Early Prenatal Diagnosis of Isolated Anal Atresia via Ultrasound and Fetal MRI

    

    

    Figure 2:  MRI demonstrates a dilated distal sigmoid colon to ten millimeters.
    
    

Discussion

Anal atresia is a rare congenital abnormality, present in approximately 1 in 5000 live births [1]. Anal atresia is rarely found in isolation, with prevalence of 1.11 in 10,000 live births in a large cohort of 4.6 million newborns in Europe [2]. Anal atresia is more frequently associated with additional congenital abnormalities such as VACTREL syndrome (vertebral defects, anal atresia, tracheoesophageal fistula with esophageal atresia, radial and renal dysplasia, and limb malformations) and caudal regression syndrome [3-7]. The ability to diagnose anal atresia prenatally is of clinical significance, as prenatal diagnosis allows difficult decisions to be made regarding the continuation of the pregnancy. Furthermore prenatal diagnosis provides time to arrange for delivery in a tertiary care facility that can provide planned immediate surgical attention at birth. Fetal defecation and the neurologic innervations of the colon and rectum provide the physiologic basis for prenatal diagnosis of anal atresia. Fetal defecation illustrated by meconium stained amniotic fluid was previously thought to indicate fetal distress and hypoxia that typically occurred late in the third trimester [7-11]. Current evidence has demonstrated that defecation is potentially a normal process of the fetus [12-15]. Abramovitch and Gray concluded that fetal defecation begins as early as 14 weeks [16]. This concept has been further supported by the detection of high levels of micro-villar intestinal enzymes, such as disaccharidases and intestinal alkaline phosphatases in amniotic fluid between 14 and 22 weeks with a sudden drop in levels following the twenty second gestational week [17]. Yuan has shown via fluorogold retrograde tracing that rats with anorectal malformations have deficient motor innervation of the anal sphincter leading to retained meconium in the fetal period [18]. This explanation provides a physiological basis for the fact that the colon is not prominent ultrasonographically until late second trimester, following adequate neural innervation of the sphincter and subsequent retention of meconium.

Prenatal diagnosis of anal atresia is challenging, with most cases diagnosed at the time of birth. Prenatal diagnosis is established by finding dilated loops of fetal bowel on ultrasound [19]. In a small cohort of twelve patients with documented anal atresia at birth, it was found that a U or V shaped segment of dilated bowel in the lower abdomen or pelvis is particularly suggestive of anal atresia [20]. This diagnosis is generally supported by the presence of additional congenital abnormalities since anal atresia rarely occurs as an isolated anomaly [3]. The youngest fetus diagnosed with sonographic evidence of dilated loops of bowel was 12 weeks of age [21]. This fetus also presented with the additional anomaly of a small perimembranous ventricular septal defect that was visualized sonographically. Harris et. al studied a cohort of patients with sonographically consistent anal atresia where the diagnosis was confirmed either at birth or fetal autopsy. Within the cohort, 11 of 12 patients (92%) with anal atresia demonstrated abnormalities associated with either VACTREL syndrome or caudal regression syndrome [22]. In this cohort, the diagnosis was not suspected until late into the second trimester. The earliest case positively diagnosed was at 26 weeks of gestation [22]. This emphasizes that early diagnosis is difficult to achieve.

Ultrasound of the anal ring can be used to rule out the presence of anal atresia. This can be seen as early as 15 weeks of gestation; however, at this gestation false negatives are greater. Generally, in the third trimester the ring can reliably be seen (figure 3).

    

    

    Figure 3:  The anal complex can be seen between the pelvic bones at the
tip of the arrow. There is a hypoechoic ring, which is the muscular portion
surrounding the hyperechoic mucosa and the central hypoechoic area being
the lumen of the anus. This may be seen from 15 weeks on to term, but may
not be seen well until after 20 weeks of gestation.