Alcoholism and biliary tract disease account for the vast majority of cases of acute pancreatitis in adults in the United States. In mild acute edematous pancreatitis, the gland is swollen and edematous but otherwise well-preserved. Normal structure and function can return after resolution. Moderate-to-severe acute pancreatitis, as in this case, is characterized by proteolytic destruction of pancreatic parenchyma, hemorrhage from blood vessel wall necrosis, fat necrosis, and accompanying inflammation which may be widespread in the abdominal cavity. Secondary bacterial infection may occur after 3 to 4 days resulting in suppurative necrosis or abscess formation. If the patient survives acute phlegmonous pancreatitis, the end result may be pancreatic fibrosis, calcification and duct dilatation. Severe upper abdominal pain in a band-like distribution sometimes radiating to the back is almost always present. Elevation of serum amylase and lipase are characteristic although not completely specific.

RADIOLOGY: The sonographic and/or CT appearance of the pancreas is normal in about 30% of patients with the clinical diagnosis of acute pancreatitis. When pancreatic abnormality is present, it may be diffuse or focal, and when focal usually involves the head or the tail.

SONOGRAPHY: The involved area is usually but not always hypoechoic due to the presence of edema. There may be pancreatic ductal dilatation associated with focal enlargement of the head of the pancreas. In mild pancreatitis the swelling of the gland and dilatation of the pancreatic duct usually subside within days to weeks as the echogenicity of the parenchyma returns to normal.

A phlegmon is a spreading diffuse inflammatory edema of soft tissues usually accompanied by pancreatic fluid and usually emanating from the body and tail. Spread usually occurs in an anterior direction into the area of the lesser sac and anterior pararenal spaces. More posterior extension is usually prevented by the anterior pararenal fascia (Gerota's fascia). The phlegmon may involve the right anterior pararenal space and may simulate gallbladder disease by producing thickening of the gallbladder wall and a pericholecystic fluid collection. Extension may also occur into the transverse mesocolon and small bowel mesentery and inferiorly into the retroperitoneum and pelvis. A phlegmonous mass is hypoechoic and has fairly good through transmission whereas a peripancreatic fluid collection is relatively anechoic with excellent through transmission. The sonographic appearance of pancreatic hemorrhage varies with its age; acutely the hemorrhage can appear homogeneously echogenic. The degree of echogenicity decreases with time and so the mass assumes a complex appearance, and by 3-5 days the area of hemorrhage is sonolucent.

CT: The gland may be focally or diffusely enlarged, and the involved portion may have a well-defined outline or its interface with the peripancreatic fat may be ill-defined. The attenuation number of the involved pancreas is reduced on non-contrast scans, presumably secondary to the presence of edema, and it is the spread of this edema into the surrounding fat that gives the pancreatic outline its indistinct appearance.

Peripancreatic phlegmons are fairly low attenuation on unenhanced scans but enhance with intravenous contrast whereas their accompanying peripancreatic fluid collections do not. The pancreatic parenchyma will also enhance with intravenous contrast, so that contrast enhanced scans give a better distinction between gland, fluid and phlegmon than unenhanced scans. CT gives a better global depiction of the extent of pancreatitis than sonography. CT can make the specific diagnosis of hemorrhagic pancreatitis when high attenuation values (50-75H) are present in the phlegmon. However, these high density numbers will only persist for 24-48 hr after the initial event. When hemorrhagic pancreatitis is suspected, a CT should be performed immediately with a preliminary non-contrast scan as intravenous contrast may obscure the high density of a hemorrhage. Pseudoaneurysms can occur secondary to acute pancreatitis and most commonly involve the splenic artery but can involve other branches of the celiac axis, the superior mesenteric artery or intrapancreatic arteries. Doppler sonography or dynamic CT should be performed whenever this complication is suspected.