Acute pancreatitis is a common inflammatory condition of the pancreas associated with various complications. Among these, pancreatic necrosis is considered one of the most significant and serves as a critical indicator of disease severity.

The Revised Atlanta Classification (RAC) standardizes terminology and defines complications of acute pancreatitis to facilitate multidisciplinary communication. Pancreatic necrosis typically develops within 24–48 hours after symptom onset. Therefore, early imaging within the first 72 hours is generally not recommended, as a CT scan performed too early may fail to accurately assess the presence and extent of necrosis or the morphological severity of the condition.

The RAC categorizes complications into acute (within four weeks of onset) and delayed (more than four weeks after onset) and further subdivides them based on pancreatic parenchymal viability. Acute complications include acute peripancreatic fluid collections (APFC) and acute necrotic collections (ANC). Delayed complications are classified as pancreatic pseudocyst or walled-off pancreatic necrosis (WOPN).

Pancreatic necrosis is associated with a prolonged clinical course, a high risk of local complications, and an increased mortality. Necrotizing pancreatitis presents in different patterns:

• The most common involves necrosis of both pancreatic parenchyma and peripancreatic tissues.
• Necrosis is limited to the peripancreatic tissues without affecting the pancreas.
• It is confined to the pancreatic parenchyma without involving the peripancreatic tissues.

Acute peripancreatic fluid collections (APFC) are commonly seen in acute pancreatitis, lack a well-defined wall or encapsulation and are generally sterile. Most APFCs resolve spontaneously within 2–4 weeks. Aspiration or drainage is discouraged at this stage, as it may introduce infection. If APFCs do not resolve, they can evolve into pseudocysts after at least four weeks.

Peripancreatic necrotic fluid collections (PNPFC) develop from the liquefaction of necrotic pancreatic tissue and/or peripancreatic fatty tissue over time. During the early stages of the disease, any fluid collection within the pancreas that replaces pancreatic parenchyma should be classified as PNPFC rather than a pseudocyst. These collections may contain sterile or infected.

A pseudocyst is a fluid collection surrounded by a well-defined fibrous wall. It typically arises in cases of interstitial pancreatitis, with the absence of necrotic tissue being essential for diagnosis. Pseudocysts often communicate with the pancreatic duct.

Walled-off pancreatic necrosis (WOPN) evolves from PNPFC and is characterized by a thick, non-epithelialized wall separating necrotic tissue from adjacent viable tissues. WOPN forms after a minimum of four weeks following the onset of acute necrotizing pancreatitis. Any fluid collection within the pancreas that persists for this period and contains necrotic material should be identified as WOPN. Its contents may be sterile or infected, depending on the clinical progression.

Severe acute pancreatitis can also result in vascular complications. Pseudoaneurysms are reported in 3.5–10% of cases, with the splenic artery (40%) and gastroduodenal artery (30%) being the most commonly affected. Another common vascular complication is thrombosis, particularly of the splenic vein, which can result in portal hypertension.