth on a routine prenatal ultrasound. The two most common congenital causes are a blockage where the ureter meets the renal pelvis and vesicoureteral reflux, a condition where urine flows backward from the bladder into the ureters and kidneys.
A particularly serious cause in male newborns is posterior urethral valves, which are abnormal folds of tissue in the urethra that block urine from leaving the body. Because this obstruction sits below the bladder, it affects both kidneys and is considered more urgent than a blockage on just one side. Doctors prioritize evaluating lower urinary tract obstructions in newborns for exactly this reason.
Symptoms to Recognize
Bilateral hydronephrosis that develops slowly, as with an enlarging prostate, may produce no obvious symptoms for months or even years. You might notice a weaker urine stream, difficulty starting urination, or a feeling that your bladder never fully empties. These signs are easy to dismiss as a normal part of aging, which is why the condition sometimes goes undetected until kidney function has already declined.
When the blockage comes on more quickly, symptoms are harder to ignore. Flank pain on both sides (the area between your lower ribs and hips), reduced urine output, nausea, and swelling in the legs or around the eyes can all signal that urine is backing up. In severe or prolonged cases, waste products accumulate in the blood because the kidneys can no longer filter effectively. This can cause fatigue, confusion, loss of appetite, and a metallic taste in the mouth.
How It’s Diagnosed
Ultrasound is the standard first step. It’s safe, widely available, and reliable for showing whether the collecting system inside the kidney is dilated. On imaging, the renal pelvis normally measures under 10 millimeters across in adults. Measurements between 10 and 20 millimeters generally indicate mild to moderate dilation, though different institutions define these cutoffs slightly differently. Ultrasound can also often reveal the level of the blockage and whether the bladder is overfull.
CT scanning is more detailed and is the preferred test when the cause of the obstruction isn’t clear on ultrasound, or when kidney stones are suspected. A non-contrast CT is especially good at picking up stones and masses. In the United States, CT is often the go-to imaging choice for acute flank pain, while ultrasound is used more commonly as a first-line test in many other countries. Blood tests to check kidney function (creatinine and related markers) are typically run alongside imaging to see how much the backup has affected the kidneys’ ability to filter.
Treatment Approaches
The immediate goal is to relieve the obstruction and let urine drain. How that happens depends on where the blockage is and how urgent the situation is.
If the problem is at the bladder outlet, the simplest intervention is placing a catheter through the urethra to drain the bladder. This alone can relieve pressure on both kidneys quickly. For blockages higher up, or when a catheter can’t be passed, doctors may place a nephrostomy tube, a thin tube inserted through the skin of the back directly into the kidney to drain urine into an external bag. Another option is a ureteral stent, a soft plastic tube threaded into the ureter to hold it open and restore normal flow.
These are often temporary measures while the underlying cause is addressed. For an enlarged prostate, that might mean medication to shrink the gland or a procedure to open the urethral passage. For a tumor compressing the ureters, treatment focuses on the cancer itself, whether through surgery, radiation, or chemotherapy. In newborns with posterior urethral valves, surgery to remove the obstructing tissue is typically needed early to protect developing kidneys.
What Happens Without Treatment
Because both kidneys are affected, untreated bilateral hydronephrosis carries a real risk of kidney failure. When urine pressure stays elevated for weeks or months, the kidney tissue gradually thins and loses its ability to filter blood. This damage, called renal atrophy, can become permanent. The longer the obstruction persists, the less likely the kidneys are to recover full function even after drainage is restored.
Chronic obstruction also makes the kidneys more vulnerable to infection. Stagnant urine is a breeding ground for bacteria, and an infected, obstructed kidney (a condition called pyonephrosis) can become life-threatening quickly. Electrolyte imbalances, particularly high potassium levels, are another complication as failing kidneys lose the ability to regulate blood chemistry. These risks are why bilateral hydronephrosis is treated more urgently than the unilateral form, where the unaffected kidney can compensate.
Recovery and Outlook
The prognosis depends almost entirely on how long the obstruction lasted and how much kidney damage occurred before it was relieved. Acute bilateral hydronephrosis caught early, such as from a sudden bladder outlet blockage, often resolves completely once drainage is restored. Kidney function can bounce back to normal within days to weeks.
Chronic cases are less predictable. If the kidneys have been under pressure for months, some degree of permanent function loss is common. These patients may need ongoing monitoring with blood tests and imaging to track kidney health over time. In the most severe cases, where both kidneys have atrophied significantly, dialysis may become necessary. Early detection makes a substantial difference in outcomes, which is one reason imaging findings of bilateral kidney swelling are taken seriously even when symptoms seem mild.