Renal pelviectasis describes the dilation of the renal pelvis, the funnel-shaped structure in the center of the kidney that collects urine before it flows down the ureter to the bladder. The condition is essentially a mild form of hydronephrosis, which is a broader term for fluid buildup in the kidney. This finding is most frequently noted during a routine prenatal ultrasound, often referred to as fetal pyelectasis, typically around the 20th week of pregnancy. Detection in a developing fetus is common, affecting between 0.6% and 4.5% of fetuses in the second trimester. Pelviectasis results from a temporary or persistent issue with the flow of urine out of the kidney.
Understanding the Causes of Kidney Dilation
Renal pelviectasis results from a disruption in the normal drainage of urine, which causes pressure to build up and dilate the renal pelvis. This disruption generally falls into two categories: obstruction and backflow. Obstruction occurs when there is a physical or functional blockage preventing the smooth passage of urine from the kidney into the ureter or bladder.
The most common obstructive cause is Ureteropelvic Junction (UPJ) obstruction, where the kidney meets the ureter. This blockage may be congenital, resulting from a structural abnormality like a narrowing of the junction or an aberrant blood vessel compressing the ureter. Backflow, known as Vesicoureteral Reflux (VUR), is another potential cause, where urine flows backward from the bladder up into the ureter and kidney during urination.
VUR can lead to intermittent or constant dilation. In many cases, particularly those detected prenatally, the cause is temporary, unknown, or physiologic, meaning it often resolves on its own. However, persistent dilation after birth suggests a need to investigate for these underlying obstructive or reflux-related causes.
How Pelviectasis is Identified
The initial identification of renal pelviectasis is made during a routine second-trimester fetal anatomy ultrasound. If the dilation persists or worsens during the pregnancy, a follow-up ultrasound is performed after birth to confirm the condition in the newborn. Postnatal ultrasound imaging measures the anterior-posterior diameter (APD) of the renal pelvis to assess the degree of dilation and determine the need for further investigation.
If the dilation is significant or does not resolve spontaneously, specialized tests are used to determine the specific cause and its effect on kidney function. A Voiding Cystourethrogram (VCUG) is a fluoroscopic X-ray procedure used to check for VUR by filling the bladder with a contrast dye. This allows doctors to visualize the urinary tract and see if the dye flows backward into the ureters or kidneys during bladder filling or voiding.
Another common test is the MAG3 renal scan, a nuclear medicine study that assesses how well each kidney is functioning and how quickly urine drains. A radioactive tracer is injected into a vein, and a camera tracks its movement through the kidneys and into the bladder. This scan is useful for diagnosing a UPJ obstruction by showing a slow washout of the tracer from the renal pelvis.
Grading the Severity of the Condition
Physicians use standardized grading systems to classify the severity of renal pelviectasis. The most common method involves measuring the anterior-posterior diameter (APD) of the renal pelvis on an ultrasound image. For a fetus in the third trimester, a measurement between 7 and 9 millimeters is considered mild pelviectasis, which often resolves without intervention.
Moderate pelviectasis is defined by an APD between 10 and 15 millimeters during the third trimester, suggesting a higher likelihood of true obstruction. Severe dilation, indicated by an APD greater than 15 millimeters, is strongly correlated with postnatal pathology and potential loss of kidney function.
The Society for Fetal Urology (SFU) classification system is also widely used, providing a qualitative assessment based on the dilation of the renal pelvis and the calyces, the cup-like structures that collect urine. The SFU system assigns grades from 1 to 4, with Grade 1 being mild dilation and Grade 4 representing gross dilation, often with thinning of the kidney tissue. This structured grading helps providers stratify the risk and determine the appropriate follow-up schedule and intervention plan.
Management and Long-Term Outlook
Management of renal pelviectasis is dictated by the severity of the condition and the underlying cause. For most mild or moderate cases, the initial approach is “watchful waiting” or active surveillance. This involves regularly scheduled follow-up ultrasounds to monitor the dilation size and ensure the kidney function remains stable over time.
If the diagnostic workup suggests Vesicoureteral Reflux (VUR) or if the child has recurrent urinary tract infections, prophylactic antibiotics may be prescribed. This prevents infection from reaching the kidney and causing damage while waiting to see if the VUR resolves spontaneously. Surgical intervention is generally reserved for severe cases, especially those with a confirmed high-grade obstruction, such as Ureteropelvic Junction (UPJ) obstruction.
The most common surgery for a significant UPJ obstruction is a pyeloplasty. This procedure removes the blocked or narrowed section of the ureter and reattaches the healthy part to the renal pelvis to restore proper drainage. Pyeloplasty has a high success rate, often exceeding 90%, in relieving the obstruction and preserving kidney function. The long-term outlook for children with renal pelviectasis is generally positive, as many cases resolve spontaneously, and surgical intervention is highly effective when required.