UNILATERAL RENAL AGENESIS MAY RESULT FROM IN-UTERO REGRESSION OF MULTICYSTIC RENAL DYSPLASIA

Renal agenesis is generally thought to result from a lack of induction of the metanephric blastema by the ureteral bud, which may be seconda ry to ureteral bud maldevelopment and/or to a problem with the formati on of the mesonephric duct. Multicystic dysplasia is thought to result from early ureteral obstruction as evidenced by the high frequency of associated proximal ureteral atresia. The recent obstetrical practice of large scale screening of large numbers of fetuses with sonography has resulted in a significant increase in the detection of this and ot her genitourinary anomalies. These findings have contributed to our un derstanding of the natural history of many of these malformations, res ulting in ongoing reassessment and refinement of current management mo dalities. Thus, it is now well established that a significant number o f multicystic dysplastic kidneys, initially detected in utero and foll owed postnatally, involute and disappear with time. We report the uneq uivocal presence of unilateral multicystic dysplasia detected during m aternal sonography in 3 fetuses, which could not be seen on subsequent studies during the fetal or immediate postnatal period. To our knowle dge, these cases constitute the first report of complete in utero regr ession of multicystic kidneys. This novel observation indicates that s ome cases of unilateral renal agenesis result from in utero regression of multicystic dysplastic kidneys. This phenomenon may explain the pr esence of an ipsilateral blind ending ureter found in some patients wi th unilateral renal agenesis. However, this infrequent observation doe s not explain all cases of renal agenesis. Thus, the etiology of renal agenesis remains multifactorial.