DCBD is an extremely rare congenital anomaly of the biliary system. Since Vesarius first reported it in 1543, only 24 cases had been reported in the western literature until 1986[3]. However, cases of DCBD are reported more often in the eastern literature. A total of 47 patients reported in the Japanese literature were reviewed by Yamashita et al[4] in 2002; furthermore, Chen et al[5], reported 24 Chinese cases in 2014.

The mechanism of this biliary system anomaly is considered to be due to an abnormality during embryogenesis. The presence of two bile ducts is normal anatomically during early human embryogenesis[6]. However, failure of the second bile duct to regress results in the persistence of an accessory extrahepatic bile duct[1].

Morphological classification of this anomaly was first suggested by Goor et al[7]. Goor’s classification was revised by Saito et al[2], and recently Choi et al[1], modified it by adding a Type V, which is characterized by single drainage of the extrahepatic bile ducts. According to the classification by Choi et al[1], the types of DCBD are as follows: Type I, CBD divided by septum; Type II, CBD bifurcates at the distal end and drains into each opening independently; Type III, duplicated biliary drainage with (Type IIIa) or without (Type IIIb) intrahepatic communicating channels; Type IV, duplicated biliary drainage with one or more communicating channels; Type V, duplicated extrahepatic bile duct rejoined as a single drainage with (Type Va) or without (Type Vb) communicating channels (Figure 4). Our case corresponds to Type Va in the classification by Choi et al[1].

Very few cases of DCBD Type V have been reported. Ever since Choi et al[1] reported the first case of DCBD Type V, only 7 cases have been reported[8]. To the best of our knowledge, this patient is the eighth case of DCBD Type V to be reported. Among the previously reported cases, 5 cases were diagnosed with choledocholithiasis[3,8-11]. There were 2 cases[9,11] that had choledocholithiasis in both extrahepatic bile ducts; one underwent surgery as the stones were not completely removed using ERCP[11], and another case successfully underwent stone extraction in one session of ERCP[9]. In our case, the patient presented with recurrent pyogenic cholangitis because a DCBD was not initially suspected due to the rarity of the anomaly. Therefore, two sessions of ERCP were needed to remove all choledochal stones in this case.

MRCP is the examination of choice to evaluate the anatomy of the biliary tree and its variations[12]. It has several advantages over ERCP such as (1) non-invasiveness; (2) no radiation hazard; and (3) less operator dependence[13]. It also has advantages over CT; it does not require intravenous contrast media and provides comprehensive anatomy of the biliary tract in a single image due to its capability to acquire images in any desired plane[14]. As very few biliary endoscopists and radiologists have ever encountered DCBD even in high-volume centres[9], in this case, the right CBD was mistaken for a residual cystic duct on initial CT because the patient had already undergone cholecystectomy. However, the precise anatomy of CBD was clarified through MRCP, and this extremely rare congenital anomaly could be diagnosed. Therefore, MRCP should always be considered in patients with suspected biliary tract anomalies.

Despite the extremely rare nature of this condition, it is crucial to consider the possibility of this congenital anomaly. Goor et al[7] reported a case of inadvertent transection of the CBD during laparoscopic cholecystectomy because of the failure to recognize the presence of DCBD. It is substantial to establish complete stone clearance during ERCP, because residual stone is a probable risk factor for recurrent cholangitis[15]. Although we performed ENBD to confirm the residual stones, the stones in the ancillary bile duct were completely concealed because only one of the two CBDs was assessed during the index procedure. It may be quite challenging to access both bile ducts in a DCBD Type V because it shares a short segment of the draining duct. In a case reported by Kim et al[11], stones in the right CBD were extracted surgically owing to inadequate access to the right duct. Although only one duct was accessible initially in our case, we were able to successfully cannulate both bile ducts after recognizing DCBD.