A review on this topic recently appeared in NEJM's Clinical Therapeutics series. Key points below.
Urgent intervention (within 24 hours) is favored in the presence of cholangitis or early intervention (within 72 hours) if suspicion of common duct obstruction exists (e.g. persistent elevation or worsening of LFTs) without cholangitis. The authors cite relevant guidelines and note that increasingly (and since much of the guideline informing research was done) MRCP and EUS have been used to help inform more precise use of ERCP.
The pathophysiology is not completely understood, but septal compression by a distended common bile duct or pancreatic duct obstruction and/or bile reflux due to obstruction of a common channel are suspected mechanisms. An unusually long common channel seen in some patients may explain severity of disease in a minority of patients with gallstone pancreatitis. All that being said it is well known, note the authors, that severe pancreatitis can occur with only microlithiasis.
Practical aspects of ERCP
In the presence of obstruction vitamin K absorption may be impaired putting the patient at risk for impaired coagulation, hence the importance of checking an INR, which should preferably be below 1.5 prior to ERCP. In addition the authors recommend that the platelet count exceed 75K. They recommend antibiotics (a quinolone or cephalosporin) pre-procedure.
If a sphincterotomy was done the patient is not likely to have recurrent pancreatitis but remains at high short term risk for other biliary events. Thus the authors recommend, if the patient can be stabilized, cholecystectomy that same admission.