Idiopathic guttate hypomelanosis (IGH) is an absolutely harmless skin condition. It appears mostly on shins and sometimes on forearms of people with lighter pigmentation. It shows up as pale to white spots of normal skin texture with sharper margins, smaller than 6 to 8 millimeters (¼ inch).

When lighter skin is not tanned, the spots are not noticeable. After exposure to sunlight (we won't include tanning beds here), the surrounding skin darkens proportionately, highlighting presence of IGH. It commonly crops up as more candles crowd the birthday cake and disappear as the number of candles requires a burning permit from the fire department.

The microscopic findings show a reduction but not absence of pigment cell numbers and the pigment granules in them. Nobody has a clue why it happens.

It has been suggested that this disorder results from an age-related somatic mutation of pigment cells. You might say that the melanocytes become too pooped to pigment.

It is a very common condition, being found in 46 percent of 400 patients in one study, more women than men.

It was first described in 1923 by Matsumoto of Japan, who saw it in the natives of Truk (not car or boat) in the Pacific South Sea Islands. It was described again in 1951 from Brazil, and finally in the U.S. by two different groups in 1966.

Why even bring this up if it is medically meaningless? Because people pay money to ask about it and to know whether it's bad for them.

It is important to be aware of its existence so it won't be misdiagnosed as other skin problems, such as vitiligo mainly, or post inflammatory pigment loss, leprosy, disseminated superficial porokeratosis (just had to throw one of those in, didn't we?), etc.

The people I have seen with IGH have never been overly distressed with the explanation or the fact that there is nothing to do for it. If you do wind up having truck with it, as do the natives of Truk, it shouldn't drive you semi-nuts with worry. On with the show - and the sunscreen.

Frank Bures is a dermatologist in Winona.