Congenital anomalies of the gallbladder can be categorized as anomalies of number, anomalies of form and anomalies of position. Anomalies of number include congenital absence of the gallbladder, duplication of the gallbladder and perhaps hypoplasia of the gallbladder. Abnormalities of form include the presence of septa or abnormal folding of the gallbladder that results in a bilobed gallbladder, hourglass deformity or a phrygian cap deformity. This category can also include diverticula of the gallbladder. Anomalies of position include an intrahepatic gallbladder, left-sided gallbladder and a floating gallbladder that is susceptible to torsion. Of the above, the most common are anomalies of form that result in a phrygian cap deformity or a bilobed deformity. In relation to anomalies of number, congenital absence of the gallbladder is rare but duplication of the gallbladder has an estimated prevalence of 1–3 per 10 000 individuals.

The patient illustrated below was a 30-year-old white woman who was referred for investigation because of discomfort in the upper abdomen and episodes of pain in the epigastrium and right upper quadrant. On examination, the only abnormality was mild tenderness on palpation over the right upper quadrant. Routine blood tests including liver function tests were within the reference range. An ultrasound study of the upper abdomen revealed two cystic structures in the gallbladder fossa lying side by side (Fig. 1). The structure on the left had the appearance of a normal gallbladder while the structure on the right had minor thickening of the wall and contained echogenic material. Magnetic resonance cholangiopancreatography revealed a true duplication of the gallbladder and showed that the two cystic ducts converged to form a common cystic duct prior to entry into the bile duct (Fig. 2). The presence of duplication of the gallbladder was confirmed at the time of laparoscopic cholecystectomy. Although a common cystic duct was demonstrated in the above patient, the majority of patients have separate cystic ducts that enter the bile duct. There are also reports of duplication where the second gallbladder is in an anomalous position such as within the liver or under the left lobe of the liver. There is no persuasive evidence that duplication of the gallbladder is associated with increased risks for gallbladder stones or gallbladder neoplasia.

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