Imaging description

A 21-year-old patient with twin pregnancy at 19 weeks of gestational age (GA) was referred for echogenic bowel on ultrasound (US) in one of the two male fetuses. On repeat US some mildly distended loops of bowel were noted and the presence of echogenic bowel was confirmed. A fetal MRI was performed at 22 weeks of GA. T1-weighted images failed to demonstrate normal high signal meconium in the rectum (Fig. 76.1a). The fluid-sensitive sequences demonstrated distended fluid-filled colon, including the rectum, and within the fluid multiple low signal intensity pellets were noted. The diagnosis of imperforate anus with urorectal fistula was made based on these findings. The follow-up US and MRI at 32 weeks of gestational age confirmed the findings (Fig. 76.1b, c). A newborn abdominal radiograph demonstrated discrete oval to rounded calcifications along the course of the colon, consistent with intraluminal calcified meconium. The infant was treated surgically for imperforate anus with urorectal fistula.

Importance

In the fetus, meconium is formed primarily by the accumulation of dehydrated amniotic fluid swallowed by the fetus, bile salts, and desquamation of enteric cells. On US, meconium has intermediate echogenicity while on MRI images meconium has high T1 and low T2 signal. Since fetuses apparently do not pass large amounts of meconium during pregnancy, meconium accumulates in the rectum, which is the largest meconium-filled area of the fetal bowel. Anal atresia is therefore difficult to diagnose prenatally by US or MRI if there is not an associated urorectal fistula. Anal atresia is typically differentiated into two types: low atresia (below the levator ani muscle) and high atresia (above the levator ani muscle). An urorectal fistula is typically associated with high anal atresia, mainly in boys (ratio boys:girls = 6:1). In girls, imperforate anus is most frequently associated with urogenital sinus or a cloaca. Cloaca usually presents with a fluid-filled bladder and vagina and with a rudimentary sigmoid colon.