What is the staging process leading to walled-off necrosis (WON) in acute pancreatitis?

Staging Process Leading to Walled-Off Necrosis (WON)

WON develops through a predictable temporal evolution over approximately 4 weeks following the onset of acute necrotizing pancreatitis, progressing from acute necrotic collections to mature, encapsulated collections with well-defined inflammatory walls. 1

Timeline and Evolution

The staging process follows a clear temporal sequence:

Week 1-4: Acute Necrotic Collection (ANC) Phase

• Necrosis of pancreatic parenchyma and/or peripancreatic tissues occurs in 10-20% of patients with acute pancreatitis during the initial inflammatory phase. 2
• During the first 4 weeks after disease onset, necrotic tissue remains unencapsulated and is classified as an acute necrotic collection (ANC). 1, 3
• This early phase is characterized by poorly demarcated necrotic debris mixed with fluid, without a defined wall. 3
• Approximately one-third of deaths occur during this early phase, primarily from multiple organ failure rather than infection. 4, 5

Week 4 and Beyond: WON Formation

• After 4 weeks or more from the onset of acute pancreatitis, the collection matures into walled-off necrosis, characterized by a well-defined, enhancing inflammatory wall that encapsulates the pancreatic and/or peripancreatic necrosis. 1
• This maturation process creates a distinct capsule that separates the necrotic material from surrounding tissues. 1, 6
• The collection becomes heterogeneous, containing varying amounts of liquid and solid necrotic material. 6
• WOPN was formerly termed "pancreatic abscess" under older classification systems. 7

Key Pathophysiologic Stages

Initial Inflammatory Response

• Acute pancreatitis triggers acinar cell destruction and acute inflammation of the pancreas. 1
• The severity determines whether necrosis develops—most patients (80-85%) develop mild disease without necrosis. 1

Necrosis Development

• In 10-20% of cases, necrosis extends into pancreatic parenchyma and/or peripancreatic tissues. 2
• The extent of necrosis directly correlates with mortality risk and complications. 5

Encapsulation Process

• Over 4 weeks, granulation tissue forms around the necrotic collection, creating the characteristic inflammatory wall. 1
• This wall formation distinguishes WON from earlier acute necrotic collections. 1

Clinical Implications of Staging

The 4-week threshold is critical because interventions for infected necrosis should be delayed until this maturation occurs whenever possible, as this timing results in lower mortality. 4

Management Based on Stage

• During the ANC phase (weeks 1-4): Focus on supportive care including fluid resuscitation, early enteral nutrition, and monitoring for complications. 4
• After WON formation (≥4 weeks): Intervention may be considered if the collection becomes infected or symptomatic, using a step-up approach starting with percutaneous or endoscopic drainage. 4, 2

Infection Risk by Stage

• Infection of pancreatic necrosis occurs in 20-40% of patients with severe acute pancreatitis, typically after 7-10 days of illness. 4, 5
• Infected necrosis with organ failure carries mortality rates of 35.2%, while infected necrosis without organ failure has mortality of only 1.4%. 5

Imaging Correlation with Staging

• Contrast-enhanced CT or MRI should be performed 72-96 hours after symptom onset to assess for necrosis development. 5
• Early imaging may underestimate the extent of necrosis before full demarcation occurs. 7
• MRI and endoscopic ultrasound are superior for evaluating solid necrotic debris within collections. 6

Common Pitfalls

Avoid intervening too early (before 4 weeks) unless absolute indications exist, such as abdominal compartment syndrome unresponsive to conservative management, acute ongoing bleeding when endovascular approach fails, or bowel ischemia. 4 Early intervention before wall maturation increases morbidity and mortality compared to delayed intervention after WON formation. 2, 7