Many urologic abnormalities are picked up on routine maternal sonography. However, the cause of the anomaly is usually not identified accurately.
Active urine production of adequate volume with sodium reabsorption begins sometime between the 12th and 15th week of gestation. By this time, the ureteral branching, bladder formation and urethral tubularization, including penile development, have been completed; and ureteral transport of urine and bladder contractions are usually in place. It is during this early to mid second trimester that the urinary system of the fetus can be initially evaluated during maternal sonography.
About 1 out of 500 maternal examinations with sonography will reveal some urologic abnormality. Of course, incidental developmental urologic anomalies without dilatation such as uninvolved duplex systems will not be detected. The most common prenatal diagnoses include: ureteropelvic junction obstruction, primary megaureter, multicystic dysplastic kidneys, posterior urethral valves, duplex anomalies, and reflux. Other developmental anomalies are less common.
The amniotic fluid is derived mainly from fetal urine production. When the urinary system is severely obstructed or there is bilateral renal agenesis, oligohydramnios results. Pulmonary hypoplasia is usually fatal and associated with oligohydramnios. Pneumothorax and pulmonary hypoplasia can also be seen in severe posterior urethral valve disease or with prune belly syndrome.
The opportunity to prevent pulmonary hypoplasia in the presence of renal agenesis or severe renal hypoplasia probably does not exist. The presence of pulmonary hypoplasia is determined well before the 15th week of gestation; and, because we cannot make a diagnosis of hydronephrosis before 16 weeks and since the urinary contribution to amniotic fluid is not significant prior to the 15th week of life, it is unlikely that any intrauterine mechanical or surgical manipulation will be effective. Therefore, most antenatal therapeutic options are academic.
If a fetal urologic anomaly is diagnosed, several questions and observations need to be determined: is the fetus male or female, is there a normal amount of amniotic fluid, is the hydronephrosis unilateral or bilateral, are the ureters dilated, does the bladder remain distended, is the bladder thick-walled, is the posterior urethra in males dilated, are there congenital anomalies present, is the amniotic fluid volume decreasing or increasing, and is the urologic process progressive?
Significant urologic abnormalities are frequently asymptomatic at birth and, if not diagnosed antenatally through sonography, may not be diagnosed until infection, trauma, bleeding or extensive renal parenchymal loss has occurred.
The number of nephrons does not increase after birth, although substantial changes in the nephrons do occur during the first few weeks of extrauterine life.
An increase in circulation from the fetal to the adult type results in a 50% increase in glomerular filtration in the first 5 days of birth and a doubling at 3 weeks of age. The counter-current mechanism matures so that the newborn kidney can concentrate its solute secretion up to 1200 milliosmoles up to two weeks. These physiologic changes occurring during the first few weeks of life are sensitive to obstruction.
When an immature kidney is obstructed, renal dysplasia may be the result, with primitive tubules and glomeruli and primitive mesenchymal tissue being present.
LIMITATIONS TO MATERNAL SONOGRAPHY
The rate of errors in prenatal diagnoses of urologic abnormalities has been reported to be as high as 30%.
Physiologic hydronephrosis, which may be present in up to 20% of the fetuses in the 3rd trimester, my lead to an incorrect diagnosis and inappropriate therapy. Hydronephrosis secondary to reflux has also been misinterpreted as obstructive uropathy. Differentiation between hydronephrosis secondary to ureteropelvic junction obstruction and multicystic dysplastic kidney is also characteristically difficult. Furthermore, the normal lucency of the fetal renal pyramids has frequently been misinterpreted as hydronephrosis or cystic disease of the kidneys.
Initially, a good physical examination is undertaken. The abdomen is examined for its musculature and whether the bladder or other masses and/or kidneys are palpable. The back and lumbosacral spine area are also examined to exclude signs of spinal dysraphism, which may suggest a neurogenic bladder.
A postnatal sonogram should then be obtained. In many cases, the prenatal diagnosis of hydronephrosis is not substantiated by the postnatal sonogram. However, there have been a number of neonates who have had a normal sonogram in the first few days of life only to have significant hydronephrosis return at a later date. This may be related to the physiologic oliguria seen in the neonate. Therefore, the so-called return to normal should always be confirmed several weeks later with a further renal sonogram.
If hydronephrosis is documented in the postnatal period, then a voiding cystourethrogram and an intravenous pyelogram should be performed.
However, the main benefit of a prenatal diagnosis is to allow early postnatal evaluation and appropriate therapy before infection and further parenchymal loss occurs.