Management of Fluid Collections in Acute Pancreatitis

David S. Pryluck, Charan K. Singh and Timothy W.I. Clark

Sterile and infected fluid collections are common local complications of acute pancreatitis. Initial diagnosis and management of these fluid collections commonly rely on image-guided techniques like fine-needle aspiration and percutaneous catheter drainage. Fine-needle aspiration (often at multiple sites) documents or excludes infection and can be used to guide antimicrobial therapy. Early diagnosis of these conditions can dramatically alter therapy; for example, a diagnosis of infected necrosis may warrant surgery or an aggressive attempt at percutaneous drainage to stabilize such patients.1,2 Percutaneous catheter drainage can also be used to relieve symptoms caused by sterile fluid collections, such as gastric outlet obstruction or abdominal pain.

Just as acute pancreatitis exists as a spectrum of severity, the fluid collections associated with the disease also exist as a spectrum. After the 1992 International Symposium on Acute Pancreatitis, a classification system describing acute pancreatitis–associated fluid collections was implemented. It defined the terms acute fluid collection, pancreatic necrosis, acute pseudocyst, and pancreatic abscess based on pathophysiology and prognostic and treatment implications.³ Ambiguity ensued in the use of these terms in medical literature and clinical practice, as well as difficulty in radiographically distinguishing among them. A revised classification scheme has been proposed that seeks to clarify the nomenclature used to describe acute pancreatitis–associated fluid collections.^4,5 It proposes that acute fluid collections be referred to as acute peripancreatic fluid collections (sterile or infected), pancreatic necrosis as acute postnecrotic pancreatic/peripancreatic fluid collections (sterile or infected), acute pseudocysts as pancreatic pseudocysts (sterile or infected), and pancreatic abscesses as walled-off pancreatic necrosis (sterile or infected).^4,5

Multidetector computed tomography (CT) is the preferred modality used to diagnose and evaluate pancreatitis and associated fluid collections. Dynamic contrast-enhanced CT depicts the distinction between necrotizing and interstitial pancreatitis by demonstrating nonenhancing, nonviable areas of pancreas with necrosis. CT can depict all but the mildest forms of acute pancreatitis, demonstrate most of the major complications, and guide percutaneous needle aspiration and catheter drainage.6,7 Ultrasonography, endoscopic retrograde cholangiopancreatography (ERCP), and magnetic resonance imaging (MRI) have secondary or adjunctive utility in pancreatitis, for example, in monitoring the size of pseudocysts or defining the relationship between a fluid collection and the pancreatic duct before percutaneous or surgical drainage.

This chapter will review current practices in percutaneous management of fluid collections associated with acute pancreatitis, both diagnosis and treatment. Sterile and infected collections will be considered. Established and new nomenclature will be used as appropriate.

Indications

Percutaneous therapy for acute pancreatitis can be effectively used to treat complications previously managed only surgically; it can be performed repeatedly and does not preclude any subsequent form of therapy. The decision to drain and the type of drainage are based on the associated CT findings, results of needle aspiration, and the patient’s clinical condition.8–11

Pancreatic necrosis is defined as diffuse or focal areas of nonviable pancreatic parenchyma, typically associated with peripancreatic fat necrosis. This definition includes both sterile and infected necrosis.⁴ Normal nonenhanced pancreas has a CT attenuation of 30 to 50 Hounsfield units (HU) and shows homogeneous enhancement to 100 to 150 HU with contrast material. A focal or well-defined zone of nonenhancing parenchyma larger than 3 cm in diameter or greater than 30% of the pancreas is a reliable CT finding of necrosis. Approximately 30% of patients with necrotizing pancreatitis develop secondary bacterial infection of necrotic debris.¹² The rationale for not using percutaneous drainage is therefore to avoid potentially converting a sterile collection into an infected one. With careful catheter care and judicious use of antimicrobial agents, secondary infection can be avoided and selected patients with sterile necrosis can benefit from percutaneous drainage. Liquefied necrosis that yields a brownish fluid containing debris is amenable to percutaneous drainage. Percutaneous catheter drainage may also decrease morbidity by avoiding surgery, or it can be used to temporize a patient before surgery.

Acute fluid collections (acute peripancreatic fluid collections per revised nomenclature) occur in up to 50% of patients early in the course of acute pancreatitis, usually within the first 48 hours. These collections result from either a rupture of a small branch of the pancreatic duct or from edema associated with parenchymal and/or peripancreatic inflammation.⁴ They contain protein-rich fluid, may or may not contain high concentrations of pancreatic enzymes, and resolve spontaneously in about 50% of cases.⁴ On CT they are low-attenuation, poorly defined collections of fluid with no recognizable capsule or wall; this radiographically distinguishes them from pseudocysts. Acute peripancreatic fluid collections can be sterile or infected, and intervention is usually unnecessary beyond needle aspiration to rule out infection.¹³ The size and location of acute peripancreatic fluid collections can vary; large fluid collections are commonly localized in the lesser sac and anterior pararenal space, left more commonly than right.¹⁴

Acute pseudocysts (pancreatic pseudocyst per revised nomenclature) are round or oval collections of pancreatic fluid enclosed by a wall of fibrous or granulation tissue that arise at least 4 weeks following acute pancreatitis.4,15 It is estimated that 30% to 50% of acute peripancreatic fluid collections progress to pseudocyst formation.⁴ Although most pseudocysts resolve spontaneously, pseudocysts larger than 5 cm and those increasing in size are less likely to resolve and can be considered for percutaneous drainage. Severe pain and gastrointestinal (GI) or biliary tract obstruction are other indications for percutaneous drainage of noninfected pseudocysts. Approximately 10% of pseudocysts become secondarily infected, presumably from bowel seeding.⁴

Pancreatic abscesses (walled-off pancreatic necrosis per revised nomenclature) are circumscribed intraabdominal collections of pus in proximity to the pancreas. They contain little or no necrosis and usually occur 4 weeks or more after the onset of acute pancreatitis. Differentiation of an abscess from infected necrosis is crucial for appropriate clinical management. Abscesses are infected fluid collections that can be drained percutaneously, whereas infected necrosis develops in relatively solid or incompletely liquefied necrotic tissue and generally requires surgical débridement. Needle aspiration is crucial because visually, the CT appearance of a low-attenuation zone of infected necrosis may be similar to that of an abscess.

Infected necrosis (infected postnecrotic pancreatic/peripancreatic fluid collection [PNPFC] per revised nomenclature), as the name suggests, is infected pancreatic or peripancreatic necrotic tissue. Associated mortality rates range from 15% to 60%, and this is considered the most severe complication of pancreatitis. Traditionally, infected necrosis is an indication for surgical débridement. The rationale against use of percutaneous drainage has been that it is ineffective because necrotic material blocks drainage catheters. Catheter drainage may temporize a patient preoperatively; it does not preclude surgery and may optimize surgical timing. A trend toward minimally invasive organ-preserving therapy with aggressive percutaneous maneuvers is supported by reports of complete success with catheter drainage alone in some cases of liquefied infected necrosis. This approach mandates use of large-bore catheters, with extensive lavage and catheter manipulations performed over multiple sessions.16–20