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Hybrid cyst
Hybrid cyst
The term hybrid cyst was originally introduced to describe a cyst in which the upper half showed features of an epidermoid cyst whereas the lower portion comprised a trichilemmal cyst.1 There was a sharp distinction between the two linings. The spectrum was subsequently expanded to include cysts with a variety of dual linings including epidermoid cyst and pilomatrixoma, trichilemmal cyst and pilomatrixoma, epidermoid with both trichilemmal and pilomatrical features, and eruptive vellus hair cyst with trichilemmal cyst.2โ5 Cystic lesions with follicular germinative differentiation usually represent cystic trichoblastomas, cystic panfolliculomas, or even cystic follicular hamartomas.6
There are also a number of reports of cysts combining the features of eruptive vellus hair cyst and steatocystoma.2,7โ10 These are intriguing, given the potentially shared molecular pathogenic features of these processes likely involving keratin 17. Epidermoid cyst with apocrine hidrocystoma and pilomatrixoma with cystic trichilemmoma have also been described.11,12 A lesion combining isthmic-catagen, pilomatrical, and syringocystadenoma papilliferum components has been described.13 Hybrid cysts with follicular and apocrine differentiation seem to be more common on the eyelid.14 Since all of these cysts are derived from various components of the hair follicle, their combination is not surprising. The characteristic cyst of Gardner syndrome, in which epidermoid features merge with pilomatrixoma, can also be regarded as a type of hybrid cyst.15 The hybrid cysts associated with Gorlin-Goltz syndrome have been described under epidermoid cyst.