Year : 2003 | Volume
: 14 | Issue : 3 | Page : 336--341
Congenital Renal Agenesis: A Review
Rajesh Yalavarthy, Chirag R Parikh
Division of Renal Diseases and Hypertension, University of Colorado Health Sciences Center, Denver, Colorado, USA
Chirag R Parikh
Division of Renal Diseases and Hypertension, University of Colorado Health Sciences Center, 4200 E Ninth Ave, Box C-281, Denver, CO- 80262, Colorado
|How to cite this article:|
Yalavarthy R, Parikh CR. Congenital Renal Agenesis: A Review.Saudi J Kidney Dis Transpl 2003;14:336-341
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Yalavarthy R, Parikh CR. Congenital Renal Agenesis: A Review. Saudi J Kidney Dis Transpl [serial online] 2003 [cited 2021 Dec 7 ];14:336-341
Available from: https://www.sjkdt.org/text.asp?2003/14/3/336/33012
Renal Agenesis: Complete absence of renal development.Renal Dysgenesis: Abnormal kidney development, which includes aplasia, hypoplasia, dysplasia, and cystic disease.Renal Aplasia: Rudimentary kidney without any functional nephrons.Renal Hypoplasia: Small non-dysplastic kidney that has less than the normal number of calyces and nephrons.Renal Dysplasia: Focal, diffuse, or segmentally arranged primitive structures, specifically primitive ducts, resulting from abnormal metanephric differentiation. Non-renal elements, such as cartilage, may be present. If cysts are present the condition is termed as cystic dysplasia.Renal Adysplasia: This term is used when aplasia and severe dysplasia are aspects of phenotypic spectrum.
Congenital renal agenesis is defined as the complete absence of renal tissue at birth. This condition includes bilateral renal agenesis (BRA) or unilateral renal agenesis (URA). BRA is not compatible with life and is associated with oligohydramnios, flat facies, and pulmonary hypoplasia, a syndrome that was first described by Potter, hence called Potter's sequence [Figure 1].  URA is the most common congenital abnormality of the urinary tract and this entity has been recognized known since the time of Aristotle. , Due to the introduction of improved imaging modalities (ultrasound, CT scan and MRI) and rigorous prenatal screening for this condition, renal agenesis is being increasingly recognized. This article reviews the embryology, epidemiology, etiology, clinical features and other malformations associated with congenital renal agenesis.
Renal development involves fusion of two embryological structures [Figure 2]:
Metanephric blastema (undifferentiated mesenchyme in nephrogenic ridge): it is the proximal component, which gives rise to glomeruli and tubules up to the distal collecting tubules.Ureteric bud: it is the distal component, which gives rise to collecting ducts, calyces, and pelvis.
During the 5th week of gestation the ureteric bud arises from the Wolffian duct and penetrates the metanephric blastema. Nephrogenesis begins at the 7 th week of gestation under the influence of the ureteric bud. By the 20 th week the ureteric bud branches into 15 generations and forms the entire collecting duct system. At this time nephrogenesis is only 30% complete. Further maturation proceeds gradually until the 36 th week.  The fetal kidney plays only a minor role in the salt and water homeostasis in utero but is an important source of amniotic fluid. Hence, in BRA and URA oligohydramnios may be present during pregnancy. This often is the first indication that the fetus has renal agenesis. In our study of URA, there was an increased incidence of oligohydramnios in pregnant women whose fetus had URA as compared to controls (17.5% vs. 1.4%, P 
The incidence of URA is between 1/500 to 1/3200 births. This wide discrepancy between various studies is due to differences in sample size, study design, diagnostic criteria, and population selection. The incidences observed in various studies are summarized in [Table 1].Males are more commonly affected than females with a ratio of 1.2-2.3:1.  This has been attributed to the earlier differentiation of Wolffian duct that takes place close to the time of ureteral bud formation. The ureteral bud is more likely to be influenced by abnormalities in the Wolffian duct than abnormalities of the Mullerian duct, which develops later in fetal maturation. The left kidney is more commonly absent in URA as compared to right side.  URA is more prevalent in the AfricanAmerican race as compared to Caucasians. 
