What is the most appropriate diagnostic strategy for evaluating a small common bile duct stone?

Common bile duct stones (CBDS) are often encountered in routine practice and can cause jaundice, cholangitis, and pancreatitis. CBDS can also cause obstruction of the bile and/or pancreatic ducts even if the size of the stone is small; therefore, they should be treated regardless of size. Endoscopic retrograde cholangiopancreatography (ERCP)-related techniques for the treatment of CBDS have advanced significantly in recent years.1-3 However, due to the risk of post-ERCP pancreatitis, appropriate decision-making is required.4 In cases where the presence of CBDS is clear, ERCP should be performed, but in cases where the presence of stones is unclear, the decision to perform ERCP becomes more difficult. Diagnostic tools for CBDS include abdominal ultrasonography (US), computed tomography (CT), magnetic resonance cholangiopancreatography (MRCP), and endoscopic ultrasound (EUS), all of which can be used to diagnose CBDS if the stones are large. However, it is often difficult to detect small stones. Although US is less burdensome to the patient and can be performed in any hospital, gastrointestinal gas and subcutaneous fat sometimes reduce its diagnostic performance. The American Society for Gastrointestinal Endoscopy and the European Society of Gastrointestinal Endoscopy guidelines recommend EUS or MRCP for the evaluation of patients with an intermediate risk of CBDS after a negative abdominal US, and they do not recommend CT scans.5, 6 However, in many cases where ERCP for the treatment of CBDS is considered, CT is performed. Therefore, CT is often performed to diagnose clinically suspected CBDS in real clinical settings. CBDS can be easily detected by CT if the stones are large. Even with small stones, the diagnosis of CBDS can be made if a high density area, similar to that of a stone in the gallbladder, can be detected in the common bile duct. A problematic case in clinical practice is one in which no stone is found in the common bile duct on CT, but obstructive cholangitis, obstructive jaundice, or cholelithiasis is suspected based on blood sampling data and physical examination. In these cases, MRCP or EUS are candidates for subsequent testing. There have been many reports on the ability of MRCP and EUS to visualize CBDS; a meta-analysis of five randomized trials to compare the accuracy of choledocholithiasis detection between EUS and MRCP showed that the aggregated sensitivity, specificity, positive predictive values, and negative predictive values of EUS were 0.93, 0.96, 0.93, and 0.96, respectively, and of MRCP were 0.85, 0.93, 0.87, and 0.92, respectively. The aggregated values were not significantly different between the groups.7 In addition, several studies have examined the usefulness of EUS in patients in whom the stones could not be detected by CT. All studies demonstrated the usefulness of EUS following negative CT scans; however, no study had compared EUS and MRCP for detection of CBDS. In this issue, Suzuki et al. compared the diagnostic accuracy of EUS and MRCP for CBDS that could not be detected by CT.8 The authors planned this comparative study by focusing on the clinical situation often seen in which the presence of choledocholithiasis was initially evaluated using CT. The perspective of this study is very interesting, and it is worthy of evaluation as it was thought out with actual clinical usefulness in mind. In this comparison, 50 patients suspected of having CBDS were enrolled and randomly allocated to the EUS or MRCP group. Upon initial examination, patients with CBDS or sludge formation underwent ERCP, while those who were CBDS-negative underwent a second examination with either MRCP or EUS, which was distinct from the initial diagnostic procedure. The results showed that the accuracy was 92.3% for EUS and 68.4% for MRCP (P = 0.055). EUS showed 100% sensitivity, 88.2% specificity, 81.8% positive predictive value, and 100% negative predictive value, while MRCP showed 33.3% sensitivity, 84.6% specificity, 50% positive predictive value, and 73.3% negative predictive value. The CBDS detection rate in the second examination was 0% for MRCP after negative EUS and 35.7% for EUS after negative MRCP (P = 0.041). It was also reported that there was no difference in adverse events occurring in both methodologies. The results of this study suggest that EUS is the most useful modality for evaluating CBDS. There are two discussion points for this excellent and clinically useful comparison study: how to identify whether a stone was truly present and the issue of test versatility. The only way to identify if a stone was truly present is to remove it with ERCP and endoscopically confirm it. However, in this study, the fact that CBDS includes not only stones but also sludge seems to make their identification difficult. CBDS treatment is performed transpapillarily using basket or balloon forceps. It is easy to endoscopically observe the stones discharged from the papilla using any method but it is sometimes difficult to endoscopically observe sludge excreted from the papilla. If a small amount of sludge is pressed against the common bile duct wall with balloon forceps, it may not be discharged from the papilla, in which case the preoperative examination evaluation is deemed to be incorrect. In this study, it was reported that 15 of the 22 cases diagnosed with stones or sludge by EUS or MRCP had either stones or sludge removed. The majority of CBDS actually removed had been identified by EUS, suggesting that many cases of CBDS identified by MRCP were false positives. However, although it is summarized in table 5 (characteristics of patients with CBDS detected using EUS for the second examination), one case reported that ERCP did not identify stones even though EUS was able to detect 8 mm stones. Moreover, it is difficult to determine whether there were no stones. In this study, a "true negative" was defined as a stone or obvious visible sludge that was not removed on ERCP. In cases where ERCP was not performed, patients who did not have recurrent cholangitis within 6 months were also defined as "true negative." Considering that there are many asymptomatic common bile duct stones, 6 months may be too short a time frame to determine this. In terms of versatility, EUS and MRCP are inferior to CT. EUS is not a procedure that can be performed at any hospital, and as the author points out, the evaluation results differ greatly depending on the skill of the endoscopist. It is, however, relatively easy to observe the common bile duct from the duodenal bulb with a curved linear-array echo endoscope during EUS. In addition, because EUS is performed under sedation, it should be kept in mind that sedation may be a risk in cases of poor cardiopulmonary status, and there is also a risk of complications, such as bleeding and gastrointestinal perforation. However, the skill of the inspection engineer does not affect MRCP readings. The number of MRCP machines installed is smaller than that of CT, and MRCP is not an examination that can be performed at any hospital. In addition, factors such as claustrophobia and cardiac pacemakers preclude the application of MRCP. CT has a radiation exposure problem, but ERCP does as well as it is performed under fluoroscopy and endoscopy.9, 10 Therefore, it is recommended to evaluate CBDS using MRCP or EUS, which do not expose the patient to radiation. Based on the results of this comparative study, the following strategies are recommended. In cases in which the patient has symptoms, CBDS evaluation should be performed by CT first. If CBDS is not identified by CT, EUS should be performed in facilities where EUS can be performed; if stones are identified, ERCP can be performed continuously with EUS. MRCP should be performed at facilities where EUS cannot be performed. However, even if no stones are detected by MRCP, there is a possibility that stones may exist, as stated in this report. In these cases, ERCP should be performed according to the symptoms. It is important to build a unique strategy for each facility based on the types of testing that can be performed at that facility and the technical experience. We thank Editage (www.editage.jp) for English language editing. Author M.T. is an Associate Editor of Digestive Endoscopy. The other author declares no conflict of interest for this article. None.