Dr. Greene's HouseCalls have been a great help to me. Just finding out more about things I have questions about is informative and helpful to me with a 6-year-old, a 10-month-old, and a baby on the way. The information on ear infections has answered lots of questions and eased concerns. Now, Dr. Greene, my daughter has geographic tongue. What is that exactly and what causes it?

Cheryl, you certainly have your hands full. What an exciting (and busy) time lies ahead! Two in diapers is always a challenge – especially when they are numbers two and three. Be sure to ask for the help you need.

Geographic tongue is a marvelous, descriptive name for one of the most common medical conditions of the tongue. Parents usually are the ones to notice several large, red, slightly depressed, unusually smooth patches on the surface of their child's tongue -- when nothing was there hours before. Often the red areas are bordered with distinct white bands. The sharp borders of these irregularly shaped lesions give the surface of the tongue the appearance of a map, perhaps a map of a group of uncharted islands. The rather dramatic appearance of geographic tongue looks to many like a burn, or like some kind of nasty infection.

How many parents must puzzle over geographic tongue? The exact prevalence varies widely from study to study, but at any given time, somewhere between 0.1 percent and 14.3 percent of otherwise healthy people have it. It has been found to be present in about 0.6 percent of Americans (Community Dental and Oral Epidemiology Aug 1994), about four percent of healthy Iraqi schoolchildren (Community Dental and Oral Epidemiology Aug 1982), and about 2 percent of young Finns (Oral Surgery, Oral Medicine, and Oral Pathology, Feb 1982).

The healthy tongue is a mass of muscle fibers covered by a mucous membrane. On the underside of the tongue, the mucous membrane is smooth. On the upper side, the tongue is covered with many tiny protrusions called papillae. These papillae come in four types with different shapes. Three of these types contain taste buds; the fourth does not. This fourth type are called filiform papillae, and they are packed tightly together over the entire upper surface of the tongue.

For some reason, medical conditions of the tongue often have picturesque names (such as "black hairy tongue" or "scrotal tongue"). Most of these conditions are abnormalities of the papillae, of one type or another. In geographic tongue, the filiform papillae are missing in the reddish areas and are overcrowded in the gray-white borders.

We still do not know exactly what causes geographic tongue, but we do know that it strongly tends to run in families (Oral Surgery, Oral Medicine, and Oral Pathology, Nov 1976). Geographic tongue has polygenic inheritance -- it is associated with several different genes. We also know that it is associated with a number of other genetic medical conditions.

It has been most closely linked to psoriasis, and is notably more common in those who have psoriasis (British Journal of Dermatology, Sep 1996).