EXPERIMENTAL URINARY OBSTRUCTION AND VESOURETERAL REFLUX MODELS

Congenital hydronephrosis is the most frequently detected abnormality in prenatal diagnosis. Renal pathologies that develop as a result of urinary obstruction and vesicoureteral reflux are among the most important causes of end-stage renal failure in children and adults. For these reasons, urinary obstruction and vesicoureteral reflux are among the most discussed topics in pediatric urology, both experimentally and clinically. Experimental and clinical studies to elucidate the diagnosis, follow-up and treatment of both conditions, as well as the etiopathogenesis of kidney damage, continue today. In this article, experimental studies and the results of studies aimed at elucidating the etiopathogenesis of urinary obstruction and vesicoureteral reflux will be briefly presented.
1. EXPERIMENTAL URINARY OBSTRUCTION MODELS

1.1. Importance of hydronephrosis

Hydronephrosis is a radiological finding that describes the dilatation of the renal pelvis and calyces. Its clinical importance is the kidney damage that can be caused by accompanying urinary obstruction. Any obstruction in urine flow, if left untreated, will result in kidney dysfunction. The location and form of obstruction are important in the effect of obstruction on kidney development and function. Obstruction occurs at either the pelviureteric junction or the ureterovesical junction, and is ultimately diagnosed as either hydronephrosis or hydroureteronephrosis. Obstruction in the bladder neck and urethra is usually accompanied by vesicoureteral reflux. Pelviureteric junction obstruction has a greater impact on renal dynamics than ureterovesical obstruction. Pelviureteric junction (PUJ) obstruction mainly occurs due to two reasons: Intrinsic problems occur in the progression of urine collected in the pelvis to the ureter and the exact cause has not yet been elucidated. The other is obstruction caused by extrinsic factors due to fibrous band or vascular compression. Often extrinsic and intrinsic factors exist together. The exact cause of many PUB obstructions is unknown, but it is thought to be obstruction as a result of developmental abnormalities of the pelvis and ureter.1 While the ureter is in a solid structure in the fourth week of pregnancy, it later turns into a tubular structure with recanalization. It is thought that obstruction may occur as a result of the inability to transform the obstruction from this solid phase into a lumen-containing tubular structure.2 Factors such as the time and place of the obstruction, whether it is complete or partial, continuous or intermittent, the compliance of the renal pelvis and collecting systems, the presence of infection along with the obstruction, and ischemia are issues that need to be emphasized and studied on the physiopathological mechanism that occurs as a result of the obstruction. These factors should be considered and evaluated as a whole in studies and evaluation of results.

We see that many experimental studies have been conducted to investigate the etiology and prognosis in pelviureteric junction obstruction. The inflammatory response that begins as a result of urinary obstruction, It has been shown in many experimental studies that it causes gloemerulosclerosis, tubular dilatation, and fibrosis. The common result of all these studies is the impairment of kidney functions. The most effective factor in the deterioration of renal functions is the intraureteral pressure rising above the threshold value and being reflected in the renal tubules and glomeruli. The renal pelvis can tolerate this high pressure to some extent. As the pressure increases with the increase in volume, kidney damage also develops. Obstructive pathology that causes hydronephrosis also causes the release of many vasoactive peptides in the kidney, and as a result, glomerular and tubular dynamics are affected. Some factors effective in kidney damage have been shown in experimental studies. These are especially vasoconstrictor substances such as Angiotensin II and Thrombaxan A2. Activation of the renin-angiotensin system stands out as the main factor in initiating this response. Preventing kidney damage with the use of angiotensin converting enzyme (ACE) inhibitors supports this view. In experimental obstructive studies, it has been shown that Angiotensin II plays a major role in the etiopathogenesis of tubulointerstitial fibrosis. These substances, which act on a single nephron, affect glomerular arteriolar vascular resistance and ultimately affect total renal clearance and glomerular filtration. It has been shown that renal perfusion changes after obstruction are not perceived equally in all nephrons, and that some regulatory mechanisms also have an effect on nephrons. After 24 hours of complete obstruction, the removal of the obstruction A study conducted after 60 days showed that GFR was preserved. Although there is a significant decrease in the total number of nephrons, the preservation of GFR is due to the increase in the workload of the remaining nephrons. The effective mechanisms here are probably the effects of eikasonoids - especially prostacyclin and PGE2 - on glomerular arteriolar vascular resistance.3 Frokier et al.4, in a study in which they created unilateral ureteral obstruction, showed that pelvic pressure increased and renal blood flow on that side decreased, and they inferred that renal prostaglandins were the most important modulators on renal hemodynamics.

