The findings in these 9 patients suggest that an eccentric mural-based filling defect in the distal common bile duct can be artifactual in nature, as documented by the transience on imaging or absence of pathology at the time of surgery. This highlights the need to interpret abnormalities of the distal common bile duct seen on static ERCP images with caution, particularly if only a limited number of images are available. If the etiology of a distal common bile duct finding is uncertain, close inspection of remaining images, repeat ERCP, or correlation with cross-sectional imaging such as CT MRI, or endoscopic ultrasound may be helpful in making this determination [1
Given that only one of the nine patients went to surgery (and no pathological abnormality was found), the anatomic basis of the pseudotumor appearance in this series is largely speculative. That said, we believe the transient nature of the finding in the 8 non-surgical patients suggests that the etiology may be related to contraction of the sphincter of Oddi. Additionally, the lack of bile duct dilatation proximal to the lesion suggests that the finding does not represent a true obstruction. We observed that many of our patients had prior hepatobiliary pathology or intervention; the extent to which this may have altered motility or contraction dynamics in the distal common duct is unknown. Another possible explanation is that patients had an adherent stone in the duct which passed in the interval between images, although the lack of stones in five patients and the mural-based nature of the imaging finding in all patients would argue against this explanation. There are also a variety of technical factors which may mimic lesions in the bile duct on ERCP including introduction of air bubbles [2
] or physical impaction of the scope into the ampulla. When the bile duct is cannulated, it is possible that the force of the cannula pushes the ampulla into the lumen of the duct and causes the bile duct to deform and buckle creating a filling defect. This filling defect may resolve with movement or removal of the scope. Communication between the radiologist and endoscopist may help clarify the nature of such artifacts.
Our study provides a contemporary update to earlier studies describing the so-called “pseudocalculus sign” in the distal common duct on endoscopic, percutaneous, intra-operative, and MR cholangiopancreatography [3
]. The “pseudocalculus sign” is a smooth or irregular convex upward termination of the distal bile duct (compared to the eccentric focal narrowing seen in the pseudotumor in our case series) that mimics a distal bile duct calculus at cholangiography. One study attributed the findings to motion artifact caused by pulsation of the inferior vena cava [7
]. Previous studies also believe that the finding is due to transient physiologic contraction of the sphincter of Oddi. Wertheimer et al. described the finding and how it could be distinguished from a true calculus on dynamic ERCP [8
]. More recently, dynamic MR cholangiopancreatography has been used to visualize real time common duct contractile activity [9
]. When reviewing static images, if the stricture resolves on later imaging, or if the inferior border cannot be visualized [4
], then the finding is more likely due to contraction of the bile duct rather than stone or stricture. We observed the latter in only two of our cases. Other causes of an artifactual filling defect or “pseudocalculus” at cholangiography include insertion of the cystic duct or posterior compression from the right hepatic artery [10
], though these etiologies will usually result in a pseudocalculus in a more proximal segment of the common duct rather than in the distal common duct.
Our report has several limitations. First, it is a retrospective review of cases collected by a single radiologist at one center. The images reviewed were static and therefore we could not evaluate any dynamic changes in the pseudotumor that might have been evident at fluoroscopy. Our study only included 9 cases. The patients were not selected systematically and we do not know how often this pseudotumor is seen on ERCP. Further research would be required to address these deficiencies.
In conclusion, an eccentric mural-based filling defect in the distal common bile duct can be artifactual in nature and may reflect transient contraction of the sphincter of Oddi.