The clinician's differential diagnosis included intradermal nevus, trichoepithelioma, and basal cell carcinoma. The tumor was excised and subjected to histopathological examination.

The clinical manifestations of ectopic hidradenoma papilliferum are similar to those of anogenital hidradenoma papilliferum. Lesions range in size from 0.5 to 6.0 cm; more than 85% of the lesions are about 1.5 cm in size⁹. The most common site of ectopic hidradenoma papilliferum is the head and neck region^6-9. Lesions have occurred on the eyelid (3 lesions), forehead (2 lesions), face (site unspecified, 2 lesions), external auditory canal (1 lesion), postauricular area (1 lesion), scalp (1 lesion), and nose (1 lesion). Other lesion sites include the axilla, arm, thigh and back. Although the reason that most cases of hidradenoma papilliferum occur in the head and neck region has not been clarified, their location may be related to the distribution of apocrine glands. Apocrine glands are heavily concentrated in the anogenital area and axillae, most cases of hidradenoma papilliferum are located in the anogenital area. Modified apocrine glands are normally found in the external auditory ear canal, eyelid and breast. Thus the location of ectopic hidradenoma papilliferum corresponds to these common sites of apocrine glands and modified apocrine glands^5-7. In our patient, it is possible that the heterotrophic apocrine gland normally found in the nasal skin may be related to the development of tumor. Therefore, the heterotrophic apocrine gland in the resection specimen may be heterotrophic apocrine glands adjacent to the tumor in the nasal skin⁸.

Histopathologic examination showed solitary, well-circumscribed, cystic spaces in the dermis surrounded by tissue with no connection to the overlying epidermis. Within the cystic spaces, multiple papillary projections into the cavity were seen. The lumen is usually lined with double or multiple rows of columnal cells or simple cuboidal cells with active "decapitation" secretion like that found in the apocrine gland. Although these invaginations that extend downward from epidermis and papillary projections can be found in syringocystadenoma papilliferum, the absence of connection in the papillary structure distinguishes hidradenoma papilliferum from syringocystadenoma papilliferum. Immunohistochemically, the hidradenoma papilliferum expresses EMA, CEA, gross cystic disease fluid protein (GCDFP)-15 and human milk fat globule membrane antigen (HMFG) and are usually positive for the markers of apocrine differentiation. Syringocystadenoma papilliferum usually express AE1/AE3, EMA and CEA, but the results of markers of apocrine differentiation including GCDFP-15 and HMFG are variable.

Although the exact pathogenesis of hidradenoma papilliferum is unclear, the fact that these tumors have only been reported after puberty suggests that estrogen receptor expression may be involved⁹. Regardless of the site and size, the treatment of choice is local excision and recurrence is unusual. Recently, some authors found similarities between hidradenoma papilliferum arising from extra-genital area and apocrine papillary cystadenoma^11,12. The similarity of histopathology and apocrine differentiation in both tumors suggest that they may be identical tumors, but further study is needed.

The differential diagnosis includes syringocystadenoma papilliferum, tubular apocrine adenoma, and clear cell (apocrine) hidradenoma. Syringocystadenoma shows some cystic invaginations extending downward from the epidermis as well as plasma cell infiltration. In tubular apocrine adenoma, some dilated tubules with papillary projections extend into the lumen. In clear cell hidradenoma, the lesions are constructed with apocrine-like tubular structures and clear cells can be seen⁶.

The prognosis for hidradenoma papilliferum is good^1-4. Malignant transformation of these tumors is rare and there are only 2 reports of carcinoma arising in a pre-existing hidradenoma papilliferum¹¹.