Abstract

Apocrine hidrocystomas are uncommon dermatologic cystic lesions, which despite their differentiation, commonly manifest on sites divergent from typical distribution of apocrine glands.  Lesions frequently present around the head or neck, rarely in the axilla.  A review by Anzai et al. reported only 7/167 cases of apocrine hidrocystomas involved the axilla.  Eccrine hidrocystomas similarly have a predilection for involvement of the face, and to a lesser degree, trunk, popliteal fossa, external ear, or vulva.  Histologic overlap exists between the two entities.  Distinguishing characteristics can be subtle and include decapitation secretion and presence of myoepithelial cells, both of which are associated with apocrine differentiation.  Immunohistochemical markers specific for myoepithelial cells can be utilized as further evidence or to elucidate etiology in ambiguous cases.  Further, hidrocystomas often present as solitary papules.  The presence of multiple papules has been associated with focal dermal hypoplasia, Grave’s disease, and raises consideration for Schopf-Schulz-Passarge or Goltz syndromes.  Our case involves a 46-year-old female who presented with two firm dermal papules in the left axilla, measuring 4x3mm and 3x3mm.  Histologic evaluation demonstrated cystic spaces, and the cyst walls were lined by unusual mixed apocrine and eccrine differentiation.  Distinct histomorphologic characteristics of each lineage were identified in each specimen, corroborated by immunohistochemical evaluation (Smooth muscle actin (SMA), S100, and Periodic acid-Schiff (PAS)/ Periodic acid-Schiff with Diastase (PAS-D)).  The distinct features of this case include the uncommon presenting location, presence of multiple lesions, unique mixed apocrine and eccrine composition within the same cyst, and inclusion of accompanying immunohistochemical staining profile.