CUBAN MEDICAL LITERATURE

Prenatal Hydronephrosis:
A Proposal for Postnatal Study & Follow-Up

By Sandalio Durán, MD

CONCEPT

The collecting system is the structure that gathers urine directly from the renal tissue and sends it to the bladder through the ureter. Prenatal hydronephrosis is the dilatation of the renal collecting system detected before birth.

Development of the Fetal Kidney & Production of Amniotic Liquid

The kidneys are formed by three separate, but interrelated, structures: the ureteric bud, the metanephric blastema and the cloaca. By 20 weeks of pregnancy, the excretory system is already completely formed, including the ureter, renal pelvis, calyxes, and papillary and collecting ducts. However, by this time, when the collecting system has finished its formation, only a third of the total number of nephrons is present; nephrogenesis continues until 36 weeks of gestation.[1]

Fetal urine contributes to the quantity and quality of the amniotic fluid depending on the gestational time. At the beginning of pregnancy, the amniotic liquid is a transudate of maternal plasma, and as the fetus grows, the amniotic liquid becomes similar to the fetal plasma, which is thought to be due to diffusion through the fetal skin. Production of fetal urine begins after the ninth week of pregnancy, and the kidneys are capable of excreting sodium and concentrating urea between 12 and 14 weeks of gestation. After 18 weeks, and especially after 20 or 22 weeks, all the amniotic liquid is constituted by fetal urine.[1,2]

For that reason, before 16 weeks of gestation, although there are no functional kidneys, the amniotic liquid can be normal because it is produced by other non-renal pathways. After 16 or 18 weeks of pregnancy and especially past 20 weeks, bilateral kidney agenesis is always associated with severe and progressive oligohydramnion.

Fetal Ultrasonographic Exploration

Maternal-fetal ultrasound has posed questions and challenges in the management of problems that were unknown a few years ago. However, it has also provided a new and splendid way of introducing ourselves in human renal pathophysiology.[3]

The first urinary tract structure that can be detected by ultrasound is the bladder, visualized between the ninth and tenth weeks of pregnancy, just after the start of urine production. The fetal bladder appears as a round or oval, echolucid, structure emerging from the fetal pelvis. It fills up and empties every 20 to 30 minutes, so that if is not visible at the beginning of the study, it is generally full by the end and may easily be detected.[1] If it is not possible to detect the bladder in several studies, vesical extrophy should be suspected.[2]

The fetal kidney may be visualized after about 14 weeks of pregnancy, but it is not routinely observed until after 16 or 18 weeks. Initially the kidneys appear as two hypoechogenic masses adjacent to the lumbar spine of the fetus and may be difficult to define, but as pregnancy progresses, they are easy to detect since retroperitoneal fat deposits around them and they are more clearly defined. Some of the internal structures of the kidneys, such as the fat of the renal sinuses and the medullar pyramids are not well defined until 20 weeks of gestation.[1]

Generalized use of maternal-fetal ultrasound has shown that, in addition to hydronephrosis, renal cystic diseases and even calculi and tumors[4] can be detected. But it has also been shown that pelvic dilatation or hydronephrosis is the most frequently found condition.[4-7]

Examination of the fetal urinary tract should include the volume of amniotic liquid, shape, echogenicity and position of the kidneys, presence or absence of renal cysts, good or poor delimitation between the cortex and the medulla, and the presence of cortex thinning.[1,8,9]

Most fetal ultrasound studies are performed before 24 weeks of gestation because the obstetric management may be changed if desired. However, since anomalies of the urinary tract are generally progressive lesions, alterations that were not detectable between 20 and 24 weeks may appear. Maternal-fetal ultrasound should be performed after 28 weeks of pregnancy to detect alterations of the urinary tract.[1,10] Many obstructive, as well as cystic, abnormalities can be detected and this is very important for giving counsel to the family,[1] and for the study and follow-up of the newborn.

