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The Sonographic Diagnosis of Infravesical Obstruction in Children: Evaluation of Bladder Wall Thickness Indexed to Bladder Filling
~22-5347/97/1573-0989$03.00/0 JOURNAL Of UROLGGY Copyright 0 1997 by AMERICAN UKOI.O(;ICAL ASSOCIATION. INC.
Vol. 157,989-991,March 1997 Printed in U.S.A.
THE SONOGRAPHIC DIAGNOSIS OF INFRAWSICAL OBSTRUCTION IN CHILDREN: EVALUATION OF BLADDER WALL THICKNESS INDEXED TO BLADDER FILLING ALAN B. RETIK
MARTIN KAEFER, CAROL BARNEWOLT,
AND
CRAIG A. PETERS
From the Division of Urology and Department of Radiology, Children’s Hospital, Boston, Massachusetts
ABSTRACT
Purpose: The pathophysiological changes that result from posterior urethral valves depend primarily on the degree and duration of infravesical obstruction. Delayed diagnosis may result in progressive bladder dysfunction, renal parenchymal loss or sepsis. Specific radiographic and clinical criteria predictive of a diagnosis of infravesical obstruction are available in the form of voiding cystourethrography and cystoscopy. However, these modalities share the disadvantage of being invasive in nature. Although noninvasive, sonographic evaluation of bladder thickness has a high degree of variability and it markedly depends on the degree of bladder filling. Materials and Methods: Bladder wall thickness and bladder diameter were assessed in 15 children with posterior urethral valves, and 10 age and sex matched children with no bladder dysfunction. Ultrasound was performed before intervention (that is catheter placement or valve ablation). Results: A positive predictive value of 93% was achieved between the 2 groups when bladder wall thickness was indexed to inner wall diameter in the anteroposterior or transverse dimension. Mean bladder wall thickness divided by mean inner bladder diameter plus or minus standard deviation was 0.402 2 0.25in the obstructed population and 0.094 -+ 0.025 in controls ( p = 0.0004). Conclusions: The bladder thickness index is a sensitive sonographic predictor of infravesical obstruction. Application of this index as a noninvasive screening tool for the patient with persistent voiding dysfunction may prove beneficial for identifylng infravesical pathology. KEYWORDS: bladder, ultrasonography, urethra Congenital infravesical obstruction leads to significant alterations in bladder structure and functi0n.l Posterior urethral valves represent the most common anatomical form of bladder outlet obstruction which, if not diagnosed in a timely fashion, may lead to continued irreversible injury to the bladder and kidneys.2 The presence of a structural infravesical obstruction is readily established by voiding cystourethrography and cystoscopy. However, situations exist in which one may prefer to avoid invasive testing (for example in a boy with daytime wetting, irritative or obstructive voiding symptoms, or persistent enuresis after age 5 years in whom outlet obstruction is deemed unlikely) if information could be obtained from a noninvasive screening tool. Therefore, we sought to identify an alternative radiographic test to help to predict the presence or absence of infravesical Obstruction. Ultrasound, which provides prognostic information in posterior urethral valves regarding renal f ~ n c t i o n also , ~ provides excellent imaging of the bladder and bladder base.4 Detrusor hypertrophy, the sequelae of increased outlet resistance, is a logical variable to consider when seeking a predictor of bladder obstruction.5 Muscle hypertrophy is related to the amount of work performed by the organ under study and it depends on a number of factors. The degree of obstruction, which occurs in a spectrum of seventy, may be the most significant determinant of bladder response.6 The degree of bladder response to obstruction also depends on the presence or absence of pop-off valves (for example high grade vesicoureteral reflw, urinoma or diverticulum).’ Although obstruction results in detrusor hypertrophy, the simple measurement of bladder wall thickness is not an Accepted for publication September 27,1996.
