With ultrasounds being done so commonly during pregnancy, one of the most common congenital abnormalities encountered is accumulation of urine inside the kidney, or in medical terms, hydronephrosis. This happens in 1-5% of pregnancies. Most cases of antenatal hydronephrosis (more than 50%) are inconsequential and transient, but on the other side of the spectrum there are cases of real urinary tract obstruction which, if untreated, could lead to infections, stones and loss of the kidney.
A well done study to determine which cases of antenatal hydronephrosis are transient versus potentially dangerous has not been done. Most of the studies to date are retrospective in nature and with substantial biases.
Measurement of the diameter of the renal pelvis (see picture) has been used to classify cases into mild, moderate, and severe. Several researchers have come up with different thresholds, some categorizing greater than 15mm as severe, others categorizing greater than 10mm as severe. Choosing a specific threshold to determine normal from abnormal has its consequences. We understand that the higher the measurement the more likely the kidney will have problems. So say you have 10 real cases: 5 are significant and their measurements are 8,9,14,16, and 18. Five are not significant and their measurements are: 7,6,8,9, and 11. If you chose 10mm as your threshold then 2 significant cases and 1 insignificant case would have been incorrectly diagnosed. If you chose 15 as your threshold, 3 significant cases and no insignificant cases would have been incorrectly diagnosed.
So what are the most common causes of antenatal hydronephrosis?
1. Transient/Physiologic:This just means that the hydronephrosis is a normal anatomic variant with no clinical consequences. This accounts for roughly 40-88% of the cases.
2. Ureteropelvic Junction Obstruction. This refers to an obstructive process happening between the renal pelvis and the ureter (see picture above). This accounts for 10-30% of the cases of antenatal hydronephrosis. Only 10-25% of children with ureteropelvic junction obstruction will eventually need an operation. The rest don’t develop an obstruction significant enough to damage the kidney, cause infections or symptoms.
3. Vesicoureteral Reflux: This means that urine will go back to the kidney from the bladder. It occurs in about 10-20% of antenatal hydronephrosis cases. The incidence of reflux is not different among children with or without antenatal hydronephrosis. In recent years, reflux has become one of the most controversial topics in pediatric urology. Some U.S. centers are aggressive at the diagnosis and management of reflux, whereas more and more centers are taking a very conservative approach at diagnosing and managing reflux. The main reason for the latter is that recent well conducted studies have failed to show much benefit in diagnosing and treating reflux, in terms of preventing renal damage, hypertension, or infections with either antibiotics or surgery.
4. There are other causes of antenatal hydronephrosis that are not common which I won’t cover in this post.
Which patients require further evaluations?
Most patients with antenatal hydronephrosis diagnosed during the second trimester will have their hydronephrosis resolved by the third trimester. If the hydronephrosis persists through the third trimester and especially if the diameter of the pelvis is greater than 10 mm, further postnatal evaluation with a renal bladder ultrasound is recommended.