The local complications of acute pancreatitis are the fluid and necrotic collections that form in and around the gland in the weeks after an attack. They matter twice over: their presence is part of what separates a moderately severe episode from a mild one, and infection of a collection is the complication that most often makes the disease life-threatening.
An acute peri-pancreatic fluid collection is a low-attenuating area of fluid around the pancreas. It is found in about 30–50% of acute pancreatitis, especially the mild form, and most such collections resolve on their own.
A collection that persists and develops a wall more than 4 weeks after the onset of acute pancreatitis becomes a pseudocyst. Because most peri-pancreatic fluid collections resolve, pseudocysts are very rare; when they do form they can sit away from the pancreas, in the pelvis or the chest.
An acute necrotic collection is the collection of dead tissue and fluid that forms in the necrotising forms of the disease, in two variants: pancreatic necrosis and peri-pancreatic necrosis. When it persists it goes on to form walled-off necrosis (WON) — pancreatic necrosis that has liquefied by about 5–6 weeks and acquired a defined wall, in the same way that a persisting fluid collection becomes a pseudocyst.
Two terms describe how much of the gland is involved. Interstitial pancreatitis is interchangeable with mild acute pancreatitis: all parts of the pancreas are well perfused and no low-attenuation areas appear. Necrotising pancreatitis covers both pancreatic and peri-pancreatic necrosis, and pancreatic necrosis means that more than 30% of the pancreatic parenchyma has low attenuation or fails to enhance on contrast CT. Among cases of necrotising pancreatitis, about 45% have both pancreatic and peri-pancreatic necrosis, about 45% have only peri-pancreatic necrosis, and pure pancreatic necrosis is very rare, at about 5%.
The systemic complications of the early phase — the organ failure driven by the systemic inflammatory response — are described in Clinical Course of Acute Pancreatitis.