adequate predictor of bladder outlet pathology because it is highly dependent on the degree of bladder filling.4Therefore, we chose to evaluate the relationship between these 2 variables as it relates to obstructed and nonobstructed states. By indexing bladder thickness to bladder filling, we provide a sensitive and specific predictor of bladder outlet obstruction. METHODS
This study included 15 boys with posterior urethral valves, and 10 controls with ureteropelvic junction obstruction and normal bladder anatomy on voiding cystourethrography.Presenting signs and symptoms included urinary tract infection, abdominal distension or hydronephrosis on a prenatal sonogram. All sonographic examinations at our institution were performed using an Acuson 128XP/10 with a 5 MHz. transducer. Ultrasound was performed in all patients before correcting the underlying pathology and all had documented negative urine cultures. Posterior urethral valves were subsequently confirmed by voiding cystourethrographyand cystoscopy. Scans were excluded when there was inadequate resolution of the bladder wall or absence of the entire bladder image on a single frame. Images were evaluated with the examiner blinded to the underlying diagnosis. Measurements were obtained of bladder diameter (anteroposterior and transverse) and wall thickness (dome,floor and 2 lateral walls, fig. 1).Statistical analysis was done using a computer software package with Welch‘s alternate t test for distributions with unequal variances. RESULTS
Separation of obstructed from nonobstructed bladders waa tested using various proportions (table 1).Indexing bladder
989
SONOGRAPHIC DIAGNOSIS OF INFRAVESICAL OBSTRUCTION
990
A
1 0 X
B
C
D
Y
b FIG. 1. a , transverse (TR) sonographic image of bladder (BL).b, measurements of bladder wall thickness and diameter TABLE1. Sensitivity, specificity and predictive value
of
various
indexes A + B ~-
C + D Y
(A+B+C+DV4 ( x + Y)B
93 90 93
100 90 94
~
X
Sensitivity
86 90 P;edictivi value 93 Values are reported as percentages. Specificity
populations are shown in table 2. This index provides better data separation than either of the simple 1-dimensional measurements. Analysis using Welch’s alternate t test demonstrated a statistically significant difference between the 2 groups (p = 0.0004). DISCUSSION
Ultrasound has proved invaluable for assessing congenital obstructive uropathies prenatally and postnatally.* This technology is effective for evaluating post-void residual urine wall thickness to diameter in the anteroposterior dimension in these children.8-9Many others have also sought t o identify allowed a reasonable level of sensitivity and specificity. Com- ultrasonographic variables predictive of obstruction. Identiparisons using the transverse dimension provided a point at fication of a distended bladder is not specific and the obwhich slight improvement in sensitivity was achieved. Com- structed bladder is not always distended. As a single varibining these 2 dimensions by indexing average bladder wall able, bladder distension cannot reliably differentiate thickness to average diameter provided a measurement that between posterior urethral valves and other nonobstructive we designated the bladder thickness index. By adjusting the conditions (for example the megalocystis-megaureter associthreshold value for the bladder thickness index, a cutoff was ation).10 A dilated posterior urethra may also suggest infound (bladder thickness index value of 0.13)at which 100% fravesical obstruction. However, posterior urethral dilatation sensitivity was achieved without compromising specificity depends on flow across the obstruction. Posterior urethral (fig. 2). Mean bladder thickness indexes for the 2 patient diameter may also fail to differentiate between simple dilatation and true obstruction (for example the prune-belly syndrome).11 A posterior ledge at the bladder neck has been found to correlate with functional bladder neck obstruction in 10 adults. Using a transrectal probe Perkash and Friedland demonstrated a long ledge in 87% of adults with detrusorn sphincter dyssynergia, which resolved in the majority after X sphincterotomy.12 This ultrasonographic finding has not 7.5 been reported in the pediatric population. Gilsanz et a1 idenY tified increased bladder wall thickness on ultrasound in X many patients with bladder outlet obstruction.13 However, 0) U this variable of bladder wall thickness depends on the state of bladder filling.4 5.0 L The inverse relationship between bladder radius and wall 0) thickness led us to evaluate an index of these 2 variables as U a predictor of obstruction (fig. 3). The bladder thickness inL dex, which includes 2 measurements of diameter (anteropos2.5 tenor and transverse) and 4 of wall thickness (dome, base and 2 lateral walls) was ultimately chosen for assessment.
or
-
=
3 m
TABLE 2. Bladder thickness indew (average wall thicknesslaverage inner diameter) in posterior urethral valve and control populations
0
UPJ
PUV
RG.2. Scattergram of bladder thickness index values relative to
diagnosis (obstructedversus nonobstntctedl. Transverse bars represent mean values. UPJ, ureteropelvicjunction. P W , posterior urethral valves.
