Abdominal abscess can follow primary intraabdominal disease such as diverticulitis, appendicitis, biliary tract disease, pancreatitis, or perforated viscus; abdominal surgery; penetrating and blunt abdominal trauma; and bacteremic spread of infection from a distant source to an intra-abdominal site, particularly in the immunocompromised patient. The mortality rate is reported as high as 40%; however, recent studies suggest a mortality rate of 20%, with the reduction most likely the result of earlier diagnosis. The three distinct anatomic locations of abdominal abscesses are intraperitoneal, retroperitoneal, and visceral, the last developing in liver, gallbladder, spleen, pancreas, and kidney. Liver abscesses are covered in Chapter 45, Pyogenic Liver Abscess and pancreatic abscesses in Chapter 46, Infectious Complications of Acute Pancreatitis.

Bacteria in the peritoneal cavity are subject to the normal influences of gravity and pressure gradients. If the patient is upright, peritoneal fluid will collect within the dependent portion of the pelvis. Patients who are sick with an intraperitoneal process such as peritonitis typically are supine, and their dependent positions are the subphrenic space and the pericolic gutters. Pressure gradients within the peritoneal cavity are due to motion of the diaphragm. With expiration, relative negative pressure beneath the diaphragm sets up a current of movement that favors fluid moving from the pericolic space to the subhepatic and subphrenic space. These currents allow the bacteria to come into contact with the diaphragmatic surface, which has lymphatic fenestrations and is an important means of clearing bacteria from the celomic cavity.