The kidneys of children share many characteristics with those of adults, although certain differences having clinical implications exist. Positioned slightly above the level of the umbilicus and located on either side of the vertebral column, these paired, bean-shaped organs are well protected in the retroperitoneum. The right kidney usually is more dependently positioned as a consequence of its displacement by the liver. Medially, the organs are buttressed by the paraspinous muscles. Posteriorly, their upper poles are shielded by the lower ribs. However, the underdeveloped abdominal musculature of children and their softer ribs offer the anterolateral surface less effective protection against trauma. Here, an intimate association with several adjacent organs readily explains the high incidence of associated visceral injuries when the kidneys themselves are traumatized (1,2).

Each kidney and its adjacent adrenal gland are contained within a thin condensation of connective tissue, the perirenal or Gerota’s fascia. This fascial envelope is circumferentially intact, with the exception of an inferior hiatus that allows for exit of the ureter. Although it does not provide mechanical protection, Gerota’s fascia is an important anatomic barrier against the extension of primary renal tumors—including Wilms’ tumor—to adjacent organs. It also effectively contains and controls the hemorrhage of most blunt trauma. Variable amounts of fat also surround the kidneys in either a perinephric (within the fascia) or paranephric (outside of the fascia) position, buttressing them in the retroperitoneum. Unfortunately, the paucity of fat in most children makes the organ extremely mobile and more susceptible to contrecoup, flexion, and deceleration injuries.

The kidneys of the child, especially at an early age, are larger with respect to overall body size than those of an adult. This results in a relative abdominal projection that, again, makes them more susceptible to injury. Nomograms have been developed that estimate normal renal size with respect to age, although correlations should be made with overall size. As a general rule, the length of a normal kidney equals that of the adjacent two and one half vertebral bodies. The absence of growth on serial examination should prompt an investigation into cause. In addition to size, the axis of the pediatric kidney provides a clue to abnormal development. Normally, each upper pole is distinctly medial to the lower pole, and lines drawn along the longitudinal
axis of the kidneys typically intersect at the 10th thoracic vertebrae. Vertical or laterally displaced axes can be caused by the upper pole hydronephrosis of a duplex collecting system, tumors of the kidney or adrenal glands, or renal ectopia from incomplete migration or rotation during embryogenesis (see below).

Sagittal sectioning allows an appreciation of the kidney’s macroanatomy (Fig. 95-1). A thick fibrous capsule that adheres to its entire surface maintains the integrity of the renal parenchyma. The capsule is used whenever possible to reinforce the closure of renal wounds because the soft friable parenchyma, like liver, lacks the consistency needed to hold sutures without tearing. Immediately beneath the capsule lies the contiguous outer layer of the kidney, the cortex, which contains the glomeruli, proximal and distal tubules, and collecting ducts. Deep to this lies the central renal medulla, composed of straight portions of the tubules, loops of Henle, the vasa recta, and terminal collecting ducts. The medulla is divided into discrete pyramid-shaped structures that have their base along the corticomedullary junction and are called, appropriately enough, the renal pyramids.

The concavity of the medial surface of each kidney is called the hilum. Here the renal vein, artery, and pelvis are positioned in an anterior-to-posterior direction. The fat-filled space surrounding the renal pelvis and overlying kidney is called the renal sinus. The renal pelvis typically branches into two or three major calyces as it enters the substance of the kidney. Further divisions result in two or three minor calyces. Each renal pyramid (and hence renal lobe) empties into a minor calyx at the papilla, which accounts for the convex impressions seen on pyelography.

There can be significant variation in renal vascularity. Normally, the blood supply to each kidney comes from a single main artery that branches off the aorta, just inferolateral to the superior mesenteric artery. Auxiliary arteries from the aorta and adrenal or gonadal arteries frequently supply the superior and inferior poles, especially to the left kidney. These vessels cross the collecting system and are sometimes implicated in obstructions of the ureteropelvic junction.

Within the renal sinus, the main artery gives off a posterior branch that supplies that segment of the kidney exclusive of the poles. Its anterior limb divides into four branches that feed the apical, upper, middle, and lower segments. Despite any variations that might occur with arterial origins and distribution, there is no collateral blood supply between the individual segments. Each segmental artery is an end artery. Brodel’s line defines a plane between the anterior and posterior branches that, in theory, allows a relatively avascular surgical approach to the center of the kidney.