Mean 2 SD 958 Confidence intervals: Ma%. Min.
Control
Posterior Urethral Valves
0.94 2 0.25
4.02 2 2.55
1.12
5.44
0.75
2.61
SONOGRAPHIC DIAGNOSIS OF INFRAVESICAL OBSTRUCTION
991
with a range of bladder volumes attests to the practical value of this index.
:
8
CONCLUSIONS
We demonstrate that ultrasonographic assessment of bladder wall thickness indexed to bladder volume is a sensitive 0.2 tool for differentiating between the presence and absence of 0 anatomical infravesical obstruction. optimal use of this meas0 1 2 3 urement w i l l be determined afier applying this index in a prospective fashion. Only by evaluating each individual at Radius several states of bladder filling will we be able to determine FIG. 3. Bladder wall thickness (x) as function of radius (R) the range of filling for which the bladder thickness index can according to formula, x = & ’- R. optimally predict obstruction.
E
0.4
:
REFERENCES
Each measurement is included, since the bladder is not a perfect sphere and wall thickness cannot be assumed to be equal throughout.8 For the purpose of this pilot study a group of children with clearly defined anatomical obstruction was compared to controls with documented normal bladder outlets. Evaluation was performed in the absence of infection, since inflammation may result in bladder wall thickening. The bladder thickness index was applied before correcting underlying pathology, since relief of obstruction can alter bladder architecture. Using this index we identified a cutoff a t which 100% sensitivity was attained while maintaining 90% specificity. Recently Close et a1 demonstrated predictable resolution of bladder wall thickness following initial valve ablation.14 In patients who have incomplete ablation of posterior urethral valves bladder thickness may be less likely to resolve. Therefore, the bladder thickness index may also prove valuable for assessing the adequacy of initial valve fulguration and need for repeat intervention. There are other forms of bladder outlet obstruction in children. Obstructive uropathy exists in the child with neurogenic bladder dysfunction associated with spinal dysraphism. Although the majority of these children c a n be identified by physical examination, a significant number may have occult forms with significant implications for bladder function.16 These patients are often initially identified when detrusor-sphincter dyssynergia manifests as enuresis. This condition is clearly an important subset of infravesical obstruction that must be identified by the bladder thickness index to make its use appropriate as a screening tool. Limitations of the bladder index may exist. We have observed that images of the bladder obtained during voiding yield thicker wall measurements than those obtained just before or aRer voiding. The reason for this observation is not yet clear. To prevent the false impression of wall thickening, measurements should be obtained in the resting state. By performing this study in a retrospective fashion, we were also limited to bladder volumes at which the images were obtained. The bladder thickness index may prove most reliable within a given range of 6lling. However, p m c t i d y speaking the fact that statistical significance was reached using scans
1. Bauer, S. B., Dieppa, R. A., Labib, K. K. and Retik, A. B.: The bladder in boys with posterior urethral valves: a d y n a m i c assessment. J. Urol., 121: 769,1979. 2. Seeds,J. W.,Mittelstaedt, C. A and Mandell, J.: Pre- and postnatal ultrasonographic diagnosis of congenital obstructive uropathies. Urol. Clin. N. h e r . , lS 131,1986. 3. Hulbrt,W.C.,Rosenberg, H. K, Cartmight, P. C., Duckett, J. W.and Snyder, H. M.: The predictive value of ultrasonography in evaluation of infanta with posterior urethral valves. J. Urol., 148:122,1992. 4. Jequier, S.and Rousseau, 0.: Sonographic measurements of the normal bladder wall in children. AJR, 149 563,1987. 