Prenatal Imaging
Despite widespread use of ultrasound, a debate exists in the field of maternal fetal medicine over the required use of gestational (in utero) ultrasound. Clear indications for sonography include discrepancies in fundal height for gestational age, elevated levels of maternal serum alpha fetal protein and a history of previous pregnancies resulting in congenital anomalies. Regardless of the controversy, when a gestational ultrasound is performed certain basic details must be covered in the examination. These include:
Estimation of fetal size and maturity
Amniotic fluid volume
Standard fetal survey of head, spine, heart, lungs, limbs and abdomen
Assessment of kidneys including position, size and texture
Appearance of ureters and collecting system
Bladder volume, wall thickness and emptying
Examination of other pelvic organs
Appearance of external genitalia
Fetal kidneys can be visualized by the 14th to 15th week of gestation. By the 20th week of gestation, the internal architecture of the kidneys can be assessed. A normal fetal ureter is rarely visualized during ultrasonography. The actual incidence of genitourinary abnormalities on prenatal ultrasound is .2%.
Hydronephrosis is the most common abnormality detected on prenatal ultrasonography. It accounts for about 50% of all prenatally detected defects. When prenatal hydronephrosis is discovered on ultrasound, the finding does not confirm the presence of obstruction. This is due to the extremely elastic nature of the fetal kidney.
Prenatal Hydronephrosis - Risks & Causes
The most common cause of fetal hydronephrosis is ureteropelvic junction (UPJ) obstruction. This finding is characterized by varying degrees of renal pelvic dilation. The most common cause of UPJ obstruction is an intrinsic narrowing of the proximal ureter. This narrowing can be likened to a scar or stricture in the ureter as it leaves the kidney. Sometimes, the ureter can be obstructed behind a band of tissue or a blood vessel supplying the lower segment of the kidney. This entity is known as a "crossing vessel."When a dilated ureter is detected along with the renal pelvis, the working diagnosis is vesicoureteral reflux or uretero-vesical junction obstruction. Males with significant hydroureteronephrosis occurring simultaneously with a large thin walled bladder and normal renal parenchyma will often have a condition of massive reflux known as megacystis-megaureter.Prenatal findings of hydroureteronephrosis can be associated with other diagnoses such as posterior urethral valves, prune-belly syndrome, or duplication anomalies such as ectopic ureter or ureterocele.The diagnosis of posterior urethral valves is reached when a male fetus has the following findings:
- Bilateral hydroureteronephrosis
- Thick-walled, moderately dilated bladder
- Dilated posterior urethra
- Kidney architectural changes
- Diminished volumes of amniotic fluid
Other genitourinary diagnoses that can be made prenatally include renal dysplasia, multicystic dysplasia, renal agenesis, renal hypoplasia, renal cystic disease, bladder/cloacal exstrophy, and external genital anomalies.
Prenatal Hydronephrosis - Evaluation
The first important step in managing patients with prenatal findings of decreased amniotic fluid, genital malformations, or other serious anomalies is to have the child treated in a tertiary care center for delivery and management of neonatal issues. Upon delivery, a directed physical examination is important to rule out associated anomalies. A newborn ultrasound will be performed at a minimum of 48 hours of life to one month. Depending on these results, the urologist may order an x-ray test called a VCUG, which involves instilling dye into the bladder and taking x-rays while the child empties the bladder. This test helps to look for bladder abnormalities and the presence of urinary reflux.
In most cases, postnatal evaluation can be performed on an outpatient basis. Children with moderate degrees of prenatal hydronephrosis can be discharged home on amoxicillin with no further inpatient evaluation. Generally an ultrasound is not performed in the first two days of life, as the neonate is usually dehydrated, creating the potential to underestimate hydronephrosis. The first ultrasound can be performed after 3-4 weeks on an elective basis while the child continues antibiotic prophylaxis.
When the postnatal ultrasound reveals severe pelvic dilation (over 1.2 cm) and caliectasis, diuretic renography is needed to assess the degree of renal obstruction and to assess renal function. Ideally, the renography is performed after the infant is at least one month of age. 99mTc MAG-3 is the radionuclide agent of choice.