"About 60 to 70 percent of my patients have CDH, and 80 percent of the CDH patients are teens," says Jack Gladstein, director of the pediatric headache clinic at the University of Maryland Hospital for Children in Baltimore, who has written extensively on CDH. Gladstein launched his pediatric headache clinic, the second in the nation, in 1989.

According to the definition first offered in the mid-1990s by [Stephen Silberstein] and fellow neurologist Richard Lipton, CDH is a headache that occurs at least 15 days a month and lasts four hours or more, a headache that doesn't fit into standard International Headache Society (IHS) classifications. "IHS criteria help distinguish a migraine from other disorders," Gladstein notes, "but [do] not address chronic headache well." Silberstein and colleagues recognize four categories of CDH, some of which share migraine symptoms.

Andrew Hershey, director of the Headache Center at Cincinnati Children's Hospital Medical Center, speculates that CDH is the fault of a gene or group of genes that causes the brain to be hypersensitive to its environment. When Hershey conducted a study of 200 youngsters who had headaches at least 15 days a month, he found that most of their headaches had the same symptoms as migraines. Noting that migraine patients generally have close relatives who also suffer from migraines, Hershey theorizes that a youngster with one "headache gene" may be disposed to get an occasional migraine, while a youngster with several such genes might end up with CDH.