The histopathologic diagnosis of vitiligo is classically understood as the absence of melanocytes and melanin in the skin biopsy.1 It is difficult for a pathologist to establish the absolute absence of melanocytes and melanin in a skin biopsy. Therefore, we need to take into consideration many variables when we face the possibility to biopsy a vitiligo lesion.
The basis of the clinical diagnosis of vitiligo is the appearance of achromic lesions in periorificial and acral areas; however, sometimes it is difficult to differentiate between an achromic or hypochromic lesion. Although Wood light is of great help in these circumstances, it still can be difficult to make the diagnosis with certainty.
In other cases, the lesions do not present a classic distribution of vitiligo, and other differential diagnoses are considered. For example, if we see a single hypochromic or achromic lesion in a young child, then the main differential diagnosis would be achromic nevus. If there are multiple lesions, then we may consider progressive macular hypomelanosis, postinflammatory hypopigmentation, and hypopigmented mycosis fungoides. In genital lesions, the differential diagnosis between initial lichen sclerosus and vitiligo also can be considered. Finally, we must always bear in mind that both sarcoidosis and Hansen disease can appear as achromic or hypochromic lesions.
The histologic diagnosis of vitiligo in a completely constituted lesion implies the total loss of melanocytes and melanin in the epidermis. Additional histologic findings are described at the edge of the advanced border, such as the presence of melanocytes that have increased in size with large dendrites and lymphoid infiltrate. In perilesional skin, vacuolated keratinocytes and Langerhans cells have increased in number and repositioned in the basal layer, with visible degeneration of nerves and sweat glands. Lymphocytes also can be found in contact with the melanocytes.2 It is important to note that in addition to these histologic findings, it is common to find spongiosis, mononuclear superficial perivascular inflammatory infiltrate, and melanophages in biopsies of vitiligo.3
Given that ensuring the absence of melanocytes is central to diagnosis and melanocytes can be difficult to identify or differentiate from repositioned Langerhans cells in the basal layer with hematoxylin and eosin stain, immunohistochemical techniques must be performed every time we are dealing with vitiligo biopsies. Although there are no studies comparing the diagnostic value of the different immunohistochemical techniques in vitiligo, dihydroxyphenylalanine (DOPA) seems to be a good option, as it will only mark active melanocytes. Human melanoma black 45 (HMB-45), anti-TYRP1 (Mel-5), and antimelanoma gp 100 antibody (NKI/beteb) also have been used. Some authors recommend the use of pan melanoma because it includes 3 markers—HMB-45, tyrosinase, and Mart-1. Currently, SRY-related HMG-box10 (SOX10) seems to be a good option, as it is a nuclear marker that makes it easier to differentiate melanocytes from pigmented keratinocytes.4
Establishing a complete absence of melanocytes in the lesions or finding there are melanocytes but they are inactivated is key to evaluating the pathogenesis of vitiligo and directly affects the histologic diagnosis and eventually even the treatment. Le Poole et al5 used a panel of 17 monoclonal antibodies and a polyclonal antibody in lesions of 12 patients with vitiligo without identifying the presence of melanocytes. They concluded that there are no melanocytes in lesions of vitiligo.5
In a subsequent study with a larger number of patients, Kim et al2 found melanocytes that marked with NKI/beteb and Mart-1 in 12 of 100 patients with vitiligo. They also showed melanocytes by electron microscopy in lesional skin of 1 of 3 patients with vitiligo.2 Tobin et al6 managed to grow melanocytes from skin with vitiligo and confirmed the presence of melanin in basal keratinocytes of lesions of stable vitiligo. From this evidence we can conclude that the absence of melanocytes and melanin in the epidermis confirms the diagnosis of vitiligo; however, the opposite is not true—that is, the presence of melanocytes or melanin in a skin biopsy does not rule out the diagnosis of vitiligo.