Dermoscopy of Porokeratosis of Mibelli.

A 24-year-old man presented with a single asymptomatic reddish-brown annular plaque of 8-months duration over the outer right arm [Figure 1]. Dermoscopy revealed a central homogenous skin-colored to pale area with brown-colored dots, globules, and an irregular double-marginated track-like border, most prominent around the 2 o'clock and 8 o'clock position. In some sections of this track, brown dots/globules coalesced to form a continuous brown-colored line within the whitish rim [Figure 2]. Histopathology confirmed the diagnosis of porokeratosis [Figure 3]. A final diagnosis of porokeratosis of Mibelli was made. Dermoscopy of porokeratosis (PK) typically reveals central white area of scarring, red-brown dots and globules, and vessels of different patterns. The dermoscopically diagnostic finding is the white peripheral border, often double-marginated, representing coronoid lamella.[1] It has been metaphorically called “white track” or “lines of volcanic crater,” and “diamond necklace” on ultraviolet light dermoscopy. Minor differences have been reported in the Mibelli and diffuse superficial actinic variant.[12]

Figure 1

Reddish-brown annular plaque with well-defined irregular boundaries over the lateral aspect of right arm

Figure 2

Central homogenous – skin-colored to pale area bounded by irregular double-marginated “white track” border (black arrow), with focal brownish discoloration (green arrows), and brown-colored dots and globules in the central area (white arrow) on dermoscopy (original magnification, ×10; polarized mode)

Figure 3

Histopathology features from the peripheral raised edge revealing the coronoid lamella composed of thin column of tightly packed parakeratotic cells within a keratin-filled epidermal invagination with a mild lymphocytic infiltrate, dilated capillaries and multiple melanophages in the papillary dermis (H and E, ×100)

PK, an epidermal keratinization disorder is characterized by annular lesions with keratotic ridge that corresponds to coronoid lamella histologically, observed dermoscopically as a whitish peripheral rim.[3] The reddish-brown globules and dots seen within the central area histopathologically represent melanophages and dilated capillaries in the dermis.[4] Common clinical differentials of PK include annular lesions such as tinea, annular variants of psoriasis and lichen planus (LP), herald patch of pityriasis rosea (PR), nummular eczema, and granuloma annulare. Dermoscopic features of psoriasis, LP, PR, and eczema are well-defined and easily distinguishable from those of PK based on the pattern of scaling and vessels, as well as specific features such as Wickham's striae seen in LP.[5] Granuloma annulare, a close differential is dermoscopically typified by peripheral, structure-less orange-reddish borders, with occasional presence of isolated, unfocussed small vessels.

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