Unlike its arterial supply, the venous drainage of the kidney freely crosses segmental boundaries. After entering the ascending vasa rectae, venous blood drains into the interlobular veins, and then retraces the arterial path to the main renal vein. Like its arterial counterpart, the renal vein exhibits a great deal of variation. In most cases, the left renal vein, which is the longer of the two, has lumbar, gonadal, and adrenal branches. In contrast, the right renal vein is notoriously short and has no collaterals because the comparable right-sided collateral veins drain directly into the vena cava. This difference in collateral drainage gives the left kidney far more resiliency after ligation or thrombosis of its main vein.

Unilateral renal agenesis is a fairly common anomaly (1 per 1,100 in autopsy series and 1 per 1,500 in radiographic reviews). The 2:1 male predominance seen with renal agenesis probably results from the delicate interplay required of the ureteral bud and the wolffian duct. The timing of the error in embryogenesis determines its effects on the urinary tract and associated organ system anomalies (Table 95-1). The most common organ systems involved include cardiovascular (30%), gastrointestinal (25%), and musculoskeletal (14%) (5). As many as one-half of patients with one kidney still have a rudimentary ureter on the affected side, an indication that the problem occurred after the ureteral bud has extended from the wolffian duct, which remains normal. The other one-half of affected males have an absence of the ureter and ipsilateral hemitrigone of the bladder, as well as absence of the wolffian structures (ejaculatory duct, seminal vesicle, and vas deferens) as indicators of an earlier insult in development. In both instances, the ipsilateral gonad is usually normal.

Similar errors in embryogenesis also affect females for apparently the same reasons. Other than giving off the ureteral bud before they involute, the wolffian ducts play a crucial role in the development of the internal genitalia by guiding the müllerian ducts into position along the urorectal septum. It is easy to implicate abnormalities of this sequence with a unicornuate uterus, where the müllerian duct is never properly positioned, or in uterine didelphys, where the normal fusion and canalization of the two ducts is somehow deterred. Not surprisingly, both anomalies are commonly associated with renal agenesis in female patients. For the same reasons, complete absence or hypoplasia of the vagina is also frequently associated with renal agenesis (the so-called Mayer-Rokitansky-Kuster-Hauser syndrome).

Unilateral renal agenesis is occasionally discovered during the evaluation children with multiple organ system anomalies [e.g., syndrome of vertebral defects, imperforate anus, tracheoesophageal fistula, and radial and renal dysplasia (VATER)], serendipitously or in concert with an associated symptomatic genital abnormality such as hydrocolpos. They are most often diagnosed by antenatal ultrasound. After delivery, renal scintigraphy can be used to confirm the diagnosis. Parents are also made aware of the possible genital implications of the finding. Ultrasound can be used to confirm the presence of a cervix and uterus in infants and young girls. Some clinicians have also recommended screening the siblings of affected children, in whom a 10% rate of asymptomatic renal anomalies has been reported.

Until more recently, it was believed that children with solitary kidneys experienced no increased nephrologic risk. Compensatory hypertrophy of the single moiety occurs, and life expectancy is assumed to be normal. However, concerns have been raised about the hyperfiltration load assumed by the remaining kidney.

Bilateral renal agenesis is less common than its unilateral counterpart (1 in 4,800 births). Male patients are more commonly affected (75%), although the wolffian derivatives are usually normal. Failure of the ureteral bud is typically implicated because an absent or atretic ureter is found in nearly 90% of cases. An increased incidence of imperforate anus and spina bifida suggests a regional disturbance of the cloaca in some children. In the remainder, a 10% incidence of testicular agenesis in the presence of intact wolffian structures points to a dual insult to the renal and gonadal anlagen in the dorsal coelom.

Anhydramnios or oligohydramnios during pregnancy is usually the harbinger of bilateral renal agenesis. Antenatal ultrasound confirms absence of the kidneys and no evidence of bladder filling after a prolonged examination. Although false-positive results have been reported, consideration is given to termination of pregnancy because of the poor prognosis. Intrauterine compression of the fetus results in the classic stigmata of Potter syndrome (clubbed feet, bowed legs, loose skin, and prominent epicanthal fold of the cheek) and severe pulmonary hypoplasia. Stillbirths result in one-half of the cases, whereas the remaining infants die during the first day of life from pulmonary distress. Anuria and gradual renal failure are predictable for the occasional child who remains alive for a slightly longer period of time. Normal newborns should void during the first 24 hours. Anuria is occasionally prolonged by tocolytic agents given the mother or by spinal flexion from traumatic birth. Renal agenesis is suggested by more prolonged anuria, especially without a distended bladder. Ultrasound is usually diagnostic, although renal scintigraphy can also be performed for confirmation. The risk of similar involvement in subsequent pregnancies has been estimated at 2% to 5%, underscoring the need for an autopsy of any baby believed to have this diagnosis.