Peritonitis

R. Brooke Jeffrey, MD

Key Facts

Terminology

Infectious or inflammatory process involving peritoneum or peritoneal cavity

Imaging

Best imaging clue: Ascites, symmetric enhancement of peritoneum with abdominal fat stranding on CECT
Peritoneal fluid, septations, thickened echogenic mesentery on US
Best imaging tool: CECT or US

Top Differential Diagnoses

Peritoneal carcinomatosis
Benign ascites
Pseudomyxoma peritonei
Hemoperitoneum

Pathology

Pus in peritoneal cavity, thickened peritoneum or mesentery

Clinical Issues

Most common signs/symptoms: Fever, abdominal pain, distension

Diagnostic Checklist

Consider peritoneal carcinomatosis
Imaging pearls
- Symmetric enhancement of thickened peritoneum
- Enlarged fallopian tube with fluid-fluid level and complex adnexal mass in PID
- Inflammatory changes with adjacent fat of mesentery and omentum

(Left) Axial NECT of bacterial peritonitis in Crohn disease shows loculations of ascites throughout the abdomen, including thick-walled collections between bowel loops in the mesentery

. A thickened parietal peritoneum

is further evidence of an inflammatory or exudative nature. (Right) Axial NECT in the same patient shows infiltration of omental

and mesenteric fat due to a generalized inflammatory process.

(Left) Axial CECT in a 50-year-old man who presented with alcoholic cirrhosis and abdominal pain demonstrates a nodular liver and splenomegaly with ascites, findings consistent with the patient’s diagnosis of cirrhosis. Note the loculated ascites and thickened, enhancing parietal peritoneum

. (Right) Axial CECT in the same patient reveals a gas bubble

evident within fluid, a classic imaging finding of bacterial peritonitis.

TERMINOLOGY

Definitions

Infectious or inflammatory process involving peritoneum or peritoneal cavity

IMAGING

General Features

Best diagnostic clue
- Ascites, symmetric enhancement of peritoneum with abdominal fat stranding
Location
- Peritoneal surface, mesentery, omentum
Size
- Variable, may be focal or diffuse
Morphology
- Symmetric thickening of peritoneum

Radiographic Findings

Radiography
- Evidence of ascites: > 500 mL fluid required for plain film diagnosis
  - Flank bulging
  - Indistinct psoas margin
  - Small bowel (SB) loops floating centrally
  - Lateral edge of liver displaced medially (Hellmer sign); visible in 80% of patients with significant ascites
  - Pelvic “dog’s ear” present in 90% of patients with significant ascites
  - Medial displacement of cecum and ascending colon present in 90% of patients with significant ascites
- ± free air
- Hydropneumoperitoneum
- Air in lesser sac with perforated gastric ulcer

Fluoroscopic Findings

Upper GI
- Perforated ulcer with contrast leak
Contrast enema
- Diverticular perforation in diverticulitis

CT Findings

CECT
- Ascites, enhancing peritoneum with smooth thickening, infiltration, and soft tissue fat stranding within mesentery
- ± gas bubbles, low-attenuation nodes in TB peritonitis

MR Findings