Renal agenesis is caused by failure of the ureteric bud to induce development of metanephric blastema or by absence of the nephrogenic ridge.  In knockout mice, experiments have shown that Wt-1, Pax-2, Emx-2, Lim-1 and many other gene deletions are associated with renal aplasia.  In humans, naturally occurring mutations in Pax-2, KAL mutation are associated with renal abnormalities. 
The etiology of URA and BRA is heterogenous and appears to be multi-factorial with environmental and genetic influences. , Reported teratogens involved in renal agenesis include retinoids, thalidomide, arsenates, and cocaine.  The incidence of URA is increased in newborns with a single umbelical artery.  URA also occurs with the caudal regression syndrome, which is seen with increased frequency in diabetes mellitus.  Our study of prenatal and perinatal factors associated with renal agenesis demonstrated that diabetes mellitus, black race, younger pregnancies, and alcohol intake are independent risk factors for development of URA.  A mother with pre-existing diabetes mellitus has a 5-fold increased risk of having a fetus with URA [Table 2].
BRA is incompatible with extra-uterine life. Death occurs shortly after birth from pulmonary hypoplasia. This condition, as noted earlier, is called Potter's syndrome [Figure 1]. The newborn has a characterstic facial appearance, termed Potter facies. The eyes are widely separated and have epicanthic folds, the ears are low set, the nose is broad and compressed flat, the chin is receding, and there are limb anomalies. These findings are attributed to fetal compression due lack of amniotic fluid. BRA should be suspected when maternal ultrasound demonstrates oligohydramnios, non-visualization of the bladder, and absent kidneys. BRA represents 20% of newborns with Potter's phenotype. Numerous other syndromes can result in oligohydramnios and produce clinical findings as described by Potter. Other causes of Potter phenotype include cystic renal dysplasia, obstructive uropathy, autosomal recessive polycystic kidney disease, renal hypoplasia, and medullary dysplasia.
URA is an asymptomatic condition usually discovered during routine prenatal ultrasonography. Before the advent of prenatal ultrasound, the diagnosis of URA was usually made incidentally when radiographic studies were performed for other reasons. According to a recent study, renal aplasia is the predominant cause of congenital solitary kidneys.  The authors suggested that the aplastic kidney would rapidly regress in size in the newborn period and finally disappear, which may result in the mistaken diagnosis of congenital renal agenesis. They concluded that most renal agenesis diagnosed clinically thus far might more correctly be termed renal aplasia. This is contrary to the previous belief that the majority of congenital solitary kidneys are due to agenesis. Further studies need to be performed to clarify this issue.
With true agenesis, the ureter and ipsilateral bladder hemitrigone are absent. The contralateral kidney undergoes compensatory hypertrophy, to some degree before birth, but primarily after birth. 3 Approximately 15 % have contralateral vesicoureteral reflux.  While vesicoureteral reflux is the most common abnormality of the contra-lateral kidney, other reported abnormalities in the remaining kidney include renal m alr otation, ectopia, ureteropelvic junction obstruction, ureterovesical junction obstruction, ectopic ureter with partial obstruction of collecting system, dysplasia, and hypoplasia. Case reports have suggested that the contralateral kidney can develop focal segmental glomerulosclerosis later in life. 
Renal agenesis is commonly associated with abnormalities of other organ systems. 
In our experience, only 36% of newborns with renal agenesis had no associated organ abnormalities. Malformations were seen in every other organ system with the pulmonary system most commonly affected. The frequency of associated malformations in various organ systems is shown in [Table 3]. Knowledge of associated abnormalities enables the clinician to have a high index of suspicion to search for other conditions. This is advantageous because it gives adequate time to consider termination of pregnancy if there is an associated lethal anomaly or to plan postnatal management.
An algorithm for evaluation of suspected URA is shown in [Figure 3]. Once URA is suspected a thorough history, examination and diagnostic testing is essential to confirm the diagnosis and to detect any other associated problems with the urinary tract and other organ systems. Prolonged, and in some cases lifelong, follow-up is essential due to higher risk of hypertension and chronic renal failure in these patients. 
The prognosis of individuals with URA depends on the presence, and severity of associated anomalies and the status of the remaining kidney. The majority of patients with URA will not experience significant problems or complications as a consequence of this anomaly. However, some physicians advise against contact sports to prevent trauma to the remaining kidney. In any case, patients with URA require life long follow-up since hypertension and renal insufficiency have been reported in longterm studies with single kidney. 
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