Unimed Hospital, Departamento de Ginecologia e Obstetricia, Capivari, Sao Paulo, Brasil

Synonyms: Mild hydronephrosis, pelviectasis

Definition: Pyelectasis is a pelvicaliceal dilatation. Fetal pyelectasis can be classified accordingly to the gestational age:

• Between 15 and 20 weeks: greater or equal to 4mm
• Between 20 and 30 weeks: greater or equal to 5mm
• Between 30 and 40 weeks: greater or equal to 7mm

Persistent fetal pyelectasis is defined as >7 mm in the third trimester (1).

Incidence: Abnormalities of the urinary tract can be found in up to 5% of newborns. They account for 25% of all prenatally congenital defects and contribute to 4% of the perinatal mortality (24). The most common sonographic abnormality found in the fetal urinary tract is collecting system dilatation, accounting for 4.5% of all examinations (3). Pyelectasis is more likely to be bilateral (57%) (7), and is more common in the left kidney (11).

Prevalence: The prevalence of fetal pyelectasis during an anomaly scan in a low-risk population is not high (1.25%) (7).

Sex ratio: Male fetuses exhibit a significantly increased frequency of renal pelvis dilatation compared with female fetuses (3:1) (14).

Etiology: The maternal hydration influences fetal renal pelvic diameter. The effects of maternal hydration on amniotic fluid volume are partially mediated via fetal urine production (6). The fetal anteroposterior renal diameter increases with maternal hydration in both normal fetuses and in those with pyelectasis and is independent of the state of the fetal bladder (16).
Maternal high level progesterone seems to influence the fetal and mother ureter smooth muscle. There is a statistically significant association between maternal and fetal collecting system dilatation during pregnancy (19).
Isolated pyelectasis can be a sign of the presence of vesicoureteral reflux (3), which is frequent in neonates with urinary tract infection. It is often a cause of moderate dilatation of the fetal renal pelvis (20) but antenatal ultrasound has a poor sensibility for vesicoureteral reflux detection (10). Fluctuation, defined as size of renal pelvis changing by more than 4 mm during the course of prenatal or postnatal scans, is strongly associated with high-grade vesicoureteral reflux (17). Antenatal isolated pelvis dilatation is a weak predictor of vesicoureteral reflux (21). Ureteropelvic junction obstruction perhaps is the most important etiology of fetal pyelectasis. Prenatal ultrasound has led to earlier diagnosis of ureteropelvic junction obstruction, allowing earlier repair (22). In some series, dilatation of the renal pelvis was caused by primary ureteropelvic junction obstruction in 65.6%.

Sonographic findings: Diagnosis is made by the measure of the renal pelvic anteroposterior diameter. There is a positive correlation between gestational age and renal pelvic anteroposterior diameter (15). The size of the fetal renal collecting system is highly variable over the course of a two hour period. Significant caution should be used when considering the implications of renal collecting system dilatation based upon a single anteroposterior measurement (18).

Differential diagnosis: The differential diagnoses include other causes of dilatation of the renal pelvis like multicystic kidney disease, duplex system, ureterovesical junction obstruction, and posterior urethral valves.

Associated anomalies: Mild pyelectasis is a common finding which is often incidental, with no significant long term sequelae. However, there is a small association with aneuploidy, in particular the trisomy 21 and postnatal renal pathology (4). The presence of isolated pyelectasis had 9.09% sensitivity, 97.6% specificity, 0.33% positive predictive value, and 99.9% negative predictive value to detect fetuses with trisomy 21 (8). The incidence of Down syndrome is 3.3% when fetal pyelectasis is present, but 25% of fetuses with trisomy 21 have pyelectasis (12).

Prognosis: Most pyelectasis resolve spontaneously in the first year of life and invasive procedures are not required. Adequate monitoring of these children can avoid urinary tract infections and their sequelae (2). Unilateral prenatal lesions are less common than bilateral, but they have a significant postnatal pathology in 47 and 26 per cent, respectively. Postnatal follow-up is required for persistent pyelectasis (9). Fetal pyelectasis of 8 mm was 91% sensitive and 72% specific in predicting subsequent hydronephrosis (11). Urological tests are performed when the prenatal renal pelvic diameter is larger than 10 mm (10). Neonatal surgery is recommended when the anteroposterior, transverse, and longitudinal renal pelvic diameters during the prenatal period are at least 20, 25, and 26 mm, respectively. Surgery is not necessary when the diameters are less than 20 mm (13). Reflux is frequent in neonates with urinary tract infection (20).

Recurrence risk: Compared with normal fetuses, mothers with fetal pyelectasis in the first pregnancy have a relative risk of 6.1 to have a recurrence of this finding in their next pregnancy. These results suggest a predisposition for pyelectasis that may be influenced by genetic and/or environmental factors (5).

Management: Data suggest that the risk of aneuploidy associated with isolated fetal pyelectasis is low, that it should not be an indication for invasive prenatal karyotyping (7)(8). The risk of chromosomal anomalies increases when the pyelectasis is diagnosed in older pregnant women (>35 years of age) or if it is associated with other sonographic markers of chromosomal anomalies (choroids plexus cysts, cystic hygroma etc.) (7).The likelihood ratio of trisomy 21 in the group of fetuses with isolated pyelectasis is 3.79. Among fetuses with pyelectasis and other associated markers/structural anomalies the likelihood ratio for trisomy 21 is 19.2 (8), which justify the performance of an amniocentesis.

References

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