Imaging for Anorectal Anomalies
An anorectal anomaly is usually identified clinically. There may be an anal dimple present on the perineum or there may be an apparently patent external orifice but with a digitally palpable membrane on examination. Meconium may be passed per urethra in boys or per vagina in girls.
It is still common that plain radiography is performed and this will show a low obstruction early in the neonatal period (Fig. 13a). A prone lateral shoot through radiograph may be helpful in determining the level of the atresia, and allow assessment of the sacrum (Figs. 13b,c)
Ultrasound has been used to delineate the distance from the distal gas-filled colon to the perineum although its use is not widespread (Donaldson et al. 1989); less than 10 mm indicates a low lesion, and 15 mm or greater indicates a high lesion that will require diversion with colostomy.
Given the high incidence of associated anomalies all patients should have a renal tract ultrasound early in the newborn period. If this shows hydronephrosis, hydroureter or other renal abnormality (Fig. 6.13d) a full urological workup should be performed before a colostomy is fashioned or immediately after (Pena 1993). It would be appropriate to perform spinal ultrasound at the same examination to assess the integrity of the spinal cord and any associated dysraphic abnormalities (Fig. 6.13e). If this is normal only plain radiographs of the lumbar spine will be required, but if any further anomalies are demonstrated MRI of the lumbosacral spine should be obtained later when clinically appropriate (Fig. 6.13f). Possible associated intraspinal pathology includes caudal regression syndrome, cord tethering, hydromyelia or lipoma of the filum terminale. Newborn females with a cloaca may need drainage of a hydrocolpos.
Many centres make a clinical decision whether to proceed to definitive primary repair by perineal ano-plasty (for low malformations) or to perform a colostomy only, deferring further imaging until definitive repair can be performed at a later date (in high malformations). Most centers routinely use a combination of a micturating cystogram (MCU) and a loopogram through the loop colostomy (high pressure distal colostogram) to outline any associated connection or 'fistula' between the distal bowel and the urinary tract (Fig. 6.13g,h). The MCU will also demonstrate the degree of any associated vesicoureteric reflux. An increasing number of centers are additionally using MR imaging, as it can help determine the presence of the puborectalis muscle and external sphincter as well as the rectal pouch prior to surgery. The definitive surgical approach for high malformations is dependent on the type of fistula demonstrated. The posterior sagittal anorectoplasty (PSARP) is most often used in this group but in cases with an additional rectovesical fistula a laparotomy is also required in addition to the posterior sagittal approach.
A PSARP is performed prone. A midline posterior sagittal incision is made extending from the mid level of the sacrum to the anterior edge of the external sphincter.
The sphincter mechanism is divided in the mid-line hence preserving the nerve fibers. The gluteal
pelvis. Note the associated deficient sacrum. b A prone lateral shoot through radiograph showing the lowest gas filled bowel which is near the radio-opaque marker place in the natal cleft. This is a 'low' anomaly. c A prone lateral shoot through radiograph showing the lowest gas filled bowel which is near the radio-opaque marker place in the natal cleft. This is a higher anomaly. d Renal US showing crossed fused ectopia in an infant with an anorectal malformation. The fused right and left kidneys are shown by the markers. e Spine ultrasound in the patient shown in (a) showing the deficient sacrum, the cord was normal. f T2-weighted sagittal sequence of the spine showing the partly absent sacrum. g A 'fistula' inserting into the posterior urethra of a male infant. h Loopogram study demonstrating distension of the distal colon with retrograde filling of the bladder via the very narrow fistula. i MR of the pelvic floor following surgery showing mesenteric fat that has inadvertently been brought down into the rectal complex muscles are opened like a book, and all internal structures including the 'fistula' are exposed. The rectum is then separated from the genitourinary tract, dissected, and freed enough to reach its normal orifice without tension. The fistula site is then closed.
With the use of an electrical muscle stimulator, the limits of the sphincter mechanism are determined and the rectum is placed in its optimal location to achieve the best functional results. Adequate placement of the neo-rectum in both the puborecta-
lis muscle and the external anal sphincter is essential in achieving an acceptable functional outcome (Fig. 6.13i) (Nievelstein et al. 1998b).
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