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PATHOGENESIS OF
BLADDER / CLOACAL EXSTROPHY |
Infraumbilical midline defects are quite rare and, strictly speaking, only cloacal exstrophy involves the gastrointestinal tract directly.
§ In the situation of bladder exstrophy, the entire anterior wall of the urinary bladder, the overlying skin, and skeletal muscle are absent. The edges of the remaining posterior bladder are joined to the skin at a mucocutaneous junction. Bladder exstrophy has a male predominance of about 3:1. Bladder extrophy is not due to developmental arrest during embryogenesis as the human embryo does not pass through any developmental stage that is compatible with either extrophy or epispadias (1).
§ The leading theoretical explanation for cloacal exstrophy is failure of fusion of the infraumbilical midline structures during the cloacal stage of embryonic development, before fusion of the urorectal septum with the cloacal membrane. In the normal early embryo, a prominent cloacal membrane occupies the ventral infraumbilical abdominal wall. This membrane normally undergoes regression allowing formation of the infraumbilical abdominal wall via a converging process of mesodermal migration toward the midline (2). This results in lengthening of the suprapubic region and increased distance between the urogenital outlet and the umbilical cord. After the cloacal membrane regresses, it ruptures, resulting in the urogenital opening (urethra). Any interference with the regression of the cloacal membrane will result in extrophy. This has been experimentally induced in chick embryos (2).
§ The persistent cloacal membrane acts as a barrier preventing normal midline mesodermal fusion and therefore incomplete closure of the infraumbilical abdominal wall.
§ The size and cranial extent of the persistent cloacal membrane at its time of rupture accounts for the severity in the epispadias-extrophy complex.
§ This results in:
o Imperforate anus,
o exstrophy of 2 hemibladders,
o a lateral cecal fissure through which ileum may prolapse (assuming an elephant’s trunk configuration).
o Complex vertebral body anomalies of the sacrum.
o In both bladder and cloacal exstrophy, the bony pelvis assumes a so-called "open book" appearance and the pubic symphysis is widely diastatic.
o The hallmark of both entities on fetal imaging is absence of visualization of a urine-filled bladder.
o Abnormal configuration of the bony pelvis should be sought and the contour of the low, anterior abdominal wall analyzed.
§ In bladder exstrophy, the everted, but otherwise smoothly contoured mucosal surface of the bladder wall, is often visible.
o Inferior rectus abdominus muscles are absent; external oblique and transversalis muscles are incomplete.
o No muscle or connective tissue develops in the anterior wall anterior to the bladder.
o The thin anterior abdominal wall and the anterior bladder wall eventually rupture and the posterior bladder is exposed beyond the abdomen.
o There is associated epispadius, pelvic skeletal
In cloacal exstrophy, the contour of the anterior abdominal wall tends to be more lobulated because of the presence of exstrophied bowel flanked by 2 hemibladders. The presence of complex vertebral anomalies of the lower spine clinches the diagnosis of cloacal exstrophy over bladder exstrophy in this setting
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