Genetic and hormonal influences in combination with ultraviolet radiation are the two most important causes of melasma, yet phototoxic and photoallergic medications and certain cosmetics have been reported to cause melasma in rare instances. Usually, no laboratory tests are indicted. Consider checking thyroid function test results. Wood s light examination will usually help to localize the pigment to the epidermis or dermis. Note that in many cases the pigment is found in both locations.
Epidermal melanin is found in keratinocytes in the basal and suprabasal area.
In most cases there is no increased number of melanocytes, yet the melanocytes that are there are larger, more dendritic, and more active. Dermal melanin is found in the superficial and mid-dermis within macrophages, which often congregate around small dilated vessels. Inflammation is sparse or absent. Melasma can be difficult to treat. The pigment of melasma develops gradually and resolution is also gradual.