A lentigo is a spot on the skin that is darker than the surrounding skin. Lentigines are more common among whites, especially those with fair skin. Exposure to the sun seems to be the major cause of lentigines. Lentigines most often appear on parts of the body that get the most sun, including the face and hands. This could also be due to genetics or by medical procedures such as radiation therapy. The best way to prevent lentigines is to stay out of the sun as much as possible. Using a sunscreen when outdoors may also help. There are several methods for treating this condition: cryotherapy, laser surgery, creams that are applied to the skin including retinoids and bleaching agents. It would be best to see your dermatologist for the proper management.
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Are you sure these are lentigines? I thought they were just spots, not lines.