5. Peters, C. A: Congenital bladder obstruction: research strategies and directions. Adv. Exp. Med. Biol., 38k 117,1995. 6. Hendren, W. H.: Posterior urethral valves in boys. A broad clinical spectrum. J. Urol., 10g: 298,1971. 7. Kaefer, M., Keating, M. A, Adams, M. C. and Rink,R. C.: Posterior urethral valves, pressure pop-offs and bladder function. J. Uml., part 2, 154: 708, 1995. 8. Beacock, C. J. M., Roberts, E.E.,Rees, R.W.M. and Buck, A C.: Ultrasound assessment of residual urine: a quantitative method. Brit. J. Urol., S f i 410,1985. 9. Cruz-Orive, L. M.and Roberta,N.:Unbiased volume estimation with coaxial sections: an application to the human bladder. J. Micmac., 170: 25,1993. 10. Mandell, J., Lebowitz, R. L.,Peters, C. A, Estroff, J. A, Retilt, A B. and Benacerraf, B. R.: Prenatal diagnosis of the megacystis-megaureter association. J. Urol., 148.1487,1992. 11. McAlister, W.H.: Demonstration of the dilated prostatic urethra in posterior urethral valve patients. J. Ultrasound Med., 3: 189,1984. 12. Perkash, I. and Friedland, G. W.:Posterior ledge at the bladder neck crucial diarmostic role of dtrasonozraphy. - - _ Urol. Rad., 8: 175,1986. 13. Gilsanz, V., Miller, J. H. and Reid, B. S.: Ultrasonic characteristics of posterior urethral valves. Radiology, . * l 143,1982. 14. Close, C. E.,Cam, M. C., Burns, M. W.and Mitehell, M. E.: Lower urinary tract changes after early valve ablation in neonates and infants: is early diversion warranted? J. Urol., part 2,156:3134 abstract 9,1996. 15. Keating, M.A,Rink,R. C., Bauer, S.B., Krarup, C., Dyro, F.M., Winaton,K R., S W t o , J., Fiacher. E. G. and Retilt, A B.: Neurourological implications of the changing approach in management of occult spinal lesions. J. Urol., part 2. 140: 1299. 1988.
-
Vol. 157,989-991,March 1997 Printed in U.S.A.
THE SONOGRAPHIC DIAGNOSIS OF INFRAWSICAL OBSTRUCTION IN CHILDREN: EVALUATION OF BLADDER WALL THICKNESS INDEXED TO BLADDER FILLING ALAN B. RETIK
MARTIN KAEFER, CAROL BARNEWOLT,
AND
CRAIG A. PETERS
From the Division of Urology and Department of Radiology, Children’s Hospital, Boston, Massachusetts
ABSTRACT
Purpose: The pathophysiological changes that result from posterior urethral valves depend primarily on the degree and duration of infravesical obstruction. Delayed diagnosis may result in progressive bladder dysfunction, renal parenchymal loss or sepsis. Specific radiographic and clinical criteria predictive of a diagnosis of infravesical obstruction are available in the form of voiding cystourethrography and cystoscopy. However, these modalities share the disadvantage of being invasive in nature. Although noninvasive, sonographic evaluation of bladder thickness has a high degree of variability and it markedly depends on the degree of bladder filling. Materials and Methods: Bladder wall thickness and bladder diameter were assessed in 15 children with posterior urethral valves, and 10 age and sex matched children with no bladder dysfunction. Ultrasound was performed before intervention (that is catheter placement or valve ablation). Results: A positive predictive value of 93% was achieved between the 2 groups when bladder wall thickness was indexed to inner wall diameter in the anteroposterior or transverse dimension. Mean bladder wall thickness divided by mean inner bladder diameter plus or minus standard deviation was 0.402 2 0.25in the obstructed population and 0.094 -+ 0.025 in controls ( p = 0.0004). Conclusions: The bladder thickness index is a sensitive sonographic predictor of infravesical obstruction. Application of this index as a noninvasive screening tool for the patient with persistent voiding dysfunction may prove beneficial for identifylng infravesical pathology. KEYWORDS: bladder, ultrasonography, urethra Congenital infravesical obstruction leads to significant alterations in bladder structure and functi0n.l Posterior