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Microliths are gallstones less than 3 mm in size and which cannot be detected on abdominal USG. Because of the lithogenic bile, microliths generally grow in size. Patients with suspected microlithiasis have been shown to develop gallstones on follow‐up abdominal USG in up to 75% of cases [29,30]. Microliths are different from biliary sludge. Biliary sludge is seen as low‐amplitude echoes in the gallbladder without acoustic shadowing on USG and layer in the dependent part. Microliths are best diagnosed on EUS with a sensitivity of 96% [35].

Earlier studies had shown a high prevalence of microlithiasis in patients with idiopathic AP; however, recent studies have shown a much lower prevalence (Table 2.2). One of the reasons could have been the reliance in earlier studies on duodenal bile microscopy to detect biliary crystals, an indirect method, in order to diagnose microlithiasis. This has largely been supplanted by EUS which detects microliths directly. Microlithiasis should be considered as a cause of pancreatitis in the presence of abnormal liver function tests within the first 24–48 hours of onset of AP as mentioned above in the context of gallstones [36].

Once a patient develops gallstone pancreatitis, cholecystectomy should be advised. Endoscopic sphincterotomy alone might prevent the recurrence by not allowing the small stone to obstruct the papilla of Vater. In a study of 5079 patients, recurrence of biliary AP in those treated with endoscopic sphincterotomy occurred in 6.7%; cholecystectomy was able to reduce this rate to 4.4%, while cholecystectomy followed by endoscopic sphincterotomy further reduced the rate to 1.2% [37].

Table 2.2 Frequency of biliary microlithiasis in recurrent acute pancreatitis.

Study	Frequency of microlithiasis
Older studies
Ros et al. 1991 [29]	37/51 (73%)
Lee et al. 1992 [30]	21/29 (72%)
Sherman et al. 1993 [31]	7/13 (54%)
Kaw et al. 1996 [32]	15/25 (60%)
Newer studies
Garg et al. 2007 [33]	10/75 (13%)
Wilcox et al. 2016 [34]	20/200 (10%)