A ureteropelvic junction (UPJ) obstruction is a blockage where the kidney meets the tube that carries urine down to the bladder. This junction, called the ureteropelvic junction, is the funnel-shaped exit point of the kidney’s collecting system. When it’s blocked, urine backs up into the kidney and causes it to swell, a condition known as hydronephrosis. UPJ obstruction is most often something people are born with, though adults can develop it too.
Where the Blockage Happens
Your kidneys filter blood and collect the resulting urine in a central basin called the renal pelvis. From there, urine funnels into a narrow tube called the ureter, which delivers it to the bladder. The spot where the renal pelvis narrows into the ureter is the ureteropelvic junction. It’s a natural bottleneck, and when something prevents urine from passing through smoothly, pressure builds in the kidney.
The blockage isn’t always a physical plug. In many congenital cases, the problem is functional: the muscle fibers in that segment of the ureter are arranged abnormally, so they can’t squeeze urine through in coordinated waves the way a healthy ureter does. The result is a segment that just sits there instead of actively pushing urine along, even though the passage isn’t physically sealed shut.
What Causes It
Most UPJ obstructions are congenital, meaning they develop before birth. The three main congenital causes are:
- Abnormal muscle development in the ureter. The smooth muscle layer doesn’t form correctly, leaving a segment that can’t contract and move urine forward.
- High insertion of the ureter. The ureter connects to the renal pelvis at an unusually high angle, making it difficult for urine to drain downward.
- A crossing blood vessel. An extra artery or vein crosses over the ureter near the kidney’s lower pole, kinking it like a garden hose under a tire. This accounts for roughly 50% of surgically treated UPJ obstructions in children with symptoms.
In children, UPJ obstruction sometimes appears alongside other congenital differences, including multicystic kidney or urine reflux (where urine flows backward from the bladder toward the kidney).
Acquired UPJ obstruction is less common and typically results from kidney stones that scar the area, chronic inflammation, prior surgery, radiation therapy, or, rarely, a tumor in the ureteral wall. Scar tissue from any of these can gradually tighten the junction into a stricture.
Symptoms in Children vs. Adults
Many congenital UPJ obstructions are detected before birth. During routine prenatal ultrasounds, doctors look for swelling of the kidney’s collecting system. The severity is graded by measuring the diameter of the renal pelvis: mild cases measure 4 to 7 mm in the second trimester, moderate cases (with visible swelling of the smaller collecting branches) measure 8 to 10 mm, and severe cases with thinning of the kidney tissue measure over 10 mm.
After birth, infants and young children may show few obvious symptoms. A urinary tract infection, especially with fever, is sometimes the first sign. In older children and adults, the most common symptoms are flank pain (often on one side of the back, just below the ribs) and blood in the urine. The pain can be intermittent and sometimes worsens after drinking large amounts of fluid, because the extra urine production overwhelms the narrowed junction. Episodes of sudden, intense flank pain with nausea, known historically as Dietl’s crisis, can mimic kidney stones.
How It’s Diagnosed
Ultrasound is usually the first test, both prenatally and in children or adults with symptoms. It shows whether the kidney is swollen and how much the collecting system is dilated. Doctors grade the severity on a 0 to 4 scale: 0 means no swelling, 1 means only the renal pelvis is dilated, 2 and 3 reflect increasing dilation of the smaller collecting branches, and 4 means the kidney tissue itself has started to thin.
To confirm that the swelling actually represents a blockage (rather than a floppy but unobstructed system), a specialized nuclear medicine scan is typically performed. A small amount of radioactive tracer is injected into a vein, and a camera tracks how quickly the kidney takes it up and then washes it out after a diuretic medication is given. The key measurement is the “half-time,” which is how long it takes for half the tracer to drain from the kidney. Under 15 minutes is normal, over 20 minutes indicates obstruction, and anything between 15 and 20 minutes falls in an indeterminate range that may need repeat testing.
This scan also measures how much of the total kidney function each kidney contributes, which is critical for deciding how aggressively to treat.
When Treatment Is Needed
Not every UPJ obstruction requires surgery. Many infants diagnosed prenatally with mild to moderate swelling are monitored with periodic ultrasounds and nuclear scans because a significant number improve on their own as the child grows. Surgery becomes necessary when the affected kidney is losing function over time, when symptoms like recurrent pain or infections persist, or when the obstruction is severe from the outset.
In adults, treatment is generally recommended when there is pain, recurrent infections, kidney stones forming behind the blockage, or measurable decline in kidney function.
Surgical Options
The standard surgical repair is called a pyeloplasty. The surgeon removes the narrowed segment of the junction, then reconnects the healthy ureter directly to the renal pelvis, creating a wider, unobstructed opening. This can be done as open surgery, laparoscopically, or with robotic assistance. All three approaches produce durable results. A large study of over 1,300 robotic pyeloplasty patients found a success rate of roughly 89%, and the procedure is considered the gold standard for UPJ repair.
A less invasive alternative called endopyelotomy involves making a small incision through the narrowed area from inside the ureter, using a scope passed through the skin or up from the bladder. Recovery tends to be faster, but it’s less effective in certain situations: patients with severe kidney swelling, very poor kidney function, or a crossing blood vessel causing the obstruction tend to have worse outcomes with endopyelotomy. It’s also generally not recommended when the affected kidney’s filtration rate has dropped below about 20 mL per minute.
When kidney function is extremely poor (below roughly 15 mL per minute) and the other kidney is healthy, removing the damaged kidney entirely is sometimes the most practical option.
Recovery After Pyeloplasty
After pyeloplasty, a small internal tube called a stent is placed inside the ureter to keep the new connection open while it heals. Traditionally, this stent stays in place for four to six weeks before being removed in a short outpatient procedure. Some pediatric centers have had success with shorter stenting periods of about one week.
Follow-up imaging typically includes ultrasounds at 3, 6, and 12 months after surgery, then annually. A repeat nuclear medicine scan is usually done around the six-month mark to confirm that drainage has improved and kidney function is stable or recovering.
Most people notice significant pain relief within the first few weeks. The kidney swelling often takes months to fully resolve on imaging, and in some cases a mild degree of dilation persists permanently even after a successful repair, simply because the kidney was stretched for so long.
Risks of Leaving It Untreated
A mild, stable UPJ obstruction that isn’t affecting kidney function can safely be monitored. But a significant obstruction that goes untreated carries real risks. Ongoing back-pressure damages the kidney tissue over time, leading to irreversible loss of function. Stagnant urine behind the blockage creates a breeding ground for infections and kidney stones. Chronic obstruction has also been linked to high blood pressure. In the worst cases, untreated obstruction can progress to kidney failure on the affected side or lead to serious bloodstream infections.