The endoscopic ultrasound features of pancreatic fluid collections: appearances can be deceptive!

Endoscopic ultrasound (EUS) evaluation of pancreatic fluid collections (PFCs) is vital for making therapeutic decisions. Imaging modalities such as magnetic resonance imaging (MRI), transabdominal ultrasound, and computed tomography (CT) can also characterize PFCs; however, the presence of the important feature of a solid component or debris within a PFC (i. e. walled-off necrosis [WON]) is better assessed by MRI and ultrasound, than by CT scanning. EUS and MRI are comparable for evaluation of the morphological features of PFCs; the former also detects collaterals. MRI scores better in detecting even minimal debris and extensions well beyond the gastrointestinal tract [1]. Diagnostic EUS can alter the management of PFCs in one-third of patients, mostly because of changes in the diagnosis (type of PFC) or the identification of vascular and/or anatomic factors that could interfere with endoscopic drainage [2]. EUS therefore offers the dual advantage of a tool for detailed inspection of the pancreas and PFC, plus its drainage.

The currently accepted management of periluminal PFCs is largely endoscopic drainage under EUS guidance. Historically, the conduit used for drainage has been one or more double-pigtail plastic stents, but more recently, for more than a decade, dedicated short-length fully covered large caliber metal stents are increasingly being preferred. The two main types of large caliber metal stent are lumen-apposing metal stents (LAMSs; e. g. Axios, Spaxus) and biflanged metal stents (e. g. Nagi, Plumber, and others). The inclination toward large caliber metal stents is mainly because of their technical ease of placement, efficient drainage of PFC contents, and reasonable ease for the performance of direct endoscopic necrosectomy (DEN). However, the downsides are cost, compulsory removal after a finite indwell period of ≤ 1 month, and the chance of adverse events if left in situ for longer.

“Paradoxically, despite poor agreement on morphology, there was moderate-to-substantial agreement regarding the choice of stent and the need for DEN, even though the respondents were blinded to the clinical details.”

In clinical practice, it is accepted that the percentage of debris relative to the volume of the PFC decides the choice of stent. Most experts believe that simple plastic stents are equally efficient for the drainage of a pure pseudocyst or walled-off necrosis (WON) that has little debris (≤ 10 %), while “heavy-duty” large caliber metal stents should be considered for WON that has significant debris (≥ 30 %); opinion remains divided for the intervening group.

Can EUS provide a percentage estimation of the debris in WON with reasonable accuracy? To answer this question, Fabbri et al. in their study focused on determining interobserver agreement for the morphology of PFCs by EUS and the subsequent management strategies [3]. The authors shared short-duration EUS videos of 50 patients with a PFC, without their clinical correlates, that were independently reviewed by 12 experts and evaluated for eight features: five diagnostic (type of collection, percentage solid debris, presence of infection, main pancreatic duct [MPD] recognition, and PFC–MPD communication) and three therapeutic (type of stent for drainage, and the requirement for DEN and its timing). The study reported poor interobserver agreement for all of the morphological features of PFCs examined (except for PFC–MPD communication), which is not unusual given the subjective nature of EUS evaluation, including crucially the assessment of debris [3]. Notably, even the landmark studies did not mention the methods for measurement of percentage debris [4] [5]. T2-weighed MRI can objectively quantify this by calculating the percentage volume of the solid debris component (length × breadth × height) with respect to volume of the PFC [6].

Interobserver agreement studies on EUS are operator- and observer-dependent, but are an essential step toward the standardization of EUS-based management algorithms for PFCs. There were no available data from interobserver agreement studies on the morphological characterization of PFCs and the subsequent therapeutic decisions. The study by Fabbri et al. address the knowledge gap on some of the parameters in this interobserver agreement study [3].

Paradoxically, despite poor agreement on morphology, there was moderate-to-substantial agreement regarding the choice of stent and the need for DEN, even though the respondents were blinded to the clinical details. Underestimation of the solid component in WON by EUS may misjudge the need for DEN, as was reported in a Dutch multicenter randomized controlled trial [4]. In that study, 57 % of patients required DEN in the endoscopic arm, although more than half had < 30 % debris. The above finding might also reflect the choice of stent based on regional practice patterns, irrespective of the morphological assessment. Resource limitations can also influence stent selection, so altering the results, as a plastic stent is much more cost-effective than a metal stent [5].

The decision to perform DEN and its timing is dependent on several factors, rather than on the morphology alone. DEN is more often performed as a secondary procedure after the index drainage using a step-up approach [7]. The gradual expansion of the metal stent allows the loose floating debris to be spontaneously expelled. It is probable that adherence of debris to the wall is more likely to require DEN, than the quantity of debris itself. This needs to be addressed in future studies.

There are however some limitations to the study of Fabbri et al., such as the lack of a gold standard, no sample size calculation, inclusion of only expert endosonographers from specialized centers, and the relatively low number of videos compared with the various parameters that were studied. Despite these limitations, this is a pivotal study to encourage further research in this unexplored area. The weaknesses can be addressed in future studies with particular focus on clinically important parameters like debris that dictate the choice of stent, along with the appropriate sample size, inclusion of non-experts, and a volumetric assessment of the PFCs by the “gold standard” MRI.

Publikationsverlauf

Artikel online veröffentlicht:
05. April 2022

• References

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