INTRODUCTION

AMN, commonly referred to as a mole, is a developmental defect of epidermal melanocytes. Multiple melanocytic nevi are a component of the normal human phenotype. It begins with the proliferation of melanocytes at the dermo-epidermal junction, which starts as a flat, pigmented macule. Over time, it goes through the process of maturation, and these melanocytes penetrate into the dermis appearing clinically as a nodule on the surface of the skin. Various congenital as well as acquired dermatological disorders are known to follow the lines of Blaschko. Congenital melanocytic nevi and nevus spilus are sometimes known to follow the lines of Blaschko. However, linear AMN following the Blaschkoid pattern is extremely rare.

CASE REPORT

A seventeen-year-old boy came to us with an asymptomatic, dark, linear lesion on the face present for two months without any preceding history of trauma, vesiculation, or pain. He was otherwise healthy, and there were no obvious predisposing factors. On examination, there was linear paramedian hyperpigmented plaque beginning from the center of the forehead extending to the nasal ala on the left side measuring around 8 cm x 0.5 cm (Fig. 1). The lesion seemed to have arranged in a Blaschkoid fashion. His family history was non-contributory. Complete blood count, liver function tests, and renal function tests were within normal limits. Tests for HIV, HBsAg, and HCV were negative. X-ray skull did not show any abnormality. We considered the differential diagnoses of Blaschkoid lichen planus (LP), lichen planus pigmentosus (LPP), and linear morphea.

Figure 1: Linear paramedian hyperpigmented macule beginning from the center of the forehead extending to the nasal ala on the left side of the face.

The histopathology showed melanocytic neoplasm extending from the lower epidermis to the reticular dermis consisting of monomorphous melanocytes involving the dermis surrounding peri-appendageal structures. Melanocytes showed a nest in the dermo-epidermal junction (Figs. 2a and 2b). HMB 45 (Fig. 2c) and Melan A (Fig. 2d) stains demonstrated melanocytes in the dermis and around peri-appendageal structures. No cellular atypia was found.

Figure 2: a) neoplasm extending from the lower epidermis to the reticular dermis. The infiltrate composed of monomorphous melanocytes involving the dermis surrounding peri-appendageal structures (H&E; 10x). b) Melanocytes showing a nest in the dermo-epidermal junction (H&E; 40x). No cellular atypia was found. c) Melanocytes at the dermoepidermal junction and melanocyte nests at the upper dermis stained by HMB-45. d) Melanocytes at the dermoepidermal junction stained by Melan A.

The diagnosis of linear acquired melanocytic nevus (AMN) in a Blaschkoid distribution was established.

DISCUSSION

Acquired melanocytic nevi are well circumscribed, round or oval lesions which are derived from altered melanocytes or nevus cells with the characteristic intra-epidermal or dermal nests of nevus cells. AMN initially appears as a dark brown macule having proliferation of melanocytes at the dermo-epidermal junction. This stage is called junctional nevus. These nevus cells then migrate to the dermis to form the compound nevus presenting as pigmented, and circular, slightly raised surface. With advancing age, it appears as a hemispherical dome-shaped nodule above the skin surface, which is referred to as intradermal nevus. Melanocytes involving the dermis and surrounding peri-appendageal structures along with intra-epidermal or dermal nests of nevus cells are the characteristic of acquired melanocytic nevus. Increased hormonal levels such as in pregnancy and exposure to UV radiation may be a possible trigger for development of melanocytic nevi. Acquired melanocytic nevi have a benign course as compared to congenital ones. Surgical excision is done usually for cosmetic purposes.

Atypical morphological patterns such as linear and Blaschkoid are reported in congenital melanocytic nevi yet not in acquired melanocytic nevi [1,2]. Unilateral and agminate patterns are reported in AMN [3,4]. The Blaschko-linear pattern is seen with linear lentiginous nevus and small congenital melanocytic nevi [5].

Cutaneous lesions that follow Blaschko’s lines reflect a mosaic condition deriving either from a single mutated clone of cells originating from a postzygotic mutation or from an X-linked mutation made evident by lionization [6]. Clonal nevus cell growth due to mutation occurring at early stages of embryogenesis is said to be an explanation of nevi following Blaschko’s lines. Melanocytic nevi that follow Blaschko lines are said to be result from a clone of nevus cells, which carries the predisposing gene.

Lines of Blaschko from the center of the face are less recognized. Restano et al. added several Blaschko’s lines to the original drawing of Bolognia et al., which includes lines running parallel to the dorsum of the nose [6]. We believe our patient’s lesion corresponded to these lines.

Blaschkoid presents with violaceous papules and plaques which may be itchy and show the typical histological features such as saw tooth rete ridges, band-like lymphocytic dermal infiltrate. Blaschkoid LPP presents with dark brown macules usually on sun-exposed skin, which would show vacuolar degeneration of the basal layer with apoptotic keratinocytes in the epidermis and dense lymphohistiocytic infiltration into the dermis with numerous melanophages and pigment incontinence. En coup de sabre morphea appears as the linear paramedian depressed groove on the fronto-parital region in adolescents and may show cicarticial alopecia. Our patient showed neither of these features.

As this patient came for the cosmetic concern, we suggested various therapeutic options to him. Reassurance was given and good follow-up was advised.

CONCLUSION

Congenital melanocytic nevi, speckled nevi, and multiple agminated melanocytic nevi are reported to follow the lines of Blaschko [2.5]. We could not find supporting literature on acquired melanocytic nevus following Blaschko’s lines on Pubmed. Thus, we present an extremely rare form of acquired facial melanocytic nevus in a blaschkoid pattern in a young adult.

REFERENCES

Notes