The Kidneys

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Renal disorders in the newborn

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Prenatal diagnosis of renal disease is usually by fetal ultrasonograms that detect signs of obstructive uropathy. Fetal hydrops may occur with congenital nephrotic syndrome. Oligohidramnios occurs with severe urinary tract obstruction or renal agenesis, which is associated with pulmonary hypoplasia  (Potter’s syndromes).

Clinical manifestations of renal disease vary with the type and severity of abnormality. Certain findings are indicative or suggestive of renal disease :

  • Potter’s syndrome

Potter’s syndrome (renal agenesis and pulmonary hypoplasia) is a fatal condition that has typical physical abnormalities: flat nose, low set ears, receding chin, arthrogryposis and , often, a bell-shaped chest. With prolonged oligohydramnios due to other causes (eg obstructive uropathy, prolonged rupture of fetal membranes), the infants may show similar physical features and the severity of pulmonary hypoplasia varies from absent to severe, depending on duration and severity of oligohydramnios

  • Dysmorphic features

Dysmorphic features sugesstive of renal disease include abnormal ears, single umbilical artery, hypospadius, anorectal abnormalities, polythelia (supernumerary nipples), vertebral anomalies and esophageal atresia (with or without tracheo-esophageal atresia)

  • Lateral abdominal mass

Polycystic or multicystic kidneys, hydronephrosis, tumor (Wilm’s tumor)

  • Ascites

Ascites (urinary) due to rupture of obstructed urinary tract

  • Suprapubic mass

Suprapubic mass may be an enlarged bladder secondary to urethral obstruction

  • Abdominal wall defects

Exstrophy  of bladder, cloacal exstrophy, “prune belly” (absence of abdominal wall muscle due to fetal urinary ascites)

  • Failure to palpate kidney

Unilateral renal agenesis, renal malposition, horseshoe kidney

  • Hypertension

Hypertension is frequently due to renal disease. The commonest cause of neonatal hypertension is renovascular disease secondary to clots or emboli from a “high”umbilical arterial catheter

  • Anuria or oliguria

However, only 90% of normal infants urinate in the first 24 hours after birth, therefore, 10% of normal infants do not urinate on the first day

 

The presence of any of the above signs should alert one to the possibility of renal dysfunction and raise the possibility of further diagnostic work-up including, in addition to careful measurements of intake and urine output, serum creatinine, blood urea nitrogen, electrolytes and abdominal ultrasound.

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Dysplastic kidney disease

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Dysplastic kidney disease is common, occuring as unilateral disease in 1 in 1000 births and bilaterally in 1 in 5000. It is the common cause of an abdominal mass in the newborn. The condition typically is diagnosed on prenatal ultrasonography. The kidney is highly echogenic or “bright”, with multiple large, thin-walled cysts that typically are seen by 20 weeks gestation, in dystiction from polycystic disease.  These cysts characteristically  separate and distributed randomly througout the renal parenchyma. Grossly, the kidneys are dysplastic, with little normal architecture evident, and the multiple large cysts are easily throughout the entire organ. Most often, an affected kidney is significantly enlarged, and this large irregular mass may be easily palpated in the newborn. Both the cysts and overall enlargement may become more pronounced as gestation proceeds, but many dysplastic kidneys involute, even during the prenatal period. Unlike cases of obstruction of the lower collecting systems, it can be difficult to identify the renal pelvis or ureter on ultrasonography.

Bilateral disease frequently is severe, and the prognosis typically poor, with progression to severe oligohydramnios and pulmonary hypoplasia, resulting in Potter sequence, and a low likelihood of survival after birth. This outcome usually associated with findings of either marked renal enlargement or involution during late pregnancy. Some cases of bilateral disease are less severe and survival is possible. Dialysis is necessary for chronic renal failure in affected children, and their require specialized care for nutrition. As the children grow, renal transplantation becomes a possibility.

Unilateral disease usually presents with no specific signs or symptoms in the neonate, other than the presence of a large abdominal mass, which rarely can cause mechanical problems. If it is, of course, diagnosed typically on prenatal ultrasonography

Following a prenatal diagnosis, postnatal ultrasonography should be undertaken to rule out associated urinary tract anomalies and to confirm the original diagnosis. Many practitioners recommend waiting for at least 1 week after birth to perform this study to avoid false-negative results due to normal oliguria, but others have found that studies performed as early as 48 hours after birth are reliable, especially if repeated at about age 6 weeks.

The major risk is the development of chronic renal failure, which develops in 12% to 50% of patients who have apparently normal contralateral kidneys. Serum creatinine is monitored routinely to identify developing renal failure. Hypertension is uncommon occurence, and blood pressure monitoring is recomended only as part of regular care.

