Ultrasound Features of Nodular Hidradenoma: A Case Series of 27 Patients.

Objective: The aim of this study was to systematically investigate the ultrasonographic features of nodular hidradenoma (NH).

Methods: A retrospective analysis was used to systematically summarize the ultrasonographic data of 27 patients diagnosed with NH by surgical pathology, including 13 eccrine nodular hidradenomas (ENH) and 14 apocrine nodular hidradenomas (ANH).

Results: All instances of NH presented as solitary, well-defined lesions that infiltrated the dermis and subcutaneous fat layer, characterized by a heterogeneous hypoechoic internal solid component on ultrasound imaging. Color Doppler ultrasound revealed blood flow signals of Grade 2 or higher within 74% (20/27) of the lesions. Solid + cystic (cystic >50%) NH (14/27, 51.4%) were typically large and predominantly had an oval shape (11/14, 78.5%). Their distinctive sonographic features included the presence of inner septa within the cystic area (8/14, 57.1%), "snow falling" sign (7/14, 50%), or "fluid-fluid level" sign (7/14, 50%). Solid + cystic (cystic ≤50%) NH exhibited a lobulated morphology in all instances (5/5, 100%). No inner septa, "snow falling" sign or "fluid-fluid level" sign was observed within the cystic regions. The solid NH (8/27, 29.7%) morphology predominantly featured lobulation (6 out of 8, 75%). Ultrasound analysis revealed distinct differences between ENH and ANH. ENH were more lobulated, while ANH were predominantly oval. ANH were mainly solid + cystic (cystic >50%), whereas ENH were mostly solid. Inner septa, "snow falling" sign, and "fluid-fluid level" sign frequencies were similar in both groups, correlating more with cystic-solid composition than pathological subtype.

Conclusions: Ultrasonographic features of lobulated morphology and the presence of inner septa, "snow falling" sign or "fluid-fluid level" sign in the cystic region provide strong support for the diagnosis of NH.