Editing Pediatrics: Anomalies of the Kidneys (section)

=== Unilateral Renal Agenesis ===
* '''Renal aplasia is the most common cause of congenital solitary kidney.'''
* '''Complete absence of one kidney occurs more frequently than BRA but is not easily detected''' on physical examination.
* '''The largest study to date of neonates with an isolated single umbilical artery did not find an increased incidence of URA or other malformations'''
* '''Incidence'''
** '''Occurs 1 in 1100 births.'''
** Males predominate (ratio 1.8:1), but lower proportion than bilateral renal agenesis
** '''Absence of a kidney occurs more frequently on the left side'''
* '''Genetic/Syndromic and Other Associations.'''
** '''Syndromes associated with URA:'''
*** '''Turner syndrome'''
*** '''Poland syndrome'''
*** '''Branchio-oto-renal (BOR) syndrome'''
*** '''DiGeorge anomaly''' (when associated with maternal insulin-dependent diabetes mellitus)
*** '''VACTERL''' association (vertebral, imperforate anus, cardiac, tracheo-esophageal atresia, renal, and limb anomalies)
** '''Maternal diabetes is associated with a 3x increased risk of renal agenesis and dysplasia'''
** '''Congenital renal agenesis results from RET mutations''' that prevent or impede the embryonic development of RET-dependent structures
* '''Embryology'''
** '''The embryologic basis for URA and BRA is thought to be similar and it is most likely caused by the ureteric bud'''
*** Complete absence of a bud or aborted ureteral development prevents reciprocal induction, which is critical for the development of the metanephric blastema into the definitive adult kidney.
*** '''The metanephros is not likely to be responsible for the majority of cases'''
* Prior to the increased use of prenatal ultrasound, a substantial number of cases thought to be URA were a dysplastic or multicystic dysplastic kidney (MCDK) that had involuted before birth
* A plain film of the abdomen (or other radiographic study such as magnetic resonance imaging) showing the gas pattern of the splenic flexure in the left renal fossa suggests left renal agenesis, ectopia, or crossed ectopia, whereas the gas pattern of the hepatic flexure positioned in the right renal fossa suggests congenital absence of the right kidney.
* '''Associated Genitourinary and Adrenal Anomalies'''
** '''In autopsy studies of unilateral renal agenesis:'''
*** '''Adrenal agenesis occurs in fewer than 10%,''' although the ipsilateral adrenal gland may be flattened or "lying down."
*** The ureter is not normally developed, and the '''ipsilateral ureter is completely absent in approximately 60% of cases.'''
*** The gonad is usually normal in both sexes.
*** The head of the epididymis is normally formed because it is derived from the mesonephric tubules that link the mesonephric duct to the gonad.
** Except for ectopia or malrotation, '''anomalies of the contralateral kidney are infrequent. However, abnormalities of the contralateral ureter are not uncommon''', including ureteropelvic and ureterovesical junction obstruction in 11% and 7%, respectively and reflux in 30%
** Reproductive tract abnormalities in females occur in ≥25-50% of cases compared with 10-15% in males.
** '''Anomalies of the male'''
*** The testis and head of the epididymis, which contain the efferent ductules derived from the mesonephric tubules, are invariably present; '''WD structures''' (the body and tail of the epididymis, vas deferens, seminal vesicle, ampulla, and ejaculatory duct), '''are absent in ≈50%'''
*** '''Zinner syndrome (3):'''
***# '''Ipsilateral renal agenesis'''
***# '''Seminal vesicle cyst'''
***# '''Obstruction (atresia) of the ejaculatory duct'''
*** '''In males evaluated for infertility, the diagnosis of URA should be suspected when the vas deferens or body and tail of the epididymis are impalpable.'''
*** '''In children, URA should be considered when vasal and/or epididymal anomalies are incidentally found at the time of scrotal ultrasonography, herniorrhaphy, or orchiopexy.'''
** '''Anomalies in the Female'''
*** '''≈1/4th-1/3rd of women with MD anomalies are found to have URA.'''
*** '''Depending on the timing of the embyrological insult, a variety of anomalies may result from incomplete Mullerian duct formation''' because of alterations in normal WD development.
**** '''Most common MD anomalies (2):'''
****# '''Unicornuate uterus with complete absence of the ipsilateral horn and fallopian tube'''
****#* Forms when the embryological insult occurs before the 4th week
****# '''Bicornuate uterus with rudimentary development of the horn on the affected side'''
***** '''Partial or complete midline fusion of the MD may result in a double (didelphys) or septate uterus''' with either a single or a duplicated cervix.  '''Obstruction of one side of a duplicated system is not uncommon, and unilateral hematocolpos or hydrocolpos associated with a pelvic mass and/or pain has been described in pubertal girls'''
****** In rare instances, this anomalous condition has been mistaken for a large or infected Gartner duct cyst.
****** If the insult occurs early in the fourth week of gestation and affects both the wolffian duct and the ureteral bud, maldevelopment of the wolffian duct affects renal development, müllerian duct elongation, contact with the urogenital sinus, and subsequent fusion. Therefore, a didelphic uterus will form with obstruction of the horn and vagina on the side of the unilateral renal agenesis.
****** If the insult occurs after the 4th week, the wolffian duct and müllerian duct elongation and differentiation proceed normally and only the ureteral bud and metanephric blastema are affected, thereby resulting in isolated unilateral renal agenesis.
****** Insert figure
***** Anomalies of other organ systems are found frequently in affected individuals. The more common sites involve the cardiovascular, gastrointestinal, and musculoskeletal systems.
*** '''Diagnosis and Radiographic Evaluation'''
**** ≈90% of fetal kidneys associated with URA or MCDK undergo compensatory hypertrophy in utero which may lead to renal disease later in life