With obstruction, bacterial overgrowth occurs. Mucus accumulates in the lumen proximal to the obstruction, and intraluminal pressure increases. Impairment of lymphatic and venous drainage leads to ischemic mucosal ulceration, bacterial invasion, and transmural inflammation. Continued inflammation and ischemia, if untreated, may lead to gangrenous appendicitis, perforation, or both. Perforation may be contained, causing a phlegmon or abscess.
Free perforation causes generalized peritonitis.
In the classic sequence, the pain is initially caused by obstruction of the appendiceal lumen. The pain of appendicitis has the qualities of midgut visceral pain and is referred to the periumbilical or epigastric areas. It may be cramping or aching but is often vague. Within 12 to 24 hours, inflammation usually becomes transmural, involving the adjacent parietal peritoneum. At this time, patients may note exacerbation of pain by coughing, sneezing, or movement.