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Polycystic kidney disease

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Polycystic kidney disease occurs as two distinc genetic disorders, autosomal dominant and autosomal recessive forms. The incidence of polycystic kidney disease is 1 in 1000 live births, making it the most common of the inherited disorders. Autosomal dominant polycystic kidney disease, in which bilateral renal cysts form and progress, rarely presents in neonate. When it does, the presentation is similar to that of the recessive form, with a broad range of severity from asymptomatic through severe renal dysfunction, along with variable degrees of hypertension. As a result, the parents of any baby who has polycystic kidney disease should themselves undergo ultrasonographic examinations to rule out the small possibility of the disease being the autosomal dominant form. any patients have no renal symptoms through childhood, but in the long term, more than 50% deteriorate progressively to end-stage disease, often over decades.

The clinical presentation includes polycystic kidneys, congenital hepatic fibrosis, and some degree of biliary dysgenesis, although hepatic symptoms are uncommon in neonatus. The renal manifestations vary in severity but generally include large kidneys that appear echogenic on prenatal ultrasonography, with initially normal amniotic fluid volume. Characteristically, the kidney filled with multiple small cysts that are not easily visible on early ultrasonography and only appear as gestation proceeds, although the kidneys are large and echogenic. The gross appearance of the kidney mirrors of the ultrasonographic findings, with numerous small cysts throughout the parenchyma.

In more severe cases of autosomal recessive polycystic kidney disease, the amniotic volume declines as gestation progresses, and in many cases, oligohydramnios is present by late in the second trimester. These more severely affected fetuses may develop the Potter phenotype, with pulmonary hypoplasia and a small chest, beaked and flattened  nose, and deformations of the extrimities and spine. This phenotype is most likely to develop when severe oligohydramnios is present by mid-gestation. In these instances, the degree of lung hypoplasia may be critical, making extrauterine survival imposible.

Among a recent large cohort of patients for whom prenatal diagnosis is available, the early mortality was slightly more then 25%, primarly as a result  of respiratory failure and sepsis.A total of 41% of patients required mechanical ventilation after birth, and almost 12% of the survivors developed chronic lung disease. Longer term, 42% developed chronic renal insufficiency, and more than 25% of patients manifested slowed or delayed growth in infancy and early childhood, related to poor renal and pulmonary function. Importantly, the median age of the patients was 5,4 years, demonstrating that survival at least through childhood is possible with this disease.

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Renal replacement therapy and prognosis in the infant

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Congenital anomalies of the kidneys and urinary tract include a wide range of defects that have their origin during the critical period of organogenesis of urinary system. The conditions includes a full spectrum and array of defects and can include minor conditions, such as mild resolving hydronephrosis, all the way to anuric renal failure due to posterior urethral valves or bilateral renal agenesis. Its etiologies can include many of genes known to be critical in urinary tract development, as well as environtmental factors that can interfere with development at key stages, however, in the majority of cases, the etiology remains unknown. Most cases can be detected and diagnosed prenatally with modern antenatal ultrasonography.

Lower urinary tract obstruction secondary to posterior urethral valves shows a biphasic peak incidence for time to progression to end-stage renal disease, with the first peak being in infancy and the second peak occuring in adolescents. In the most severe cases, the kidneys are so dysplastic and poorly developed that there is severe oliguric kidney failure at the time of birth  or very soon afterwards. Many of the seemingly less severe cases still result in highly dysplastic kidneys, which display poor growth  over time, eventually culminating in end-stage renal disease.

Infant dialysis is technically challenging and costly, therefore, it would be helpful to have a clear idea of which patients would be at highest risk for early renal failure.  Dialysis can be problematic in the newborn and requires strong nephrologic expertise, which may not available at all institution. However, over  the last 20 years, renal replacement therapy has improved outcomes in infants and children, and success has been reported.

For children  with end-stage renal disease, dialysis is generally considered a bridging step until kidney transplantation is possible. Assesing the odds for successful kidney transplantation, 10 kg is generally agreed to be the smallest size. To support a newborn with end-stage renal disease to the 10 kg mark, intensive nutritional therapy, often with tube feeding, is required. These infants generally have oral aversions and have failure to thrive due to frequent vomiting and an increased catabolism.

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Prenatal diagnosis of congenital anomalies of the kidneys and urinary tract

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The widespread use and sensitivity of fetal ultrasonography has resulted in antenatal detection of the majority of renal malformations. Ultrasonographic screening for fetal anomalies can detect renal agenesis, multicystic dysplastic kidney, hydronephrosis, and abnormally shaped bladders from midway through gestation.

Fetal urinary production starts at 9 weeks gestation, therefore, the fetal bladder should be visualized from 13 weeks onward. By 20 weeks.fetal urine produces 90% of amniotic fluid. References curves for renal volume and the amount of amniotic fluid are available. Fetal kidneys can be visualized at 12 weeks, and by 25 weeks, the renal cortex and medulla are distincly demonstrated on ultrasonography. Parameters for expected length (appropriate growth) based on gestational age are also available.

