Biliary atresia (BA) is a disease involving sclerosing inflammation and obstruction of the bile ducts in early infancy that causes the liver to be unable to excrete bile into the duodenum [1,2]. BA is the most common cause of liver cirrhosis and hepatic failure in young children and often leads to liver transplantation [3]. However, the existence of multiple variants of BA can make it difficult to diagnose BA. One such variant, cystic BA (cBA), accounts for approximately 5%-10% of BA cases [4,5]. When considering the three types of BA, cystic dilatation of the bile duct could be present at the point above or between the obliteration [2,6,7]. Therefore, a cyst of varying size could be present anywhere along the bile ducts, so this condition can mimic a choledochal cyst (CC) on imaging at the first presentation.

The diagnosis of BA early in infancy is important for the effective drainage of bile flow with the Kasai operation [8]. Early bile flow diversion before 60 postnatal days leads to better outcomes in BA patients, even though cBA cases were found to have higher native liver survival rates than patients with conventional BA [7]. Moreover, the surgical methods differ between cBA and CC (hepaticojejunostomy and choledochojejunostomy, respectively) [9]. Therefore, early and accurate differentiation between cBA and CC is essential.

The typical sonographic findings for BA include a dysmorphic gallbladder (GB), an absent extrahepatic bile duct, a positive triangular cord sign, increased hepatic subcapsular flow, and an increased hepatic artery diameter [10-13]. In addition to ultrasonography (US), magnetic resonance imaging (MRI) and hepatobiliary scintigraphy can aid in the preoperative diagnosis of BA [14-16]. A microcyst in the porta hepatis is also known to be a specific sign for diagnosing BA [17]. However, distinguishing cBA from CC using imaging is particularly difficult when the cyst is large and extends into the extrahepatic space, resembling CC type Ia/b according to the Todani classification [18]. Differentiation is also difficult in prenatal assessments [19,20]. Some authors suggested that cBA and CC could be interim entities belonging to the same pathologic continuum [21]. However, limited studies have focused on the differentiation of cBA with a large extrahepatic cyst from CC type Ia/b using various imaging modalities and prenatal images.

Therefore, the purpose of this study was to compare the clinical and radiologic differences between cBA and CC type Ia/b, including both prenatal and postnatal imaging.

cBA can be misdiagnosed as CC, especially when the cyst is large and not limited to the periportal tract. However, it is important to differentiate these two disease entities early in infancy because they have different clinical outcomes without an early intervention [24]. Our study showed that an increased triangular cord thickness (cutoff value of 4 mm on US, the same cutoff as in the original research on BA [10]), smaller cyst size, GB mucosal irregularity, and non-visibility of the distal bile duct on US or MRI were the key diagnostic features for differentiating cBA from CC type Ia/b, with excellent diagnostic performance, even though T.bil and γGT levels were not significantly different between the two groups. These imaging features should be carefully examined when encountering infants with extrahepatic biliary dilatation.

In a previous study, the most frequently described difference between cBA and CC was the cyst size. Several studies mentioned that cysts were significantly smaller on postnatal US in cBA than in CC, with cutoff values ranging from 1.5 to 3.5 cm [5,24-26]. Even though the cyst size in cBA can be variable, and cysts can extend below the hepatic hilum, the cysts in infants with cBA were also significantly smaller than those in infants with CC in our study. In addition, on prenatal studies, the cysts in the CC group were all larger than 1 cm and showed a tendency for gradual growth after birth, while the cyst in the cBA group for which information was available was smaller than 1 cm and stable in size. Other studies also mentioned that the cyst size in infants with cBA was stable during follow-up in the prenatal period [20,27,28] and smaller than that reported in a postnatal study [4], even though the size itself varied considerably across infants (0.5-4.0 cm). Obliteration between the intrahepatic and extrahepatic bile ducts in BA, rather than the normal development of the bile ducts, could contribute to the stable cyst size in utero [29]. Another theory suggested that overexpression of CD56 at the portal area represented dysregulation of the cell or intercellular matrix interactions in ductal plate remodeling in prenatal cBA patients [30]. These immune and inflammatory responses could affect various features of the size and growth patterns of cysts in cBA and CC prenatally. The differences in patterns of changes in cyst sizes from the prenatal to postnatal period could assist in the early diagnosis of cBA and the provision of appropriate interventions.