urethral valves represent the most common anatomical form of bladder outlet obstruction which, if not diagnosed in a timely fashion, may lead to continued irreversible injury to the bladder and kidneys.2 The presence of a structural infravesical obstruction is readily established by voiding cystourethrography and cystoscopy. However, situations exist in which one may prefer to avoid invasive testing (for example in a boy with daytime wetting, irritative or obstructive voiding symptoms, or persistent enuresis after age 5 years in whom outlet obstruction is deemed unlikely) if information could be obtained from a noninvasive screening tool. Therefore, we sought to identify an alternative radiographic test to help to predict the presence or absence of infravesical Obstruction. Ultrasound, which provides prognostic information in posterior urethral valves regarding renal f ~ n c t i o n also , ~ provides excellent imaging of the bladder and bladder base.4 Detrusor hypertrophy, the sequelae of increased outlet resistance, is a logical variable to consider when seeking a predictor of bladder obstruction.5 Muscle hypertrophy is related to the amount of work performed by the organ under study and it depends on a number of factors. The degree of obstruction, which occurs in a spectrum of seventy, may be the most significant determinant of bladder response.6 The degree of bladder response to obstruction also depends on the presence or absence of pop-off valves (for example high grade vesicoureteral reflw, urinoma or diverticulum).’ Although obstruction results in detrusor hypertrophy, the simple measurement of bladder wall thickness is not an Accepted for publication September 27,1996.
adequate predictor of bladder outlet pathology because it is highly dependent on the degree of bladder filling.4Therefore, we chose to evaluate the relationship between these 2 variables as it relates to obstructed and nonobstructed states. By indexing bladder thickness to bladder filling, we provide a sensitive and specific predictor of bladder outlet obstruction. METHODS
This study included 15 boys with posterior urethral valves, and 10 controls with ureteropelvic junction obstruction and normal bladder anatomy on voiding cystourethrography.Presenting signs and symptoms included urinary tract infection, abdominal distension or hydronephrosis on a prenatal sonogram. All sonographic examinations at our institution were performed using an Acuson 128XP/10 with a 5 MHz. transducer. Ultrasound was performed in all patients before correcting the underlying pathology and all had documented negative urine cultures. Posterior urethral valves were subsequently confirmed by voiding cystourethrographyand cystoscopy. Scans were excluded when there was inadequate resolution of the bladder wall or absence of the entire bladder image on a single frame. Images were evaluated with the examiner blinded to the underlying diagnosis. Measurements were obtained of bladder diameter (anteroposterior and transverse) and wall thickness (dome,floor and 2 lateral walls, fig. 1).Statistical analysis was done using a computer software package with Welch‘s alternate t test for distributions with unequal variances. RESULTS
Separation of obstructed from nonobstructed bladders waa tested using various proportions (table 1).Indexing bladder
989
SONOGRAPHIC DIAGNOSIS OF INFRAVESICAL OBSTRUCTION
990
A
1 0 X
B
C
D
Y
b FIG. 1. a , transverse (TR) sonographic image of bladder (BL).b, measurements of bladder wall thickness and diameter TABLE1. Sensitivity, specificity and predictive value
of
various
indexes A + B ~-
C + D Y
(A+B+C+DV4 ( x + Y)B
93 90 93
100 90 94
~
X
Sensitivity
86 90 P;edictivi value 93 Values are reported as percentages. Specificity
populations are shown in table 2. This index provides better data separation than either of the simple 1-dimensional measurements. Analysis using Welch’s alternate t test demonstrated a statistically significant difference between the 2 groups (p = 0.0004). DISCUSSION
