Vermiform appendix and peritoneum

A) Pathology of the Appendix

Acute Appendicitis

• Acute bacterial infection of the appendix; usually precipitated by obstruction of the lumen
• Affects mainly older children and young adults (20–30 years)

Pathogenesis:

• Obstruction: fecalith (stone made of feces), gallstone, tumor, or ball of worms
• Obstruction + secretion of mucinous fluid → ↑ intraluminal pressure → collapse of draining veins
• Obstruction + ischemic injury → bacterial proliferation + edema + exudate

Morphology — 3 stages:

1. Acute early appendicitis: focal erosions in mucosa covered with fibrin; serosa becomes dull, granular, red
2. Acute suppurative appendicitis: abscess formation within the wall + ulceration + foci of mucosal necrosis
3. Acute gangrenous appendicitis: large areas of hemorrhagic ulceration + green-black gangrenous necrosis through the wall to the serosa

Clinical:

• Progression: mild periumbilical discomfort → anorexia, nausea, vomiting → RLQ tenderness → deep, constant pain in the right lower quadrant
• Fever + leukocytosis appear early
• Differential: mesenteric lymphadenitis, gastroenteritis, pelvic inflammatory disease, rupture of ovarian follicle, ectopic pregnancy, Meckel diverticulitis

Tumors of the Appendix

Carcinoids (most common appendiceal tumors)

• Arise from enterochromaffin (neuroendocrine) cells
• All carcinoids are potentially malignant (can invade + metastasize)
• Often discovered incidentally in appendices removed for appendicitis

Mucocele

• Not a neoplasm; dilation of the appendiceal lumen by mucinous secretion
• Caused by non-neoplastic obstruction (usually associated with a fecalith)

Mucinous neoplasms

• Benign: mucinous cystadenoma
• Malignant: mucinous cystadenocarcinoma → invades wall → disseminated intraperitoneal cancer = pseudomyxoma peritonei

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B) Pathology of the Peritoneum

Acute Peritonitis

• Inflammation of the peritoneum; starts localized → becomes diffuse (generalized)
• Causes:
  • Bacterial: perforation of abdominal organs, acute pancreatitis, spontaneous bacterial peritonitis (in liver cirrhosis)
  • Chemical: bile (gallbladder perforation), pancreatic juice (fat necrosis), gastric juice (perforated ulcer), blood (rupture of spleen/liver/ectopic pregnancy)
• Morphology: serosal surface hyperemic; glistening sheen lost; creamy fibrinopurulent exudate covers intestines
• Clinical: acute abdomen (nausea, vomiting, abdominal pain/distention, fever, septic shock); consequences: adhesions, abscesses, shock, death; high mortality especially in elderly

Pneumoperitoneum

• Accumulation of air/gas in the peritoneal cavity
• Most common cause: perforated abdominal organ
• Causes: perforated peptic ulcer, bowel obstruction, ruptured diverticulum, penetrating trauma, ruptured IBD (megacolon), necrotizing enterocolitis, bowel cancer

Ascites

• Collection of excess fluid in the peritoneal cavity
• Clinically detectable after ≥500 mL; can reach several liters → massive abdominal distention
• Serous fluid: ~3 mg/dL protein (albumin); same electrolyte concentrations as blood
• Fluid analysis:
  • Neutrophils → infection
  • RBCs → possible disseminated intra-abdominal cancer
• Long-standing ascites → hydrothorax, typically on the right side
• Most common cause: cirrhosis/severe liver disease or heart failure → ↑ venous pressure + hypoalbuminemia + Na⁺ retention → portal hypertension → ascites

Neoplasms of the Peritoneum

• Primary neoplasms are rare; most typical = metastatic tumors
• Benign: mesothelioma (asbestos exposure)
• Malignant: peritoneal carcinosis — from stomach, pancreas, colon, ovary

💡 High-yield: Appendicitis = fecalith obstruction; 3 stages (early → suppurative → gangrenous). Carcinoid = most common appendiceal tumor; potentially malignant; incidental finding. Pseudomyxoma peritonei = mucinous cystadenocarcinoma. Peritonitis: acute abdomen + fibrinopurulent exudate; high mortality. Ascites: neutrophils = infection; RBCs = cancer; most common cause = cirrhosis. Pneumoperitoneum = perforated organ.