Diffuse cutaneous mastocytosis: Identification of KIT mutation and long‐term follow‐up with serum tryptase level

Diffuse cutaneous mastocytosis (DCM) is the least common subtype of cutaneous mastocytotis and is generally more severe than other subtypes. We herein report a case of DCM with the consequence of a long‐term follow‐up. A 4‐month‐old boy visited with a 3‐month history of diffuse erythema that gradually worsened. Darier's sign was positive. The plasma histamine level was 4.95 ng/mL, and the serum tryptase and c‐Kit (CD117) levels were 33.3 and 27.4 ng/mL, respectively. Histopathology of the biopsied specimen showed dermal papillary edema and infiltration of mast cells identified by c‐Kit and toluidine blue staining. Amplification and direct sequencing of genomic DNA extracted from the skin biopsy specimen revealed the presence of a deletion of codon 419 in exon 8 (c.1255_1257delGAC [p. Asp419del]). There was no evidence of systemic infiltration of mast cells in this case, and we started topical corticosteroid and oral antihistamine with the diagnosis of DCM. Diffuse erythema subsided constantly with age in parallel with chronological decline of serum tryptase level, and it is no longer apparent presently at the age of 7 years, leaving only faint brown spots. Blister formation did not occur throughout the course. Our case indicates that spontaneous resolution can be expected even in DCM after a long period of time, and that serum tryptase level serves as a good surrogate marker to monitor the clinical course.