Melasma is usually a clinical diagnosis. Microscopy studies suggest that there may be two main types of melasma9: the epidermal type, characterised by increased melanin pigmentation in the suprabasal layers of the epidermis, and the dermal type, characterised by increased melanin in the dermal macrophages, with associated milder epidermal hyperpigmentation. With mixed type, some areas show enhanced pigmentation, and some do not when examined with a Wood's light, revealing a mixture of dermal and epidermal melasma in the same person (see below).
This distinction may provide a clue to the expected treatment response. Dermal type has been found to be less responsive to conventional therapy.5 An alternative way of establishing the type of melasma clinically is by using a source of ultraviolet A light such as a Wood's lamp.15 Ultraviolet light lamps enhance the lesions in light-coloured skins (i.e. skin phototypes I—IV; see Box 41.1),12 but may be of little use in dark-skinned people, in whom the enhancement is less prominent.
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