Fetal hydronephrosis is most commonly detected during routine ultrasonography between 18 and 22 weeks gestation. Several systems has been developed to diagnose and grade the severity of hydronephrosis, but there is no consensus on the most appropriate criteria. In general, the likelihood of having a significant renal anomaly correlates with the severity of hydronephrosis. Because of renal immaturity, hydronephrosis detected before 25 weeks warrants repeat scanning. As expected, studies performed in the third semester are the most helpful in predicting the postnatal outcome of congenital anomalies of the kidneys and urinary tract. Thinning of the renal parenchyma and/or cortical cyst may be seen with hydronephrosis. They indicate injury or impaired development of the renal cortex. Increased echogenicity of the renal cortex may indicate anbormalrenal parenchymal development. These findings are associated with poor postnatal renal function when combined with hydronephrosis.

Normally, the fetal ureters are not seen on ultrasonography, so when they are visualized, ureteric or bladder obstruction or vesikooreteral reflux may be indicated. Bilateral involvement increases the risk of significant renal abnormality and of impaired postnatal renal function. The bladder wall is normally thin. If the bladder wal is thick, urethral obstruction such as posterior uretral valves in a male fetus may be present. If the bladder is not seen, consider the diagnosis of bladder extrophy.

Oligohydramnions at or beyond the 20th week of gestation is the most reliable predictor of abnormal fetal renal functions. Because amniotic fluid is predominantly composed of fetal urine, biochemical analysis is useful in further assesing fetal renal function. Sodium and chloride concentration greater than 90 mEq/L (90mmol/L) and urinary osmolality less than 210 mOsm/kg H2O (210 mmol/kg H2O) in the amniotic fluid are indicative of fetal renal tubular impairement and poor renal prognosis. Under these circumstance, more invasive testing such as vesicocentesis (bladder taps) may be undertaken.

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Embryonic development of the kidney and urinary tract

 

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Because of the complex nature of renal development, there are multiple opportunities for interference, through mutations in crucial genes or environtmental stresses, affecting various stages in the ontogeny of the kidney, which unfortunately lead to the outcome of a congenital anomalies of the kidneys and urinary tract disorders. In humans, the kidneys develop through a multistep process, progressing from an anterior to posterior tract. To produce mature kidneys, 2 transient phases must be navigated. These are characterized by partial development, followed by regression of primitive kidney, and then a third and final phase that results in a pair of functioning kidneys that serve to filter blood, excrete metabolic waste products, and regulate plasma electrolyte concentration and osmolality.

The process begins with formation of the nephritic duct (day 22 in humans) in the intermediate mesoderm at an anterior position. The duct induces nearby intermediate mesoderm cells to form the primitive tubules of the first kidney, known as the pronephros. In humans, the pronephros quickly degenerates, never forming a functioning renal organ. As the tubules of the pronephros dissapear, the nephritic duct grows caudally, following parallel to the tract of intermediate  mesoderm.  At a location caudal to the pronephros, the signals of the nephric duct transform the intermediate mesoderm into the next  primitive kidney, the mesonephros. Like the pronephros, the mesonephros forms a series of tubules (day 25 in humans),  but degenerates after a short time.  However, in the case of mesonephros, parts of tubules remain, becoming components of the male reproductive tracts (vas deferens), but do not contribute to the final kidney.

Still further caudal, near the level of the developing hind limb, is the metanephric mesenchyme, which will eventually develop into the final kidneys if the proper signaling  cascade is achieved. The metanephric mesenchyme cells produce and secrete glial cell line-derived neurotrophic factor, once the nephric duct reaches the level of the metanephric mesenchyme and is exposed to the chemical gradient of glial cell line-derived neurotrophic factor, it responds by producing a diverticula that homes in on, and invades, the metanephric mesenchyme tissue. This diverticula coming of the nephric duct is known as the ureteric bud, and it reciprocates the chemical communication with the  metanephric mesenchyme by secreting a number of transcription factors, including Fgf2 and BMP7, which are necessary to prevent the cells of the metanephric mesenchyme from undergoing apoptosis, as occured with the pronephros and mesonephros. The metanephric mesenchyme is the only tissue that can respond to ureteric bud signaling because of expression in this tissue.

As  the ureteric bud invades the metanephric mesenchyme, it will begin a series of branching events influenced by glial cell line-derived neurotrophic factor. Cells from the metanephric mesenchyme that condense onto the tips of the ureteric bud (known as cap mesenchyme) will eventually form the nephron. The proper number of branching events by ureteric bud is critical to achieve the appropriate number of nephrons in the final kidney (with normal variation ranging from 300.000 to 1.000.000).

The branches of the ureteric bud eventually form the collecting ducts, renal pelvis and the ureters. The cap mesenchyme that condensed onto the branches of the ureteric bud go on to become  the glomerulus, proximal tubule, loop of Henle and distal tubule. The bladder itself originates from a portion of the cloaca, and the nephric duct empties into the bladder, under the direction of Hox genes. All these events need to be precise in both time and space in order for the kidneys, ureters, and bladder to form properly.

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