we reviewed medical records at seoul national university children 's hospital from january 2000 to december 2004 and identified 193 infants with a diagnosis of ia that underwent surgical treatment . of these infants , 46 infants that underwent transperineal us prior to corrective surgery were included in the study population . the mean patient age at the time of diagnosis was 3.5 months ( age range , 1 day to 12 months ) . the institutional review board of seoul national university children 's hospital approved this retrospective study and waived the requirement for informed consent . transperineal us was performed by one of four board certified diagnostic radiologists using a 5 - 15 mhz linear transducer with the patient in a modified lithotomy position . a thick layer of gel was applied over the perineum to prevent artifacts due to the presence of intervening air . an us transducer was placed in the midsagittal plane through the perineum . in this plane , the pubic synchondrosis , posterior urethra , anterior rectal wall and the bladder base , and in girls , the vaginal region between the urethra and rectum could be appreciated ( fig . two radiologists with 20 and four years of clinical experience , respectively , retrospectively reviewed the us images by consensus on a 2000  2000 picture archiving and communication systems monitor ( pacs ; marotech , seoul , korea ) . during the review process , on transperineal us images , an internal fistula was defined as a hypoechoic linear tract , sometimes possessing an internal echogenic line due to the presence of intervening air within the fistula . these hypoechoic tracts course anteriorly , interrupting the echogenic fat plane between the rectum and urethra ( vagina or bladder ) ( fig . second , the shortest distance between the distal rectal pouch and the perineum was measured in each infant . a scatterplot of " pouch to perineum ( p - p ) " distances was constructed versus surgically proven ia types . the mean p - p distance and range were calculated and optimal threshold p - p distances to discriminate between a high - type ia and a low - type ia were determined using a scatterplot . third , the ia type was determined by considering the fistula type and p - p distance . the international classification of 1970 was used to match ia type and fistula ( 5 ) . in male infants , an ia with 1 ) a recto - vesical fistula or 2 ) an ia with a recto - bulbar urethral fistula was classified as an intermediate - type . in female infants , an ia with a recto - upper vaginal fistula was classified as a high - type . an ia with 1 ) a recto - vestibular fistula ( where the fistula opened at the vestibule of the vagina and the narrow fistular tract coursed cephalad up to the level of the levator ) or 2 ) a recto - lower vaginal fistula was classified as intermediate - type . an ia with 1 ) an anovestibular fistula ( a fistular opening that presented in the vestibule and where the fistula connected with the vestibule and the distal rectal pouch , located below the level of the levator ) or 2 ) an anocutaneous fistula was classified as a low - type . anal stenosis and a thickened skin - covered anus were classified as low - type ias ( fig . threshold p - p distances previously determined were used to discriminate between high - type and low - type ias . in all patients , surgical and radiological examination findings obtained using distal loopography or voiding cystourethrography were viewed as reference standards . the diagnostic accuracies of transperineal us to determine ia type were evaluated by comparing us findings with reference standards . of the 46 ia cases enrolled this study , 17 cases were confirmed as high - type ias , three cases were confirmed as intermediate - type ias and 26 cases were confirmed as low - type ias . fourteen of the 17 high - type ia cases had a recto - prostatic urethral fistula and the other three cases had a recto - vesical fistula . two cases of an intermediate - type ia had a recto - bulbar urethral fistula . nineteen cases of a low - type ia had an anovestibular fistula ( n = 11 ) or an anocutaneous fistula ( n = 8) . seven cases had no fistula , and were confirmed as having anal stenosis or an anomalous membrane obstruction . all 14 cases of recto - prostatic urethral fistulas were correctly identified by the use of transperineal us ( fig . however , fistula tracts were not demonstrated in all three recto - vesical fistula cases . of the 17 high - type fistula cases , 14 cases were correctly demonstrated ( 14 of 17 , sensitivity 82% ) . for intermediate - type ias , one of two recto - bulbar urethral fistulas 4 ) . among the 19 cases of a low - type ia with an anovestibular or anocutaneous fistula , 13 cases of an anocutaneous or anovestibular fistula were correctly identified ( 13 of 19 , sensitivity 68% ) ( fig . two cases of anocutaneous fistulas were misdiagnosed as recto - prostatic urethral fistulas due to apparent anterior beaking of the distal rectal pouch . for the other four cases of low - type ias with fistulas ( three anocutaneous fistulas and one anovestibular fistula ) , a scatterplot of p - p distances was constructed versus the ia type ( fig . the mean p - p distances for high , intermediate and low - type ias were 18.2 mm ( range , 6 - 30 mm ) , 11 mm ( range , 6 - 16 mm ) and 5.3 mm ( range , 0 - 14 mm ) , respectively . some overlap in p - p distances was observed among the ia types , but all patients with a p - p distance > 16 mm had a high type ia and all patients with a p - p distance < 5 mm had a low type ia ( fig . 