Follicular cyst in a patient with myotonic dystrophy: a case of cyst with differentiation toward follicular infundibulum, isthmus, inner root sheath, and hair.

We report a case of follicular cyst, which developed in a patient with myotonic dystrophy (MyD). Histopathologically, the cyst showed infundibular and trichilemmal keratinization, inner root sheath differentiation, aggregation of basaloid cells, and pilomatricoma-like changes in the pericystic connective tissue. These findings have been reported in follicular cysts with Gardner's syndrome (GS). Interestingly, pilomatricoma is known as one of the skin diseases associated with MyD, though there have been no reported cases of cyst formation with differentiation toward portions of hair follicle in a MyD patient. In our case, we hypothesized that the cyst might be derived from embryonic follicular germinative cells or follicular stem cells under the genetic influence of the MyD gene, as observed in follicular cysts in patients with GS.