Evaluation of whether the ultrasonographic onion skin sign is specific for the diagnosis of an appendiceal mucocele

Purpose To evaluate whether the onion skin sign on ultrasonography (US) of the lower abdomen is specific for the diagnosis of an appendiceal mucocele. Methods Our study included 231 lesions from 229 patients in whom transabdominal US detected lesions in the lower abdomen and who had definitive pathological diagnoses in the small bowel, ileocecal valve, cecum, appendix, mesentery, adnexa uteri or pelvic cavity outside the major organs. Patients with metastatic tumors were excluded. We reviewed the reports and images from transabdominal US and checked for the presence of the onion skin sign. The sensitivity, specificity, and accuracy of the sign for the diagnosis of an appendiceal mucocele were calculated. Results Five (2.2 %) of the 231 lesions showed the onion skin sign on US and all were diagnosed with an appendiceal mucocele. The sensitivity, specificity, and accuracy of the sign for the diagnosis of an appendiceal mucocele were 63, 100, and 99 %, respectively. Conclusions The onion skin sign in the lower abdomen appears to be specific for the diagnosis of an appendiceal mucocele. In cases where the onion skin sign is obviously present in the lower abdomen on US, an appendiceal mucocele should be considered as the leading diagnosis.