Unless it is radiolucent, a CBD stone is not difficult to diagnose on CT. Biliary stones can appear in various attenuations on CT, and only about 20% show high attenuation[5]. Most biliary stones are either round or amorphous-shaped. In addition to the imaging findings from the biliary stone itself, low attenuated bile surrounding the stone and associated ductal dilatation or narrowing can improve the likelihood of correct diagnosis of biliary stone[5]. In addition, a signal void in the CBD or a fluid level with low signal intensity in the dependent portion of the CBD on MR imaging can suggest a CBD stone[6]. Even though ERCP is an invasive procedure, it has the benefit of simultaneous diagnosis and treatment[7]. Moreover, EUS has excellent diagnostic ability when it is performed with ERCP, and a biliary stone can be characteristically seen as a hyperechoic lesion accompanying a posterior acoustic shadow in the CBD on EUS[8,9].

On the other hand, Ascaris can appear as an elongated, high-density structure in the biliary tree on CT, although this modality is only used as complementary for diagnosis[6]. On MR cholangiogram, Ascaris demonstrates a “double-tube” appearance, a hypointense tubular defect inside the CBD with central hyperintense material representing the fluid-filled gut of the worm[10]. On ERCP, the worm is seen as an intraductal filling defect with motility in the bile duct[4,11]. The difference between CBD stone and Ascaris is even more obvious on EUS, in which the parasite appears as a spherical-shaped hyperechogenic lesion without posterior acoustic shadowing[12].

To the best of our knowledge, there has been only one case report of CBD stone mimicking Ascaris lumbricoides, and the report only described the sonographic findings[11]. Thus, it is not thought to be difficult to distinguish CBD stone from Ascaris infestation on imaging or clinical findings. Although they share radiologic findings of a non-enhancing high-density lesion on CT and a filling defect on T2-weighted images on MR, these two diseases rarely are part of the same differential diagnoses because their typical presentations are different. However, if these typical imaging features are not seen, differential diagnosis for tubular structures in the CBD could be confusing. Although we had never previously experienced an elongated-shaped CBD stone, we did experience an ascariasis patient who manifested similar imaging findings to the present case (Supplementary Figure 1).

In the present case, not only did the stone appear very similarly shaped to an Ascaris worm on CT and MR cholangiography, but it was also mistaken for Ascaris on EUS due to its elongated shape with a central hypoechoic area, rather than the nodular or angular appearance of a typical CBD stone. However, Ascaris could have been distinguished from a CBD stone in the present case by the relatively longer (15-49 cm) and thicker (2-6 mm in females)[13] body size of Ascaris compared to CBD stones, even if no motility was observed[14].

Although this was a rare case of CBD stone mimicking biliary ascariasis, it serves as a reminder that patients complaining of biliary colic or imaging finding of an elongated filling defect occupying the CBD should not be diagnosed by shape alone, and a CBD stone can mimic biliary Ascaris.