The Onion Skin Sign: A Specific Sonographic Marker of Appendiceal Mucocele

Objective. To evaluate the association of the onion skin sign as a sonographic marker for appendiceal mucocele. Methods. The sonographic onion skin sign was considered specific for the preoperative diagnosis of appendiceal mucocele. Therefore, detection of this sign in a mass located in the right lower abdomen, unrelated to the female reproductive organs, indicated surgical intervention with a presumptive diagnosis of appendiceal mucocele. From 1998 through 2001, female patients who were found to have atypical cysts containing this sign underwent surgery. The cases were closely followed, and intraoperative findings and final histologic diagnoses were recorded. Results. Appendiceal mucocele was the final diagnosis in all 7 patients in whom the onion skin sign was observed. One additional patient had an appendiceal mucocele with a sonographic picture of a clear tubular cystic structure. Conclusions. A sonographically layered cystic mass in the right lower quadrant of the abdomen in the presence of a normal ovary strongly suggests the diagnosis of appendiceal mucocele. Recognition of the sonographic onion skin sign in a cystic mass in the right lower quadrant may facilitate the accurate preoperative diagnosis of appendiceal mucocele.