Ultrasound has proved invaluable for assessing congenital obstructive uropathies prenatally and postnatally.* This technology is effective for evaluating post-void residual urine wall thickness to diameter in the anteroposterior dimension in these children.8-9Many others have also sought t o identify allowed a reasonable level of sensitivity and specificity. Com- ultrasonographic variables predictive of obstruction. Identiparisons using the transverse dimension provided a point at fication of a distended bladder is not specific and the obwhich slight improvement in sensitivity was achieved. Com- structed bladder is not always distended. As a single varibining these 2 dimensions by indexing average bladder wall able, bladder distension cannot reliably differentiate thickness to average diameter provided a measurement that between posterior urethral valves and other nonobstructive we designated the bladder thickness index. By adjusting the conditions (for example the megalocystis-megaureter associthreshold value for the bladder thickness index, a cutoff was ation).10 A dilated posterior urethra may also suggest infound (bladder thickness index value of 0.13)at which 100% fravesical obstruction. However, posterior urethral dilatation sensitivity was achieved without compromising specificity depends on flow across the obstruction. Posterior urethral (fig. 2). Mean bladder thickness indexes for the 2 patient diameter may also fail to differentiate between simple dilatation and true obstruction (for example the prune-belly syndrome).11 A posterior ledge at the bladder neck has been found to correlate with functional bladder neck obstruction in 10 adults. Using a transrectal probe Perkash and Friedland demonstrated a long ledge in 87% of adults with detrusorn sphincter dyssynergia, which resolved in the majority after X sphincterotomy.12 This ultrasonographic finding has not 7.5 been reported in the pediatric population. Gilsanz et a1 idenY tified increased bladder wall thickness on ultrasound in X many patients with bladder outlet obstruction.13 However, 0) U this variable of bladder wall thickness depends on the state of bladder filling.4 5.0 L The inverse relationship between bladder radius and wall 0) thickness led us to evaluate an index of these 2 variables as U a predictor of obstruction (fig. 3). The bladder thickness inL dex, which includes 2 measurements of diameter (anteropos2.5 tenor and transverse) and 4 of wall thickness (dome, base and 2 lateral walls) was ultimately chosen for assessment.
or
-
=
3 m
TABLE 2. Bladder thickness indew (average wall thicknesslaverage inner diameter) in posterior urethral valve and control populations
0
UPJ
PUV
RG.2. Scattergram of bladder thickness index values relative to
diagnosis (obstructedversus nonobstntctedl. Transverse bars represent mean values. UPJ, ureteropelvicjunction. P W , posterior urethral valves.
Mean 2 SD 958 Confidence intervals: Ma%. Min.
Control
Posterior Urethral Valves
0.94 2 0.25
4.02 2 2.55
1.12
5.44
0.75
2.61
SONOGRAPHIC DIAGNOSIS OF INFRAVESICAL OBSTRUCTION
991
with a range of bladder volumes attests to the practical value of this index.
:
8
CONCLUSIONS
We demonstrate that ultrasonographic assessment of bladder wall thickness indexed to bladder volume is a sensitive 0.2 tool for differentiating between the presence and absence of 0 anatomical infravesical obstruction. optimal use of this meas0 1 2 3 urement w i l l be determined afier applying this index in a prospective fashion. Only by evaluating each individual at Radius several states of bladder filling will we be able to determine FIG. 3. Bladder wall thickness (x) as function of radius (R) the range of filling for which the bladder thickness index can according to formula, x = & ’- R. optimally predict obstruction.
