The clinical significance of an empty renal fossa on prenatal sonography.

OBJECTIVE: The purpose of this study was to characterize the significance of an empty renal fossa on prenatal sonography.

METHODS: A hospital database was reviewed retrospectively to find all fetuses with an empty renal fossa diagnosed on prenatal sonography between 1989 and 2003. For each case, prenatal and postnatal data were recorded.

RESULTS: Ninety-three fetuses had an empty renal fossa: 49 on the left, 41 on the right, and 3 unspecified. Forty-four fetuses were male and 29 were female. For 20, the sex was not recorded. The average gestational age at diagnosis was 29.5 weeks. Final diagnoses were made on the basis of prenatal and postnatal imaging. The missing kidney was ectopic in 39 cases (42%): 34 in the pelvis, 4 fused to the contralateral kidney, and 1 in the thorax due to a congenital diaphragmatic hernia. The absent kidney was never located in 44 cases (47%) and presumed to be congenitally absent. Ten kidneys (11%) originally thought absent were normally located, 7 of which were dysplastic, 2 normal, and 1 infiltrated by a tumor. In 39 patients (42%), other anomalies were identified, sometimes involving multiple systems, most commonly genitourinary (29) and cardiovascular (13). Of the 77 cases with the number of umbilical cord vessels recorded, 68 (88%) were normal and 9 (12%) had only 2 vessels.

CONCLUSIONS: If a kidney is not found in the renal fossa, most are either ectopic (42%) or congenitally absent (47%). An empty renal fossa is often (42%) associated with other congenital anomalies.