Hydronephrosis is abnormal dilation of the urinary tract. Prenatal hydronephrosis is detected in 1-5% of all fetuses by prenatal ultrasound and is the most common prenatal abnormality detected. Prenatal hydronephrosis can be a sign of a wide variety of conditions ranging from transient dilation to severe life threatening obstruction to urine flow. Thankfully, the vast majority of fetuses with prenatal hydronephrosis will have a minor transient dilation.
Tests and Diagnosis
Prenatal hydronephrosis is diagnosed and assessed by prenatal ultrasound usually around 20 weeks. The hydronephrosis can be present on one or both sides. The severity of the hydronephrosis can be assessed. Since the amniotic fluid, or the fluid in the uterus, is mostly urine created by the fetus, the amount of fluid in the uterus is also assessed. The presence of a dilated bladder and ureters can also be assessed by ultrasound. All of this information will help guide further tests and determining the severity of the condition.
Depending on the severity of the findings and preferences of the parents, further testing may be recommended before the baby is born. Follow up ultrasound may be obtained, genetic testing may be considered, and even MRI studies of the fetus may be performed in severe cases.
After the baby is born further testing is also determined by the severity of the findings. If the hydronephrosis is mild and only on one side, an ultrasound around 4-6 weeks of age is usually recommended. If the hydronephrosis is severe and on both sides or if there is bladder dilation, further tests may be recommended immediately after birth.
For the vast majority of fetuses with prenatal hydronephrosis, no prenatal interventions are needed. For a very small number of fetuses with severe hydronephrosis on both sides, a severely dilated bladder, and decreased amniotic fluid prenatal intervention may be discussed. The prenatal intervention that is discussed is a vesico-amniotic shunt (VAS). A VAS is a small tube that is inserted prenatally with one end in the bladder and one end outside of the fetus in the amniotic cavity. This allows the urine to drain from the bladder into the amniotic cavity. This relieves the blockage to urine flow and increases the amniotic fluid volume which allows the lungs of the fetus to develop.
The majority of infants born with a history of prenatal hydronephrosis will have mild transient dilation of the urinary tract and no surgical intervention is needed. For specific diagnosis listed below surgical intervention is needed at varying rates.
- Bladder exstrophy
- Cloacal anomalies
- Ectopic ureter
- Posterior urethral valves (PUV)
- Prenatal hydronephrosis
- Prune belly syndrome
- Spina bifida
Almost all infants with a history of prenatal hydronephrosis will have imaging studies performed after birth. The types of imaging studies and the timing also depends on the severity of the hydronephrosis. An infant with mild hydronephrosis on one side will usually get a renal ultrasound at 4-6 weeks of life and have a visit with a pediatric urologist then. In contrast, an infant with severe hydronephrosis on both sides will likely get a renal ultrasound and a voiding cystourethrogram in the first few days of life, see a pediatric urologist soon after birth, and possibly get a diuretic renogram at 4-6 weeks of life. Further follow up and interventions would be determined by these tests.
Infants with severe urinary tract dilation or other findings such as a dilated ureter are at an increased risk for urinary tract infections. Based on results from prenatal and postnatal imaging studies, a low dose antibiotic may be prescribed to try and prevent infections in these patients at increased risk for urinary tract infections.