First described by Vater and Ezler, choledochal cysts (CDCs) are abnormal congenital cystic dilatations of the biliary tract. Although rare among Western populations (~1 in 100,000–150,000), it disproportionately affects Asian populations (~1 in 1000). ⁷ Most cases of CDCs present in infancy and childhood, with only about 20% of cases presenting in adulthood, with a female predominance of 4:1 to 3:1. ⁸ , ⁹

The most widely accepted classification of choledochal cysts among clinicians was established by Todani et al. whose breakdown into five subtypes expanded upon Alonso‐Lej's initial work, with the inclusion of both multiple (Type IV) and intrahepatic cysts (Type V). ⁷ Type 1 choledochal cysts are extrahepatic cysts and make up the most common type of CDCs (80%). ¹⁰ This is broken down into three further subdivisions, with the overarching principle being a dilatation of the common bile duct (CBD) with normal intrahepatic ducts. The subdivisions include: (1a) diffuse dilatation of the CBD, (1b) isolated dilatation of the distal CBD, and (1c) fusiform CBD dilatation, which the patient had presented with. ¹⁰ This is illustrated by Figure 2 below.

Although its etiology remains unknown, the most widely accepted theory was postulated by Babbitt et al. whose work suggested an anomaly of the pancreaticobiliary ductal system during embryonic development predisposed to the formation of CDCs. ¹² In particular, the junction of the CBD and pancreatic duct occurs outside of the ampulla of Vater at a right angle, favoring the retrograde reflux of bile and pancreatic contents up the ductal system. ¹² , ¹³ Consequently, a long common channel of the Sphincter of Oddi forms (>15 mm). ⁸ , ¹³ Over time, the reflux of caustic pancreatic enzymes causes chronic inflammation, damaging the biliary epithelium and, ultimately, leading to the formation of CDCs. ¹³ Thus, when diagnosed in adulthood, CDCs at biopsy often highlight mucosal ulceration and hyperplasia, correlating with the histopathology findings of the patient's resected gallbladder and CDC. ¹

Studies have shown that this anomalous pancreaticobiliary duct union accounts for 30%–70% of CDCs at diagnosis, increasing up to 96% in the pediatric population. ⁸ The suggested cause of a fusiform CBD dilatation (Type 1c), which was measured at 11–12 mm on ultrasonography, is the chronic reflux of pancreatic enzymes, linking with Babbitt et al. and their proposed pathogenesis of CDCs. ¹² , ¹³ Furthermore, the patient's presentation of severe colicky pain, together with a history of intense dieting with minimal water consumption over the last few years, explains the findings gathered; the 12 mm calculi at the neck of the gallbladder, alongside asymptomatic bilateral nephrolithiasis on ultrasonography.

When investigating a patient with unexplained right upper quadrant pain, ultrasonography is the preferred first‐line imaging modality. Not only is it a useful imaging modality in emergency scenarios due to its accessibility, but it has a sensitivity of 71% to 97% for identifying and localizing CCs. ¹ Although ultrasonography remains a good imaging modality, it is known that if a CBD dilatation is found, it fails to identify the underlying cause in over 30% of patients, like this patient. ¹ , ⁸ Indeed, a CBD measuring >10 mm on ultrasonography should indicate that a cystic dilatation or an obstruction has occurred. ⁸ This should prompt the use of other imaging modalities such as MRCP and ERCP, which both possess higher sensitivities (70%–100%) and specificities (90%–100%) for diagnosis of CDCs, with MRCP preferred for its reduced rate of complications. ¹ , ⁸ , ⁹

The established gold standard treatment for Type 1 CDC is excision of the CDC with cholecystectomy, after which bile flow is restored by hepaticoduodenostomy (HD) or Roux‐en‐Y hepaticojejunostomy (RYHJ). ⁸ The patient underwent a CDC excision with RHYJ and cholecystectomy. As reported in literature, an RYHJ has more favorable outcomes to an HD across a spectrum of measures. ⁸ , ⁹ These are not limited to decreased rates of gastric and biliary cancer and significantly decreased rates of postoperative reflux. ⁸ , ⁹

Importantly, this case report highlights an atypical presentation of a CDC and underlines the diagnostic difficulty when CCs are present in adults. An increase in reported CDC cases in adults in recent years, with more than 70% of cases attributed to adults, necessitates greater awareness, especially since it may be frequently overlooked. ¹ , ¹⁴ Without typical features of CDCs, which are more commonly seen in children, such as the classic triad of pain, jaundice, and an abdominal mass (85% of children present with 2/3 clinical features compared to 25% in adults), diagnosis heavily relies on imaging modalities, alongside clinical acumen. ¹⁴ , ¹⁵ The patient, who had unremarkable systemic findings at presentation, alongside normal LFTs and serum amylase and lipase markers, emphasizes this. However, this is not a rarity given that CDCs are frequently only first suspected in adults when presenting with vague upper abdominal pain. ¹⁴

The importance of prompt diagnosis of CDCs to achieve optimal patient outcomes in adults is underlined by the complications arising from untreated CDCs, which are often seen as pre‐malignant. ¹⁶ The chronic inflammatory process is thought to induce dysplastic changes, ultimately leading to CC‐related carcinomas—a well‐known long‐term complication of untreated CDCs in adults. ⁸ This risk reaches up to 50% when CDCs are found in 51‐ to 70‐year‐old patients. ¹⁷ Other complications of CDCs, which may be present on initial presentation, include ascending cholangitis, biliary cirrhosis, pancreatitis and portal hypertension. ⁸ , ⁹ , ¹⁶ , ¹⁷

Even with prompt surgical intervention, the risk of postoperative complications remains. Despite the patient's follow‐up showing no immediate complications, except for mild pneumobilia, long‐term follow‐up will be required. Whilst the risk of anastomotic strictures is reduced with a high anastomosis (RHYJ), malignancy can develop years after CDC excision, with late complications (>30 days) seen in over 40% of adults. ¹ , ⁹ It goes without saying that the long‐term prognosis of resected CDCs in adults remains excellent, with more than 85% of patients having no long‐term complications, but regular monitoring is essential to detect potential biliary malignancy. ⁸