It has been shown in many experimental studies that tubular functions are also affected by obstruction. However, the effect of obstruction on tubular functions is not as serious as its effect on the glomerulus.
1.2. Selection of appropriate laboratory animals for ureteral obstruction

The most common clinical problem in urinary obstruction is postnatal kidney pathologies caused by congenital hydronephrosis. For this reason, the model to be chosen in the laboratory animal is to create the clinical situation appropriate to that in humans. In experimental studies on urinary obstruction, we see that it has been studied in rats, guinea pigs, rabbits, pigs and sheep. The fact that nephrogenesis in rats continues in the postnatal period indicates that newborn rats are suitable for the obstruction model. By using laboratory animals in which nephrogenesis continues in the postnatal period, a timing of nephrogenesis corresponding to the last tremester in humans can be achieved. The fact that nephron development in pigs continues for up to three weeks after birth makes it suitable for investigating the effect of ureteral obstruction on kidney development in newborn pigs. The histological structure of the pig kidney is similar to the human kidney, making pigs a more suitable laboratory animal. In addition, the urodynamic functions of the porcine renal pelvis and ureter were determined to be similar to those in humans. With all these features, pigs are considered a suitable laboratory animal for late-stage fetal urinary obstruction. 5.6

1.3. Ureteric obstruction methods
Ureteral obstruction can be done in two ways in experimental studies. Complete or partial obstruction. In case of complete obstruction, ligation or clipping of the ureter is required. In partial obstruction, the ureter can be buried in the psoas muscle or enclosed in a catheter, as described by Ulm and Miller. In the process of embedding the ureter into the psoas muscle, the ureter is explored transperitoneally and after the ureter segment is buried into the psoas muscle, the psoas muscle is closed with sutures over the ureter. 7.8

In most of the children operated on due to pelviureteric junction obstruction, it has been shown that the obstruction is functional and the ureteral lumen is open, as a result of examination of the ureteropelvic junction in the samples taken during the surgery. For this reason, partial obstruction model instead of complete obstruction should be preferred in experimental studies on the pathophysiology of PIU obstruction. In partial obstruction methods, varying results can be obtained depending on the degree of obstruction and the method chosen. However, not all studies in the partial obstruction model result in progressive hydronephrosis or progressive renal impairment. The compliance of the renal pelvis functions here as a protective mechanism for the kidney.

Another problem waiting to be solved in the clinic is to answer the question to what extent kidney functions improve after the obstruction is corrected. In this regard, there is a multifactorial interaction that we tried to mention above. Studies continue to search for the answer to this question. 9
1.4. Diagnostic procedures

Tests used for the diagnosis of obstruction in clinical and experimental studies are pressure-flow study and diuretic renography. Since obstruction cannot be precisely determined physiologically, it is difficult to decide which test is the ideal method. In addition, diagnostic procedures such as ultrasonography used in the clinic can also be used in experimental studies when it is desired to obtain information about the kidney structure and dimensions. Pressure-flow study

Pressure-flow study, also known as the Whitaker test, came into use for diagnostic purposes in hydronephrosis in 1973. Since then, it has been used in both experimental and clinical studies. The basis of this test is the need for higher pressures to transport urine in case of obstruction and its determination. Measurement of high pressure in the renal pelvis is an indicator of obstruction. At the same time, measuring the intra-bladder pressure by catheterizing the bladder via the urethral route allows the diagnosis of obstruction in the lower urinary system. 10,11
1.4.2. Diuretic renography

Diuretic renography has been used as a safe and noninvasive method in the diagnosis of obstruction in hydronephrosis since the 1970s. Technically, it is based on the principle of monitoring the passage of the radioisotope through the urinary system using a computerized gamma camera after radioisotope injection. Although it is essentially a physiological method, its accuracy depends on some biological factors. Normal kidney functions and adequate glomerular and tubular functions are important factors affecting the accuracy of this test. These tests, which are used in the clinic, can also be used easily and safely in experimental animals with urinary obstruction and in experimental studies. Ultrasonography, Computed Tomography and Magnetic Resonance Imaging

Information can be obtained about kidney dimensions, degree of hydronephrosis, renal blood flow, evaluation of the contralateral kidney, ureteral dilatation and bladder status. 12
1.5. Fetal obstruction methods