Prenatal ultrasound has given rise to new situations in nephrourology that require adequate management. Many of these children may benefit from early diagnosis and prevention of secondary complications, mainly infections.[2]

Hydronephrosis

As already mentioned, pelvic dilatation or hydronephrosis is the alteration of the urinary tract most frequently detected by maternal-fetal ultrasound.[2,4-7]

Hydronephrosis is not a diagnosis; it is an image finding. The diagnosis is the cause that produces it.[2]

Since pelvic dilatation is relatively frequent in the normal fetus, and the definition and diagnosis of hydronephrosis are difficult, once it is detected, adequate follow-up is required. Postnatal study has shown that mild dilatation of the higher urinary tract does not indicate an obstruction; mild hydronephrosis may be transient; and spontaneous improvement can occur.[11,12] The idea of considering every dilatation of the urinary system as a disorder for which surgery is the solution, is no longer an axiom.[13]

Prenatal pelvic dilatation that is not found after birth or disappears quickly has been explained as a process of embryonic development of the ureter. The developing ureter undergoes an obstruction and re-channeling process at the middle level; some researchers have speculated that this obstruction may occur due to an incomplete re-channeling of the cephalic and caudal ends of the developing ureter, which is later resolved. If the ureter is not permeable when urine production begins, the pelvis may suffer a transient dilatation. Fetal ureters are redundant and tortuous, but they stretch out afterwards during the longitudinal growth of the body and the ascent of the kidneys to an upper retroperitoneal position. This could be an alternative explanation for transient dilatation.[14]

Different criteria have been used to classify the degrees of prenatal hydronephrosis based on the measurement of the anteroposterior diameter of the pelvis.

In our studies we have followed the criteria of Blachar and Blachar,[11,12] while others have followed very similar criteria[16] ( Table 1 ). Some authors[17] have used the criterion of the larger pelvic diameter measuring over 4mm before 33 weeks and 7mm after 33 weeks of gestation to suggest dilatation. Others consider that the anteroposterior diameter of the pelvis should be less that 6mm before 20 weeks of gestation; less than 8mm between 20 and 30 weeks and less than 10mm after 30 weeks.[18] The Society for Fetal Urology has proposed a classification in four groups or categories ( Table 2 ).[9,19,20] Although different criteria have been used, it is generally accepted that a renal pelvis of 10mm or more, during the second quarter of pregnancy, is an indication of hydronephrosis.

Table 1: Classifications Used for Grading Prenatal Hydronephrosis According to the Anteroposterior Diameter of the Pelvis

Table 2: Degrees of Ultrasonographic Hydronephrosis According to the Society for Fetal Urology[9]

Of the parameters used for diagnosing hydronephrosis, the most useful seem to be those used by the Society for Fetal Urology. However, degrees 1 and 2 do not represent true hydronephrosis, but a mild or moderate degree of pyeloectasia.[20,21] Real hydronephrosis always includes significant pelvic dilatation of the pelvis and calyxes. Classifications based solely on pelvic diameter can be misleading since pyeloectasia without calyx dilatation is generally not a sign of significant obstruction.[21]

Congenital obstructions of the utero-pelvic union have historically been classified as intrinsic and extrinsic. The intrinsic ones are attributed to changes in the quantity and orientation of the muscular fibers of the utero-pelvic union, collagen increase, and alterations of peristalsis. Extrinsic obstructions are attributed to anomalous, ectopic or aberrant, vessels, when a branch of the renal artery that compresses the ureter irrigates the lower pole of the kidney. Fibroepithelial polyps and ureteral valves are rare causes of obstruction.[21] Pelvic dilatations of fewer than 10mm in the third quarter of gestation are more frequently due to non-obstructive vesicourethral reflux or pyeloectasia, both conditions being more frequent in male infants.[6,7,15]

STUDY AND FOLLOW-UP

When maternal-fetal ultrasound detects a dilatation of the renal pelvis or hydronephrosis during any period of pregnancy, the neonate must be studied by renal ultrasonography in the first week of extrauterine life. The need to act fast may vary depending on the degree of dilatation or prenatal hydronephrosis; whether it is unilateral or bilateral; whether there is a palpable renal mass; or the presence of only one kidney or a urinary infection in the neonatal period.