Triangular cord thickness is the most useful US feature for diagnosing BA and MRI can show comparable diagnostic performance to US using triangular cord thickness [10,14,23]. The combination of a thickened triangular cord sign, abnormal GB morphology, and non-visibility of the common bile duct showed excellent diagnostic performance for differentiating BA using both US and MRI [23]. Our study showed that these features could also help in the differentiation of cBA from CC. Zhou et al. [19] suggested that a positive triangular cord sign (cutoff, 4 mm), smaller cyst size without internal sludge, hepatic artery enlargement, and GB mucosal irregularity could represent cBA on US. Those results are comparable with the findings of our study, except for sludge in the cyst, because one cBA patient in our study had sludge in the cyst. Furthermore, our study is meaningful since no other study has demonstrated imaging differences between US and MRI, including prenatal US. Even though US has benefits due to the better spatial resolution of neonatal abdominal US compared with MRI, both US and MRI showed excellent diagnostic performance for diagnosing cBA based on the triangular cord sign and cyst size. In addition, MRI has certain benefits, including clear visualization of the distal bile duct and its usefulness for preoperatively assessing vessel and bile duct anomalies, including anomalous pancreaticobiliary ductal union. In our study, a combination of US and MRI features, including triangular cord thickness ≥4 mm, cyst size ≤2.2-2.8 cm, GB mucosal irregularity, and a nonvisible distal common bile duct, showed excellent diagnostic performance.

The morphology of GB plays an important role in both prenatal and postnatal imaging. A small or nonvisible GB in the second trimester increases the suspicion of BA on prenatal US [28,29]. Postnatally, the GB also shows an atretic or abnormal shape in cBA patients [19,25,26]. In our study, the case of cBA with prenatal US had a nonvisible GB, whereas three of the four CC patients with available prenatal US showed normal GB. Postnatal US also showed GB mucosal irregularity in all cases of cBA and no cases of CC. Even though the GB size was not significantly different between the cBA and CC groups in our study on either US and MRI, atretic GB (<15 mm) was only seen in the cBA group. Moreover, abnormal GB morphology (a narrower GB and larger GB ratio) was a characteristic finding of cBA, rather than CC, on MRI. Not only an atretic or small GB, but also abnormal morphology (e.g., a tubular or elongated shape) can be an important marker for differentiating cBA from CC.

There are several limitations in this study. First, there was a small number of patients in each group because of the rarity of these diseases, even given the long study period at a tertiary hospital. We could not statistically compare the patterns of changing prenatal and postnatal cyst size because prenatal US images were only available for five patients, including one patient in the cBA group. Further research is needed on prenatal cyst size criteria. Second, because this was a retrospective study, there was a lack of information on some imaging features. A retrospective analysis of obtained images might identify differences in the hepatic artery diameter and cyst size depending on the imaging modality, even though the measurement difference was not significant between the groups. Third, neither pathologic correlations with the imaging findings nor clinical outcomes were assessed. Further evaluation through a multicenter study, including prenatal images, is needed for the clinical validation of these results.

Even though the cysts in cBA can resemble CC morphologically, key imaging features can aid in the differentiation of cBA from CC with excellent diagnostic performance. For cBA, a small cyst size (<1 cm) on prenatal US, triangular cord thickening ≥4 mm, and GB mucosal irregularity on postnatal US, as well as a small cyst size (≤2.2 cm) and invisible distal common bile duct on MRI can discriminate cBA from CC type Ia/b in infancy. Careful evaluation and consideration of cBA are needed when we encounter a CC-like lesion in an infant.