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Hyperpigmentation has multiple causes and may be focal or diffuse. Most cases are due to an increase in melanin production and deposition.
Focal hyperpigmentation is most often postinflammatory in nature, occurring after injury (eg, cuts and burns ) or other causes of inflammation (eg, acne. lupus ). Focal linear hyperpigmentation is commonly due to phytophotodermatitis, which is a phototoxic reaction that results from ultraviolet light combined with psoralens (specifically furocoumarins) in plants (eg, limes, parsley, celery—see Chemical photosensitivity ). Focal hyperpigmentation can also result from neoplastic processes (eg, lentigines. melanoma ), melasma. freckles, or café-au-lait macules. Acanthosis nigricans causes focal hyperpigmentation and a velvety plaque most often on the axillae and posterior neck.
Diffuse hyperpigmentation can result from drugs and also has systemic and neoplastic causes (especially lung carcinomas and melanoma with systemic involvement). After eliminating drugs as a cause of diffuse hyperpigmentation, patients should be tested for the most common systemic causes. These causes are Addison disease. hemochromatosis. and primary biliary cholangitis. Skin findings are nondiagnostic; therefore, a skin biopsy is not necessary or helpful.
Melasma consists of dark brown, sharply marginated, roughly symmetric patches of hyperpigmentation on the face (usually on the forehead, temples, cheeks, upper lip, or nose). It occurs primarily in pregnant women (melasma gravidarum, or the mask of pregnancy) and in women taking oral contraceptives. Ten percent of cases occur in nonpregnant women and dark-skinned men. Melasma is more prevalent among and lasts longer in people with dark skin.
Because melasma risk increases with increasing sun exposure, the mechanism probably involves overproduction of melanin by hyperfunctional melanocytes. Other than sun exposure, aggravating factors include
Autoimmune thyroid disorders
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