Considering these particularities in each case, the general follow-up proposal, shown in Figure 1 , may be modified. Some recommend that if hydronephrosis is moderate or severe the study must be carried out according to our proposal, but if the degree of hydronephrosis is less (mild), they prefer waiting for a few days to ensure an adequate hydration and to minimize the incidence of false negative ultrasonographic studies.[22]

Between the third and seventh days of life an ultrasound must be performed to determine precisely whether there is dilatation or not, and to measure the pelvis, calyxes and renal parenchyma. It will also enable adequate evaluation of the contra lateral kidney (when hydronephrosis is unilateral), to look for ureteric dilatation and visualize the bladder, and to rule out the possibility of ureterocele as the cause of dilatation of the upper urinary tract. It is also important to measure the size of the kidney, which grows 1mm in height every week of the gestational period.[22]

In the first three days of life, renal ultrasound should not be performed, except in very special situations, since at this stage oliguria and hypohydration may conceal pelvic dilatation.

If, during the first week of life, the renal ultrasound shows no evidence of pelvic dilatation and the kidney size is normal, the parents must be given reassuring information, but the child has to be reevaluated ultrasonographically between 3 and 6 months of age. If this second kidney ultrasound is normal, it is not necessary to continue the follow-up, if the baby remains asymptomatic. If during the second study pelvic dilatation is found, even if it is mild, micturitional ureterocystography must be performed to rule out vesicoureteral reflux. Mild dilatations (less than 10mm in diameter) are frequently due to vesicoureteral reflux,[23, 24] and it has been proven in children without postnatal dilatation detectable by ultrasound.[15,24,25] A valuable sign for suspecting vesicoureteral reflux and performing a micturitional ureterocystography is the cyclic dilatation of the pelvis during the ultrasonographic study.[26]

In all cases with dilated pelvis, micturitional ureterocystography should be performed. If the dilatation is severe or moderate, antibiotic prophylaxis should be used to try to avoid urinary infection. In the neonatal period, preferred antibiotics and chemotherapy are cephalexin (2-3 mg/kg/day). Nitrofurantoin (1-2 mg/kg/day) is used after the first month of life, and trimethropin-sulphamethoxazole (sulphaprim at 1-2 mg/kg/day of trimethoprim) after two months of age.[27] In case vesicoureteral reflux is detected, the conduct advocated for this condition shall be followed[28] and congenital or prenatal kidney damage must be ruled out by static gammagraphy.

If vesicoureteral reflux is not found and hydronephrosis is of grade 1 or 2, renal ultrasound follow-up shall be carried out every 6 months. If hydronephrosis progresses, the conduct proposed for higher degrees will be followed. In hydronephrosis of grades 3 and 4, dynamic gammagraphic studies (MAG 3 + furosemide) should be performed. If a non-obstructive renographic pattern appears, ultrasound will be performed every 6 months, carefully measuring the pelvis, calyxes and the renal parenchyma. If the result of the gammagraphic pattern is uncertain, it will be repeated after 3 months, and if the pattern is obstructive it should be repeated after 4 or 6 weeks. In any of these two situations (obstructive or uncertain pattern), if the differential kidney function decreases, especially more than 10% (40% or less) and the pelvic diameter or the calyx dilatation in the ultrasound increase, surgery must be considered (pyeloplasty).