6 ) . among 17 cases of high - type ia , 15 cases were correctly determined as high - type ias ( 15 of 17 , sensitivity 88% ) and 14 cases were diagnosed as high - type ias due to the identification of a recto - prostatic urethral fistula ( fig . one case of an ia with a recto - vesical fistula , where a fistula tract was not demonstrated was diagnosed as a high - type based on the p - p distance ( 19 mm ) . in the other two high - type ias with a recto - vesical fistula , no fistula tract or distal rectal pouch was demonstrated due to poor transperineal us tissue penetration , and the two cases remained undetermined . for intermediate - type ias , only one case was correctly diagnosed as an intermediate - type by demonstrating the presence of a recto - bulbar urethral fistula ( fig . the other two cases remained undetermined due to a non - visualized fistula tract and indeterminate p - p distances ( 6 mm and 11 mm , respectively ) . of the 26 cases of low - type ias , 23 cases were correctly diagnosed as low - type ias ( 23 of 26 , sensitivity 88% ) . thirteen cases of low - type ias were correctly determined by identifying anocutaneous or anovestibular fistulas ( fig . 5 ) and the other 10 cases were correctly diagnosed as low - type ias due to a short p - p distance ( < 5 mm ) , even though the fistula tract was not demonstrated on us . two cases of low - type ias with an anocutaneous fistula were misdiagnosed as high - type ias with a recto - prostatic urethral fistula . only one low - type ia was undetermined due to an invisible fistula tract and an indeterminate p - p distance ( 7 mm ) . overall , the ia type was correctly diagnosed by the use of transperineal us in 39 of the 46 study subjects ( 85% ) . of the 46 ia cases enrolled this study , 17 cases were confirmed as high - type ias , three cases were confirmed as intermediate - type ias and 26 cases were confirmed as low - type ias . fourteen of the 17 high - type ia cases had a recto - prostatic urethral fistula and the other three cases had a recto - vesical fistula . two cases of an intermediate - type ia had a recto - bulbar urethral fistula . nineteen cases of a low - type ia had an anovestibular fistula ( n = 11 ) or an anocutaneous fistula ( n = 8) . seven cases had no fistula , and were confirmed as having anal stenosis or an anomalous membrane obstruction . all 14 cases of recto - prostatic urethral fistulas were correctly identified by the use of transperineal us ( fig . however , fistula tracts were not demonstrated in all three recto - vesical fistula cases . of the 17 high - type fistula cases , 14 cases were correctly demonstrated ( 14 of 17 , sensitivity 82% ) . for intermediate - type ias , one of two recto - bulbar urethral fistulas 4 ) . among the 19 cases of a low - type ia with an anovestibular or anocutaneous fistula , 13 cases of an anocutaneous or anovestibular fistula were correctly identified ( 13 of 19 , sensitivity 68% ) ( fig . two cases of anocutaneous fistulas were misdiagnosed as recto - prostatic urethral fistulas due to apparent anterior beaking of the distal rectal pouch . for the other four cases of low - type ias with fistulas ( three anocutaneous fistulas and one anovestibular fistula ) , a scatterplot of p - p distances was constructed versus the ia type ( fig . intermediate and low - type ias were 18.2 mm ( range , 6 - 30 mm ) , 11 mm ( range , 6 - 16 mm ) and 5.3 mm ( range , 0 - 14 mm ) , respectively . some overlap in p - p distances was observed among the ia types , but all patients with a p - p distance > 16 mm had a high type ia and all patients with a p - p distance < 5 mm had a low type ia ( fig . among 17 cases of high - type ia , 15 cases were correctly determined as high - type ias ( 15 of 17 , sensitivity 88% ) and 14 cases were diagnosed as high - type ias due to the identification of a recto - prostatic urethral fistula ( fig . 3 ) . one case of an ia with a recto - vesical fistula , where a fistula tract was not demonstrated was diagnosed as a high - type based on the p - p distance ( 19 mm ) . in the other two high - type ias with a recto - vesical fistula , no fistula tract or distal rectal pouch was demonstrated due to poor transperineal us tissue penetration , and the two cases remained undetermined . for intermediate - type ias , only one case was correctly diagnosed as an intermediate - type by demonstrating the presence of a recto - bulbar urethral fistula ( fig . the other two cases remained undetermined due to a non - visualized fistula tract and indeterminate p - p distances ( 6 mm and 11 mm , respectively ) . of the 26 cases of low - type ias , 23 cases were correctly diagnosed as low - type ias ( 23 of 26 , sensitivity 88% ) . thirteen cases of low - type ias were correctly determined by identifying anocutaneous or anovestibular fistulas ( fig . 