[Folie à deux: update of an old concept regarding two cases]

Folie à deux or induced delusional disorder is a rare mental disorder. It was initially described by the French Lasègue and Falret in 1877. Two subjects, who live in a close relationship, in isolation, share delusional ideas based on the same themes. Various classifications exist. Its epidemiology remains unclear, because most of the data have been extrapolated from case reports.

In this paper, we describe and comment two cases of shared paranoid disorder: in the first case report, a husband shares the paranoiac delusion of his wife; the second case report describes a shared paranoid disorder between a schizophrenic daughter and her mother.

A review of the existing literature is also presented. Some clinical characteristics arise, such as frequent mother-daughter associations and diagnosis of schizophrenia in inducing subject. Particular social and psychopathological conditions for the occurrence of a shared delusional disorder are described, such as personality traits and genetic influences. This article also reviews some forensic issues, which may be of importance, since this disorder is underdiagnosed. Data concerning the principles of its treatment are sparse, but most authors consider that the separation of the two subjects has to be the basis of any intervention. The inducing subject has to be treated with specific medical interventions, including the prescription of antipsychotics. Sometimes, the separation is enough to eliminate the delusional ideas from the induced subject, who, according to the ICD-10 and DSM-IV, is the only one to meet the criteria for shared delusional disorder. The case reports are discussed in light of the review, and some propositions for their treatment are made.

As shared delusional disorder is a rare disease, only few data exist on its pathophysiology and mechanisms, and controlled studies are needed in order to understand its specific implications better and to define recommendations for its management.