These white spots are probably part of a skin disorder known as vitilgo. It occurs when the skin pigment disappears over a period of time and the disorder is extremely rare. It can affect anybody regardless of age and skin color. The start of vitilgo generally begins at childhood and (like you mentioned) it starts first at the joints of the body like the knuckles, knees and elbows. These spots are very few and eventually spread over a period of time.
The exact cause of vitilgo is not known or even the reasons why the skin pigment starts to disappear. A theory suggests that the body's own defense system starts to attack the cells within the body. Some studies indicate that it passed on through genetic factors and is probably hereditary. The symptoms of vitilgo do not include itching and hence it is definitely not a allergic reaction or infection cause due to bacteria. Therefore there is not much to worry about except maybe for the fact that the disorder destroys the body's natural defense against the sun which the cover of pigmented skin. Generally when your child is going to be out in the sun for an extended period make sure to make him put on a good sunscreen lotion (a minimum of 15 SPF).
answered by M W