E
0.4
:
REFERENCES
Each measurement is included, since the bladder is not a perfect sphere and wall thickness cannot be assumed to be equal throughout.8 For the purpose of this pilot study a group of children with clearly defined anatomical obstruction was compared to controls with documented normal bladder outlets. Evaluation was performed in the absence of infection, since inflammation may result in bladder wall thickening. The bladder thickness index was applied before correcting underlying pathology, since relief of obstruction can alter bladder architecture. Using this index we identified a cutoff a t which 100% sensitivity was attained while maintaining 90% specificity. Recently Close et a1 demonstrated predictable resolution of bladder wall thickness following initial valve ablation.14 In patients who have incomplete ablation of posterior urethral valves bladder thickness may be less likely to resolve. Therefore, the bladder thickness index may also prove valuable for assessing the adequacy of initial valve fulguration and need for repeat intervention. There are other forms of bladder outlet obstruction in children. Obstructive uropathy exists in the child with neurogenic bladder dysfunction associated with spinal dysraphism. Although the majority of these children c a n be identified by physical examination, a significant number may have occult forms with significant implications for bladder function.16 These patients are often initially identified when detrusor-sphincter dyssynergia manifests as enuresis. This condition is clearly an important subset of infravesical obstruction that must be identified by the bladder thickness index to make its use appropriate as a screening tool. Limitations of the bladder index may exist. We have observed that images of the bladder obtained during voiding yield thicker wall measurements than those obtained just before or aRer voiding. The reason for this observation is not yet clear. To prevent the false impression of wall thickening, measurements should be obtained in the resting state. By performing this study in a retrospective fashion, we were also limited to bladder volumes at which the images were obtained. The bladder thickness index may prove most reliable within a given range of 6lling. However, p m c t i d y speaking the fact that statistical significance was reached using scans
1. Bauer, S. B., Dieppa, R. A., Labib, K. K. and Retik, A. B.: The bladder in boys with posterior urethral valves: a d y n a m i c assessment. J. Urol., 121: 769,1979. 2. Seeds,J. W.,Mittelstaedt, C. A and Mandell, J.: Pre- and postnatal ultrasonographic diagnosis of congenital obstructive uropathies. Urol. Clin. N. h e r . , lS 131,1986. 3. Hulbrt,W.C.,Rosenberg, H. K, Cartmight, P. C., Duckett, J. W.and Snyder, H. M.: The predictive value of ultrasonography in evaluation of infanta with posterior urethral valves. J. Urol., 148:122,1992. 4. Jequier, S.and Rousseau, 0.: Sonographic measurements of the normal bladder wall in children. AJR, 149 563,1987. 5. Peters, C. A: Congenital bladder obstruction: research strategies and directions. Adv. Exp. Med. Biol., 38k 117,1995. 6. Hendren, W. H.: Posterior urethral valves in boys. A broad clinical spectrum. J. Urol., 10g: 298,1971. 7. Kaefer, M., Keating, M. A, Adams, M. C. and Rink,R. C.: Posterior urethral valves, pressure pop-offs and bladder function. J. Uml., part 2, 154: 708, 1995. 8. Beacock, C. J. M., Roberts, E.E.,Rees, R.W.M. and Buck, A C.: Ultrasound assessment of residual urine: a quantitative method. Brit. J. Urol., S f i 410,1985. 9. Cruz-Orive, L. M.and Roberta,N.:Unbiased volume estimation with coaxial sections: an application to the human bladder. J. Micmac., 170: 25,1993. 10. Mandell, J., Lebowitz, R. L.,Peters, C. A, Estroff, J. A, Retilt, A B. and Benacerraf, B. R.: Prenatal diagnosis of the megacystis-megaureter association. J. Urol., 148.1487,1992. 11. McAlister, W.H.: Demonstration of the dilated prostatic urethra in posterior urethral valve patients. J. Ultrasound Med., 3: 189,1984. 12. Perkash, I. and Friedland, G. W.:Posterior ledge at the bladder neck crucial diarmostic role of dtrasonozraphy. - - _ Urol. Rad., 8: 175,1986. 13. Gilsanz, V., Miller, J. H. and Reid, B. S.: Ultrasonic characteristics of posterior urethral valves. Radiology, . * l 143,1982. 14. Close, C. E.,Cam, M. C., Burns, M. W.and Mitehell, M. E.: Lower urinary tract changes after early valve ablation in neonates and infants: is early diversion warranted? J. Urol., part 2,156:3134 abstract 9,1996. 15. Keating, M.A,Rink,R. C., Bauer, S.B., Krarup, C., Dyro, F.M., Winaton,K R., S W t o , J., Fiacher. E. G. and Retilt, A B.: Neurourological implications of the changing approach in management of occult spinal lesions. J. Urol., part 2. 140: 1299. 1988.
-