The nonvisualized appendix: incidence of acute appendicitis when secondary inflammatory changes are absent.

OBJECTIVE: The objective of our study was to assess the importance of nonvisualization of the appendix and its association with acute appendicitis on helical CT when secondary inflammatory changes are absent.
MATERIALS AND METHODS: After we received institutional review board approval, CT scans of 366 consecutive patients obtained for lower abdominal or right lower quadrant pain and to rule out appendicitis were retrospectively reviewed. Images were reviewed by an experienced abdominal radiologist and compared with the formal interpretation. The amount of right lower quadrant and pericecal fat was quantified on a scale of 0 (none) to 2 (abundant). Patients with a nonvisualized appendix and other findings compatible with acute appendicitis-such as abscess formation, localized perforation, periappendiceal fat stranding, or appendicolith-were excluded.
RESULTS: The appendix could not be visualized by both reviewers in 46 (13%) of 366 cases. CT findings indicated another cause for the patient's symptoms in 12 cases (26%), including gastrointestinal and genitourinary processes. An alternate diagnosis was subsequently reached in 11 additional patients (24%) with follow-up imaging or clinical evaluation. Only one patient (2%) with a nonvisualized appendix had acute appendicitis, proven by surgical pathology. In this patient, there was paucity of fat in the right lower quadrant.
CONCLUSION: In the absence of a distinctly visualized appendix and secondary inflammatory changes, the incidence of acute appendicitis is low. Nonvisualization of the appendix even when a small amount of fat is present in the right lower quadrant may safely exclude acute appendicitis if no secondary CT findings are present.