ANALYSIS OF MANAGEMENT PROPOSED

Micturitional ureterocystography should be performed on every newborn in which prenatal renal pelvic dilatation is detected and confirmed after birth, even if the dilatation is mild. This is due to the frequency of renal pelvic dilatations secondary to vesicoureteral reflux.[15] Also because there is no correlation between the degree of dilatation detected by ultrasound and the degree of vesicoureteral reflux.[29] This general indication is more specifically for male infants, since it has been demonstrated that vesicoureteral reflux is up to six times more frequent in males than in females.[30] It is even more necessary in male infants with bilateral dilatation due to the possibility of valve obstruction of the posterior urethra.[31]

In practice, the presence of vesicoureteral reflux determines the need for and duration of antibiotic prophylaxis[21] that we use in severe and moderate dilatations, and should be used in those cases classified as mild, if they are secondary to vesicoureteral reflux. If vesicoureteral reflux is detected, a static gammagraphic study should be carried out because of the possibility of congenital renal damage.[29,30,33]

In every child with grade 3 or 4 hydronephrosis, a dynamic gammagraphic study should also be carried out to provide information on the differential renal function and the clearance rate of the renal pelvis. For this study, two substances shall mainly be used: mercaptoacethylglicine (MAG 3) and dimethyltriaminopentacetic acid (DTPA) both labeled with 99m technetium (99m Tc-MAG 3 and 99m Tc-DTPA).

Ninety percent of MAG 3 combines with plasma proteins and is mainly excreted by tubular secretion with an alternative hepatobiliary excretion pathway. DTPA scarcely combines with plasma proteins and is excreted by glomerular filtration, so it should not be used when there is an immature kidney. Due to these characteristics MAG 3 is better for this study,[32] especially in nursing babies.

Furosemide is used for the diuretic gammagraphy, since it produces an abrupt increase in urine flow, reaching a maximum effect between 15 and 18 minutes after intravenous administration. The dose recommended for this purpose is 1 mg/kg of body weight in nursing babies, 0.5 mg/kg between 1 and 16 years of age and 40 mg as the maximum dose.[32]

Surgical recommendations have been controversial and they remain unchanged for asymptomatic patients.[34] The decision becomes difficult in some cases.

Eighty-five percent of the children with a prenatal diagnosis of hydronephrosis do not have a real obstruction and so they do not require surgery and will improve spontaneously. But the true ureteropelvic obstruction should be operated on as soon as possible to avoid kidney damage. The difficulty of the decision can be minimized with a very strict protocol.[35] The degree and severity of prenatal hydronephrosis is determined after birth by evaluating the general condition of the child, the degree of dilatation by echography, and the radioisotope excretion curve after administering furosemide. Dilatation alone does not imply obstruction. However, if all the remaining parameters indicate the presence of obstruction, early surgical treatment is recommended.[36] Surgical criteria used by the Society for Fetal Urology are concurrence of the increase of the hydronephrosis and a worsening of the radioisotopic clearance of over 10% between studies.[37] Obstruction has been defined as such a state of urinary drainage that if not corrected, it will limit the final functional potential of the developing kidney.[38] Early repair is recommended if obstruction is demonstrated,[39] because the delay in alleviating the obstruction may allow rupture of the basal tubular membrane and promote the transition of the epithelium to myofibroblasts, a process that is probably irreversible.[39,40] Surveillance and control of the patient before a decision is made must be very strict in some cases.

Based on different criteria found in the literature and on the clinical evolution of our patients, we think that surgery (pyeloplasty) is indicated if the renographic pattern is obstructive or uncertain, the relative function falls below 40% or hydronephrotic dilatation increases in follow-up ultrasound studies. The obstructive cases that do not receive the benefits of timely pyeloplasty may progress to a renal function deterioration, which may become irreversible, requiring the substitution of reconstructive surgery (pyeloplasty) by mutilating surgery (nephrectomy).

Whichever the treatment used (expectant or reconstructive), hydronephrosis must be followed-up for years to evaluate the growth and functioning of the kidneys. Another important element is the control of arterial hypertension.[22] In surgically treated cases, non-steroid anti-inflammatory drugs should not be administered, and if hypertension or significant proteinuria appear, they must be treated resolutely.[39]

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THE AUTHOR

Sandalio Durán, MD, is a Pediatrics Consulting Professor at the Higher Institute of Medical Sciences of Havana and in the Nephrology Department of the William Soler Teaching Pediatric Hospital, San Francisco 10112, Habana 8, Ciudad de la Habana. E mail: sduran@infomed.sld.cu