Prenatal Imaging and Counseling: Cloaca and Complex ARMs 2015

On ultrasound, the bladder is the first cystic structure seen from the abdominal cord insertion, outlined by umbilical arteries; a cystic structure posterior/inferior to the bladder reaching into the abdomen is hydrocolpos in a cloacal malformation.

Echogenic concretions (meconium) in the hydrocolpos or bladder are clues for a recto-urinary fistula and anorectal malformation.

Fetal MRI shows meconium as dark on T2-weighted imaging and bright on T1-weighted imaging; fluid in the fetal bowel is bright on T2 and dark on T1.

Meconium reaches the rectum by 20 weeks gestation and fills the entire colon by 26 weeks.

Long-common-channel cloaca on fetal MRI presents with a high-positioned, dilated rectum that funnels distally but does not reach far enough, and a thick-walled bladder due to outlet obstruction.

Imperforate anus with recto-urinary fistula can show fluid distention of the rectum and enterocolitis on fetal MRI; increased fluid content (bright T2 signal instead of dark meconium) in a dilated rectum is a clue for recto-urinary fistula.

Urogenital sinus on fetal MRI shows a rectum following a normal posterior course (not dilated, reaching far enough), with hydrocolpos located between the bladder and rectum; this distinguishes it from short-common-channel cloaca.

Cloacal exstrophy typically presents with persistent absent bladder visualization, normal amniotic fluid, low omphalocele, skin-covered spinal defects, and sometimes a prolapsed terminal ileum (elephant trunk sign) protruding through the abdominal wall defect.

Cloacal exstrophy on fetal MRI shows no meconium signal in the expected distribution of the rectum, distinguishing it from bladder exstrophy (which has a normal rectum with dark T2/bright T1 meconium signal).

Many cloaca cases are not diagnosed prenatally; retrospective review of donated ultrasounds from mothers of babies born with cloaca revealed abnormalities that were misdiagnosed as ureterocele, double bladder, ovarian cysts, or bladder diverticulum.

Hydrocolpos is not well described in the radiology literature, so many non-specialized radiologists are unfamiliar with the finding and may misinterpret it.

If a female fetus has a prenatal diagnosis of ovarian cyst, double bladder, or ureterocele—especially with abnormal vertebrae, hydronephrosis, or dilated bowel—suspect cloaca.

Prenatal diagnosis of cloaca allows transfer to a specialized center for proper colostomy and hydrocolpos drainage, and gives the family time to prepare emotionally and logistically for surgery within 24 hours of birth.

Pediatric surgeons cannot yet predict long-term bowel control, urinary control, or sexual function from prenatal imaging alone; key information (common channel length, sacral anatomy) is still limited on prenatal studies.

At Sick Kids (Toronto), an increasing number of cloaca cases have prenatal diagnosis, but the majority still do not.

Findings of cloaca on routine 20-week ultrasound may be very subtle; community obstetricians may not suspect the diagnosis unless they have a high index of suspicion.

False-positive prenatal diagnoses of ARM occur, causing parental alarm when the baby is born without any issues.