The symptom you describe is called formication, from the Latin formicare, meaning to creep like an ant. If you are a woman in midlife, it could be related to the hormonal changes of menopause. I've heard the incidence of formication during perimenopause described as "rare," but I have also read that as many as 20 percent of all women experience it. It's no surprise that your doctors are stumped because there are no scientific studies on treating this unpleasant sensation.
The symptoms are thought to be a manifestation of menopause, much like a hot flash, and the cause is believed to be a reaction of estrogen receptors in the skin to reduced levels of this hormone. The good news is that formication usually does eventually disappear on its own. According to the North American Menopause Society, hormone replacement therapy and antihistamines may help. I've also read that acupuncture and traditional Chinese medicine can help, so you might explore these options.
Sometimes, severe itching also occurs among menopausal women. If this is part of the problem, consult a dermatologist. Formication also occurs in alcoholism and can be a symptom of drug toxicity, particularly from overuse of cocaine and amphetamines.
Andrew Weil, M.D.