5 ) and the other 10 cases were correctly diagnosed as low - type ias due to a short p - p distance ( < 5 mm ) , even though the fistula tract was not demonstrated on us . two cases of low - type ias with an anocutaneous fistula were misdiagnosed as high - type ias with a recto - prostatic urethral fistula . only one low - type ia was undetermined due to an invisible fistula tract and an indeterminate p - p distance ( 7 mm ) . overall , the ia type was correctly diagnosed by the use of transperineal us in 39 of the 46 study subjects ( 85% ) . an ia is a relatively common anomaly , which occurs in approximately one of 500 live births , with a slight predilection for male infants ( 5 ) . an ia is classified according to the international classification of 1970 and the proposed " wingspread " modification of 1986 as a high - type , intermediate - type and low - type ( 5 , 6 ) . these classifications are based on the level of the distal rectal pouch relative to the puborectalis sling of the levator ani muscle . the differentiation of ia types is critical to determine surgical treatment and to predict continence . various radiological modalities have been used to determine the level of the distal pouch in infants with ia . the modalities include inverted lateral radiography ( invertography ) , distal colostography ( loopography ) , us , ct and mri ( 1 , 4 , 7 - 11 ) . the role of the use of a preoperative radiological modality for ia is to determine the level of the distal rectal pouch , to identify the presence and location of internal fistulas and to diagnose any associated anomalies . conventional transperineal us is both rapid and straight - forward to perform and us is not associated with any radiation hazard . previously , differentiation of low - type and high - type ias with the use of us had been indirectly performed by measurement of the distance between the distal rectal pouch and the perineum ( 1 , 4 ) . however , the use of this method has been limited by measurement overlaps between high and low - type ias and with variable cut - off points among studies . ( 1 ) have reported that a p - p distance of 1.0 cm or less is suggestive of a low - type ia and a p - p distance of 1.5 cm or greater is suggestive of a high - type ia . ( 4 ) have stated that a p - p distance of less than 1.5 cm is consistent with a low - type ia . in our study , different cut - off values were obtained and some overlap in p - p distances was observed among ia types . such discrepancies in p - p distance measurement might be caused by varying degrees of pouch distension , pressure on the perineum by the probe and may be related to the age of the infant at the time that the examination is performed . accurate preoperative identification of an internal fistula between the distal pouch and the urogenital tract is very important for optimal surgical management and prevention of a potential injury to the genitourinary tract . in addition , the ia type can be determined based on the internal fistular type . with recent improvements in us resolution , transperineal us has become an excellent diagnostic modality to define the type of the internal fistula for an ia ( 3 ) . of the 38 cases with fistulas , 28 fistulas were correctly identified by the use of transperineal us . only three recto - vesical fistulas and one recto - bulbar urethral fistula with a very high location but , of 19 anovestibular or anocutaneous fistulas of the low - type , only 13 cases were visualized by transperineal us , because the low - type fistula tracts were shallow and short . however , as low - type fistulas such as anocutaneous or anovestibular fistulas can be easily determined on a clinical examination , this low detection rate of us for low - type fistulas is not of clinical concern . lastly , we considered two sonographic findings in combination ( the fistula type and p - p distance ) and we attempted to use these findings to determine the ia type . the ia type was correctly diagnosed by the use of transperineal us alone in 39 of the 46 patients ( 85% ) . if physical examination findings had been taken into consideration , the diagnostic accuracy of transperineal us would have increased . considering the easy accessibility , good performance and the lack of any radiation hazard , transperineal us can be performed as an initial modality to determine the type of ia and to determine the type of early surgical management . in the present study , a female cloacal anomaly was not included as this anomaly can be determined by inspection of the perineum alone . females with only a single opening on the perineum and no visible hymen must have a cloacal anomaly , which is indicative of a high - type anomaly , which obviates the need for a us examination to determine type . in conclusion , our study results show that transperineal us is a good modality to define the internal fistula type and to predict the ia type based on consideration of the fistula type and p - p distance .
objectivethis study was designed to assess the usefulness of transperineal ultrasonography ( us ) for the determination of imperforate anus ( ia ) type.materials and methodsfrom january 2000 to december 2004 , 46 of 193 patients with an ia underwent transperineal us prior to corrective surgery . 
 sonographic findings were reviewed to identify the presence of internal fistulas and to determine " distal rectal pouch to perineum ( p - p ) " distances . 
 ia types were determined based on the sonographic findings , and the diagnostic accuracy of transperineal us was evaluated based on surgical findings.resultsof the 46 patients , 17 patients were surgically confirmed as having a high - type ia , three patients were confirmed as having an intermediate - type ia and 26 patients were confirmed as having a low - type ia . 
 the ia type was correctly diagnosed by the use of transperineal us in 39 of the 46 patients ( 85% ) . in 14 of the 17 patients with a high - type ia , internal fistulas were correctly identified . all cases with a p - p distance 
 > 16 mm were high - type ias and all cases with a p - p distance < 5 mm were low - type ias.conclusiontransperineal us is a good diagnostic modality for the identification of internal fistulas in cases of high - type ia and for defining the ia level .