Melasma manifests as increased pigmentation in the face, typically symmetric patches or macules are found on either side, with a tendency to occur on the sunexposed
areas of the face (cheeks, upper lips). This disorder is much more common in women than men (a ratio of 9:1) and in persons with high-grade phototype.
Major etiologic factors include genetic influences, female sex hormones, and exposure to UV radiation. Histopathology findings indicate hyperpigmentation in all epidermal layers caused by an increase in melanin and also the number of active melanocytes. A flattening of the epidermal rete ridges has been reported, suggesting that keratinocytic proliferation may not be involved in melasma. Pathogenesis is poorly understood. A recent study indicated that the high expression of α-MSH in keratinocytes of the melasmic lesions was a major factor. Treatment options are prevention using good broad-spectrum sunscreen, retinoids, α-hydroxy acids, and hydroquinone.

Source: Dermatologic, Cosmeceutic and Cosmetic development - Kenneth A. Walters, Michael S. Roberts

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