The majority of hydronephrosis seen in children is hydronephrosis that begins prenatally and continues in the postnatal period. Studies on the course of congenital hydronephrosis, effective elements in treatment, and the kidney damage it will cause continue today. Fetal models are being created to explain this issue, and the results of these studies determine clinical applications. The response of the fetal kidney to obstruction differs from obstruction after birth in many ways. Since the fetal kidney is in the process of development, it is more sensitive to obstruction, but it recovers more quickly when the obstruction is eliminated. In the developing kidney, nephrons form intrauterine 11-12. It begins to function from the 34th week onwards, and nephronogenesis continues until the 34th week. In cases of renal agenesis or obstructive events, lung development is also affected as a sequela of oligohydramnios, and in severe cases, pulmonary hypoplasia can lead to consequences that are incompatible with life. In case of unilateral obstruction, situations such as the location, time and duration of obstruction, the development of compensatory hypertrophy in the contralateral kidney, the different intrauterine hormonal environment of the fetal kidney compared to the postnatal period, the presence and function of the placenta are issues that should always be considered and taken into account in experimental fetal urinary obstruction models. We see that many experimental studies have been conducted on the effect of obstruction on the developing kidney and the effectiveness of inutero interventions to correct the damage caused by obstruction.

Rabbit, sheep, pig fetuses and chick embryos are the most commonly used laboratory animals in the experimental fetal urinary obstruction model. Obstruction can occur in the ureter or bladder neck.

Unilateral ureteral ligation in rabbit fetuses in the third trimester of pregnancy was first performed by Thomasson et al. 7, but no dysplastic changes were detected. The obstruction in the later stages of pregnancy was probably effective here. In Beck's study13, renal obstruction caused by obstruction in the early stages of pregnancy was reported. changes were investigated. Beck performed ureteral ligation in sheep fetuses at the beginning of the second trimester - corresponding to the 60th day of pregnancy - and investigated the possible changes in two groups of fetuses. Dysplastic findings were detected in the kidneys in ureteral ligation performed before the 70th day of pregnancy, and hydronephrotic findings were detected in ligation performed after the 70th day of pregnancy. Later studies also supported Beck's findings. 14.15 In fetal obstruction models, early obstruction is more effective on kidney development.

In addition to studies investigating the effects of bladder outlet obstruction and congenital hydronephrosis on kidney development, studies investigating the effectiveness of inutero corrective procedures in the period after obstruction occurs We see that a lot of work has been done. Harrison et al. 16 are one of the teams working on this issue. In this regard, different experimental applications have been carried out such as urethra ligation, urachus ligation with urethra, urachus ligation and intermittent urethra ligation, urethra and urachus ligation together with vesicoamniotic decompression. In all these studies, it has been shown that obstruction in the early period is more effective on renal dysplasia, but no dysplastic findings were detected in obstruction in the late period. In their urinary obstruction methods (bladder outlet obstruction and urachus ligation, bilateral ureteral ligation, unilateral ureteral ligation) in 55-60 days old sheep fetuses, Peter et al.17 showed that renal dysplasia developed in early obstruction. In the study of Harrison et al. regarding the results of prenatal removal of the obstruction created in utero, it was shown that kidney functions and glomerular numbers were preserved by removing the obstruction.

Peters and Mandell18 made the following conclusions as a result of their literature review on experimental congenital obstructive uropathy:
1. Although generalizing the results of our experimental studies on animals to humans is the most criticized issue, it is an inevitable necessity. However, in such inferences, there is always a difference between humans and animals. should be taken into consideration. In the study, the experimental animal that is anatomically, physiologically and biologically closest to humans should be preferred.

2. In rabbits, kidney development continues for another two weeks after birth, while in humans, glomerulogenesis is completed by the 34th week of gestation. A rabbit exposed to the external environment after birth There are many differences between the offspring and the fetus in the intrauterine environment before birth, and this should be taken into account in experimental studies.
3. The kidney development of sheep follows a process similar to that of humans, but the sheep kidney structure is different from that of humans and responds to trauma differently. Lung alveolus development occurs faster in sheep, but both lung and kidney development is slower in rabbits.

4. The time to create obstruction is also important. 16–18 in humans. It is known that obstructive lesions develop together with oligohydramnios between weeks and weeks. This time is 55–65 in the bay. days, and in rabbits it corresponds to the 18th day. The obstruction that will be created after these days is not compatible with the human period in terms of renal dysplasia and pulmonary hypoplasia.
5. In experimental studies, obstruction occurs suddenly, whereas in humans this condition probably starts slowly and has a progressive course. The search for a method continues in this regard.
Despite all these controversial situations, the partial congenital obstruction method will continue to be used as an important model to explain the etiopathogenesis of congenital hydronephrosis in humans.