Saturday, July 14, 2012

DILATATION OF THE UPPER URINARY TRACT (URETEROPELVIC JUNCTION AND URETEROVESICAL JUNCTION OBSTRUCTION) part one


Background
Dilatation of the upper urinary tract still presents a significant clinical challenge in determining which patient may gain benefit by therapy.

A- Ureteropelvic junction (UPJ) obstruction is defined as impaired urine flow from the pelvis into the proximal ureter with subsequent dilatation of the collecting system and the potential to damage the kidney. It is the most common cause of neonatal hydronephrosis. It has an overall incidence of 1:1500 and a ratio of males to females of 2:1 in newborns.

B- Ureterovesical junction (UVJ) obstruction is an obstructive condition of the distal ureter as it enters the bladder, commonly called a primary obstructive megaureter. Megaureters are ranked as second in the differential diagnosis of neonatal hydronephrosis. They occur more often in males and are more likely to occur on the left side.

Much more difficult is the definition of obstruction. Creating a divide between ‘obstructed’ and ‘nonobstructed’ urinary tracts, as if entities could be as clearly differentiated as ‘black’ and ‘white’, is impossible.
 Currently, the most popular definition is that obstruction represents any restriction to urinary outflow that, if left untreated, will cause progressive renal deterioration.

Diagnosis
Due to the widespread use of ultrasonography during pregnancy, antenatal hydronephrosis is being detected with increasing frequency. The challenge in the management of dilated upper urinary tracts is to decide which child can be observed, which one should be managed medically, and which one requires surgical intervention. There is no single definitive test able to distinguish obstructive from non-obstructive cases (Figure 1).


Figure 1: Diagnostic algorithm for dilatation of the upper urinary tract

                                       Postnatal ultrasound
Dilatation (uni- or bilateral)                                         No dilatation
Voiding cystourethrogram (VCUG)*         Repeat ultrasound after 4 weeks
Diuretic renography

* A diagnostic work-up including VCUG has to be discussed with the parents since a possibly detected reflux might have absolutely no clinical impact. On the other hand, a reflux rate of up to 25% in cases of prenatally detected and postnatally confirmed hydronephrosis is reported in the literature and might therefore have some forensic impact as well.

Definition:  ANH as an anteroposterior diameter (APD) of the renal pelvis greater than 5 mm but noted the lack of consensus on the definition of ANH (Scott and Renwick, 1993, 1999; Scott et al, 1995). One of the disadvantages of the APD can be the failure to describe pelvic configuration, calyceal dilation, and the laterality of findings, which should be included. The APD can be affected by gestational age, hydration status of the mother, bladder hypertonicity, and degree of bladder distention. Because the dimensions of the renal pelvis may normally increase with gestational age, most investigators have adjusted threshold APD values for early and later gestational age. Unfortunately, a simple threshold APD value that separates normal from abnormal does not exist, because even severe cases of ANH have the potential to resolve without incident, whereas mild degrees of ANH have the potential to progress (Pates and Dashe, 2006).

Varying the minimal APD threshold can significantly alter the specificity and sensitivity of APD as a measure of ANH  and postnatal pathology. To date there is no consensus on the optimal APD threshold for determining the need for postnatal follow-up.

An APD cutoff of 15 mm for determining obstruction yielded a postnatal sensitivity of 73% and specificity of 82% (Coplen et al, 2006). A late gestational age APD cutoff of 10 mm would detect aproximately 23% of abnormal kidneys, whereas a cutoff of 7 mm detected 68% (Ismaili et al, 2003). One large systematic review estimated that only 11.9% of total pathology presented with late gestational age APD less than 9 mm, whereas 39% of total pathology was noted at APD levels less than 15 mm (Lee et al, 2006).

What appears certain is that lower cutoffs will be more sensitive in detecting postnatal pathology but will incur a higher false-positive rate.


Categorizing ANH by APD
There is near agreement that APD greater than 15 mm represents severe or significant hydronephrosis, and some would also agree that a lower threshold of 4 to 5 mm is an appropriate value for considering APD to be abnormal (Feldman et al, 2001; Ahmad and Green, 2005; Wollenberg et al, 2005; Lee et al, 2006; Coelho et al, 2007, 2008). Taking these limitations into consideration, we define ANH in the second and third trimesters using APD thresholds for which the best available evidence provides prognostic information; along with these definitions the estimated distribution of the ANH based on the previous defined definition of APD is outlined in Table 1

 Definition of Antenatal Hydronephrosis by Anterior Posterior Diameter and Estimated Severity Percentage Range
DEGREE   
                     Anteroposterior Diameter
SEVERITY
Second Trimester     
Third Trimester
Mild 
4 to <7 mm               
4 to <9 mm               
56.7%-88%
Moderate
7 to 10 mm             
9 to 15 mm             
10.2%-29.8%
Severe
>10 mm             
>15 mm                    
1.5%-13.4%

Table 1 Data from Feldman et al, 2001; Ahmad and Green, 2005; Wollenberg et al, 2005; Lee et al, 2006; and Coelho et al, 2007, 2008.


1- Antenatal ultrasound
Usually between the 16th and 18th week of pregnancy, the kidneys are visualised routinely, when almost all amniotic fluid consists of urine. The most sensitive time for foetal urinary tract evaluation is the 28th week. If dilatation is detected, ultrasound should focus on the laterality, severity of dilatation, and echogenicity of the kidneys, hydronephrosis or hydro-ureteronephrosis, bladder volume and bladder emptying, sex of the child,and amniotic fluid volume, respectively (Figure 2, 3).

Figure 2 Ultrasound appearance of normal fetal kidney with echolucent medullary pyramids distinguishable from the more echogenic cortical parenchyma. The cortical parenchyma should be of lower echogenicity than adjacent liver or spleen.

Figure 3 Severe fetal hydronephrosis with diffuse calyceal dilation arrayed around the markedly dilated renal pelvis. The renal parenchyma is stretched over the dilated collecting system, but this does not mean loss of functional potential. Corticomedullary differentiation is difficult to see in this configuration.
2- Postnatal ultrasound
Since transitory neonatal dehydration lasts about 48 hours, imaging should be performed after this period of postnatal oliguria. In severe cases (bilateral dilatation, solitary kidney, oligohydramnios), immediate postnatal sonography is recommended. During ultrasound examination, the antero-posterior diameter of the renal pelvis, calyceal dilatation, kidney size, thickness of the parenchyma, cortical echogenicity, ureters, bladder wall and residual urine are assessed.

 3- Voiding cystourethrogram (VCUG)
In newborns with identified upper urinary tract dilatation, the presence of primary or important associated factors that must be detected include vesicoureteral reflux in up to 25% of affected children, urethral valves, ureteroceles, diverticula and neurogenic bladder. Conventional VCUG is the method of choice for primary diagnostic procedures.

 4- Diuretic renography
Diuretic renography is the most commonly used diagnostic tool to detect the severity and functional significance of urine transport problems. 99mTc-MAG3 is the radionuclide of choice. It is important to perform the study under standardized circumstances (hydration, transurethral catheter) between the fourth and sixth weeks of life.
Oral fluid intake is encouraged prior to examination. Fifteen minutes before injection of the radionuclide, it is mandatory to give normal saline intravenous infusion at a rate of 15 ml/kg over 30 minutes and then at a maintenance rate of 4 ml/kg/hour throughout the whole time of the investigation. The recommended dose of furosemide is 1 mg/kg for infants during the first year of life, while 0.5 mg/kg should be given to children aged 1 to 16 years up to a maximum dose of 40 mg.







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