In a study of a group of depressed women, researchers found that some recovered and remained well only with the help of psychotherapy, others only by taking antidepressant drugs. These groups could not be distinguished by their symptoms alone, the history of their illness, or any other background information. It is a familiar problem in the treatment of psychiatric disorders: different therapies are effective for different patients, and it seems impossible to tell in advance what will work for whom. But this time the researchers may have found a way. Using positron emission tomography (PET) scans, they found different patterns of energy consumption and blood flow in the brain corresponding to the different treatment responses. This is the kind of pattern that could serve as an endophenotype.
"Pheno-" means showing or appearing. The phenotype of a disorder is its immediately observable signs and symptoms. "Endo-" means internal or inside; an endophenotype, also called an intermediate phenotype, refers to a characteristic that is not easily observed on the surface. People with a given endophenotype are more susceptible to the disorder, and that fact reveals something about the biological processes that underlie the disorder.
The concept was introduced in the 1970s, but mental health professionals have become much more interested in it recently, partly because of developments in brain imaging and genetics. More than half of the scholarly articles on endophenotypes have been published since 2004.