Dilated CBD of indeterminate etiology is frequently observed on cross-sectional imaging. The sensitivity and specificity of MRCP for detecting choledocholithiasis are 92% and 97%, respectively, while for biliary malignancies, they are 88% and 95% [5]. In comparison, the sensitivity of CT for detecting CBD stones ranges from 69% to 88% [6]. For biliary malignancies, CT demonstrates a sensitivity and specificity ranging from 84% to 90% [7].

The limitations of MRCP in the evaluation of biliary tree pathologies include issues with visualizing small stones (less than 5 mm), difficulty in differentiating between benign and malignant strictures, potential for misinterpreting artifacts or normal variants as pathology, and the ampulla of Vater sometimes becomes a blind spot, leading to failure of detection of ampullary neoplasms [8-10]. CT also has similar issues with additional problems of radiation exposure and inferior characterization of choledocholithiasis.

EUS, with its anatomical proximity to the pancreaticobiliary system, serves as a valuable diagnostic modality in assessing unexplained CBD dilatation. EUS, apart from being a diagnostic tool in finding a cause for dilated CBD, also provides an opportunity in some of these cases to biopsy mass lesions to provide a histological diagnosis. EUS has advantages as it can detect smaller stones (< 5 mm). [11] EUS facilitates accurate assessment of ampullary neoplasms, given its spatial closeness to the duodenum and the ability to perform tissue sampling concurrently [10,12].

Several prospective as well as retrospective studies in the past have assessed the efficiency of EUS in patients with undiagnosed dilated CBD noted on CT or MRI [2,13]. Mahajan et al. reported a diagnostic yield of approximately 56% [1], and Rana et al. reported a yield of 50% [3], both of which are comparable to the 54.7% yield observed in our study.

In the present study, 34 patients (45.3%) remained undiagnosed even after EUS, including nine with normal CBD size and 25 with dilated CBD without an identifiable cause. Previously conducted studies showed similar results, where the majority of patients were undiagnosed after EUS [1,3]. In our study, some of the patients with unexplained CBD dilatation (n=25) may have had spontaneously passed stones, with EUS performed before the CBD had returned to normal size. This highlights a potential advantage of EUS, as it may help avoid unnecessary invasive procedures like ERCP. In a retrospective study by Quispel et al., 169 (63%) patients were able to avoid ERCP following EUS evaluation [14]. Similar benefits have been demonstrated in other studies, where EUS-guided ERCP has been associated with reduced complication rates related to ERCP [15-17].

The most common pathologic etiology in our study was choledocholithiasis, present in 25 (33.3 %) patients, followed by benign biliary stricture seen in eight (10.7%) patients. A similar etiological profile was reported in a study by Rana et al., wherein choledocholithiasis was the most common etiology [3]. In a study by Sotoudehmanesh et al., the most common pathology seen was CBD calculi (21.1%) [18].

On multivariate logistic regression analysis, age, alkaline phosphatase, and CBD diameter on cross-sectional imaging were found to be significant. Some studies have given prediction models for selecting the right patients for EUS examination and reducing unnecessary EUS examination, but have not been validated in larger cohorts. In a retrospective study by Raza et al., a predictive model incorporating age, history of ERCP, abdominal pain, and CBD diameter demonstrated a sensitivity of 92-100% and a specificity of 91-93% for diagnosing etiologies on EUS [19]. Similarly, in a study by Mahajan et al., serum bilirubin, CA 19-9, and CBD diameter were predictive of a positive diagnosis on EUS [1].

Strengths and limitations

This study has several strengths. First, its prospective design allowed for systematic and standardized data collection, reducing the risk of recall and selection bias. The inclusion of patients based on strict criteria, CBD dilatation without identifiable etiology on CT or MRCP, ensured a homogenous study population and enhanced internal validity. EUS was performed within two weeks of cross-sectional imaging, minimizing diagnostic delays and potential changes in disease status. Additionally, all procedures were conducted by a highly experienced endo-sonographer, which reduces variability in interpretation and increases the reliability of findings. Finally, the use of multivariate logistic regression and AUROC analysis enabled the identification and validation of independent predictors of a positive EUS diagnosis.

However, the study has certain limitations. It was conducted at a single tertiary care center, which may limit generalizability to broader clinical settings. Despite a large initial cohort, the small sample size met the inclusion criteria, potentially reducing statistical power. Lastly, the use of both CT and MRCP (with some patients undergoing both) introduced heterogeneity in baseline imaging, which may have influenced the diagnostic outcomes.