Appendicitis is the most common cause of surgical abdomen affecting approximately 10% of the population. It can occur at any age; however, it is most common between the ages of 10 and 30 years. Appendicitis is caused by luminal obstruction from lymphoid hyperplasia, fecalith, foreign body, stricture, or tumor. In the United States, virtually every patient with suspected appendicitis is evaluated with computed tomography (CT) or ultrasonography.

Historically the diagnosis of acute appendicitis was primarily based on clinical information, and radiologic evaluation was performed only in ambiguous cases. This approach resulted in discovery of normal appendix at surgery in 10-20% of patients, and in 30-45% of young women, in whom numerous gynecologic processes may mimic the symptoms and signs of appendicitis. Appropriate imaging can significantly reduce rate of false-negative appendectomies to less than 4%.

Appendicitis usually begins with vague periumbilical or epigastric pain caused by distension of the appendix. As inflammation extends to anterior abdominal wall peritoneum, the pain shifts to the right lower quadrant and the physical examination should demonstrate presence of peritonitis with right lower quadrant rebound tenderness, guarding, and rigidity.

The presentation of appendicitis may vary considerably if the appendix is not in contact with anterior abdominal wall. Retrocecal appendicitis may cause flank pain and psoas sign on physical examination. Pelvic appendicitis can present with discomfort on pelvic examination. Urinary frequency, dysuria, microscopic hematuria, and pyuria will occur if the appendix is adjacent to bladder. Diarrhea may be present if the appendix is adjacent to sigmoid colon.

Nearly all patients experience anorexia and majority have nausea with few episodes of vomiting. Appendicitis is unlikely if the patient is hungry (look for full stomach on CT).

Low-grade fever is common. Leukocytosis and/or abnormal cell differential counts are found in 96% of patients. High fever and leukocytosis above 20,000 cells/µL suggest perforation.

These symptoms and signs may be very subtle in elderly or immunocompromised patients.

Appendicitis can be mimicked by a wide variety of disorders. The most common conditions discovered at surgery when acute appendicitis is erroneously diagnosed are nonspecific abdominal pain, mesenteric lymphadenitis, acute pelvic inflammatory disease, ruptured ovarian follicle or corpus luteum cyst, and acute gastroenteritis. Clinical differential diagnosis also includes inflammatory bowel disease, acute cholecystitis, perforated ulcer, acute pancreatitis, acute diverticulitis, strangulating intestinal obstruction, urolithiasis, pyelonephritis, and typhlitis in immunocompromised patients. In women, ectopic pregnancy and ovarian torsion should also be considered.

CT is very accurate and is the study of choice for evaluation of suspected appendicitis for most patients. Young children can initially be evaluated with ultrasonography (followed by CT if ultrasonographic findings are ambiguous). CT can accurately diagnose complications of appendicitis as well other pathologic conditions that may mimic appendicitis.

CT with intravenous (IV) contrast should be performed 2.5-3 hours after administration of oral contrast to fill distal ilium, cecum, and hopefully the appendix with oral contrast. Appendicitis can be diagnosed without oral or IV contrast; however both are very helpful, especially for diagnosis of other pathology. If the cecum is not yet opacified by contrast and findings are ambiguous, CT scan can be repeated after appropriate delay. Rectal contrast can be helpful in select cases as it may increase the likelihood of appendicial filling by contrast. Routine use of rectal contrast is not indicated due to patient discomfort and absence of clear advantage in accuracy of diagnosis.