When exposed to sunlight, specialized skin cells called melanocytes (see Overview of Skin Pigment) produce increased amounts of the pigment melanin, causing the skin to darken, or tan. In some fair-skinned people, certain melanocytes produce more melanin than others in response to sunlight. This uneven melanin production results in spots of pigmentation known as freckles. A tendency to freckle runs in families. Other factors besides sunlight can cause increased melanin in spots or patches (localized) or in widespread areas of skin. Rarely, other substances besides melanin cause darkening of the skin.
Localized hyperpigmentation can be caused by
Some people develop hyperpigmentation in areas of skin that have been exposed to sunlight. Some plants (including limes, celery, and parsley) contain compounds called furocoumarins that make some people's skin more sensitive to the effects of ultraviolet light. This reaction is called phytophotodermatitis (see Chemical photosensitivity).
People who have a disorder called acanthosis nigricans develop darkened and thickened skin in the underarms, on the nape of the neck, and in skinfolds. Acanthosis nigricans can be a symptom of diabetes.
Lentigines (commonly called liver spots or age spots) are flat, tan to brown, oval spots on the skin. A single spot is called a lentigo. They are a type of localized hyperpigmentation.
There are two types:
Solar lentigines are caused by sun exposure and are the most common type of lentigo. They occur most frequently on areas that are exposed to the sun, such as the face and back of the hands. They typically first appear during middle age and increase in number as people age. Lentigines are noncancerous (benign), but people who have them may be at higher risk of melanoma.
Nonsolar lentigines are not caused by sun exposure. Nonsolar lentigines sometimes occur in people with certain rare hereditary disorders, such as Peutz-Jeghers syndrome (characterized by many lentigines on the lips and polyps in the stomach and intestine), xeroderma pigmentosum, and multiple lentigines syndrome (LEOPARD syndrome).
If people do not have too many lentigines, doctors can remove them with freezing treatments (cryotherapy) or laser therapy. Bleaching agents such as hydroquinone are not effective.
Widespread hyperpigmentation can be caused by
Hormonal changes may increase melanin production and darken the skin in Addison disease, in pregnancy, or with hormonal contraceptive use. A liver disorder called primary biliary cholangitis (previously called primary biliary cirrhosis) may also cause increased melanin production.
Some cases of hyperpigmentation are caused not by melanin but by other pigmented substances that are not normally present in the skin. Diseases such as hemochromatosis or hemosiderosis, which are caused by too much iron in the body, can cause hyperpigmentation. Some drugs, chemicals, and metals that are applied to the skin, swallowed, or injected can cause hyperpigmentation.
Drugs, chemicals, and heavy metals that can cause hyperpigmentation include the following:
The areas of hyperpigmentation are usually widespread, but some drugs can specifically affect certain areas. For example, some people develop fixed drug reactions, in which certain drugs (for example, certain antibiotics, nonsteroidal anti-inflammatory drugs [NSAIDs], and barbiturates) cause localized skin eruptions in the same place every time the drug is taken. This eventually leads to hyperpigmentation of the affected skin.
Depending on the drug, chemical, or metal and where it is concentrated in the skin, hyperpigmentation may be violet, bluish black, yellow-brown, or shades of blue, silver, and gray (see also Color Changes in the Skin). In addition to the skin, the teeth, nails, white of the eyes (sclera), and lining of the mouth (mucosa) may be discolored. With many of these drugs, the hyperpigmentation often fades after the drug is stopped, but with some of them, the hyperpigmentation can be permanent.