La folie à deux a toujours interrogé sur la possible transmission des troubles délirants. Les cas de folie à deux chez des jumeaux monozygotes, ayant un terrain génétique identique et une relation gémellaire unique, posent la question de l’étiologie du trouble. Cet article propose l’étude d’un cas de folie à deux chez des jumelles monozygotes, dont l’une était enceinte. Après la présentation clinique, il rappelle les définitions du trouble, décrit historiquement par Lasègue et Falret (1877). La deuxième partie présente des articles de la littérature concernant des cas de folie à deux chez des jumeaux monozygotes. Ces publications font part de plusieurs formes cliniques. La folie imposée serait l’apparition d’un délire chez une personne vulnérable et au contact d’un sujet délirant. La folie simultanée serait deux schizophrénies à début concomitant, sur un terrain génétique prédisposant. La folie communiquée reste une entité plus floue, avec une origine génétique controversée. La rareté du trouble et le manque de données épidémiologiques entraînent des biais qui limitent ces études. Une troisième partie rappelle quelques données issues des études de jumeaux dans la schizophrénie. Le taux de concordance de la maladie chez les jumeaux homozygotes varie de 41 à 65 %, contre 0 à 28 % chez les hétérozygotes. Il existe donc un terrain génétique fort associé à des composantes environnementales. Ces notions se recoupent donc avec les données sur la folie à deux et la folie simultanée. La quatrième partie précise quelques notions concernant les troubles psychotiques pendant la grossesse avant de revenir sur l’histoire clinique présentée. Ce cas de folie à deux chez des jumelles homozygotes est une belle illustration des interactions entre un terrain génétique identique, des histoires de vies communes et individuelles, et une relation interpersonnelle unique.
Folie à deux (or shared delusional disorder) has always questioned about a possible transmission of delusional disorders. Cases of folie à deux in monozygotic twins with a common genetic and environmental history, raise the issue of the different etiological hypotheses. Folie à deux is a rare disease. Ninety percent of the cases occur in the same family, with a majority of women (sisters association). Twinship is a specific relationship, where a common history is combined with a genetic background and a dual and unique relationship. Many twin studies have been published in the 1970s, regarding schizophrenia. The concordance rate of the disease is from 41 to 65% in monozygotic twins versus 0 to 28% in dizygotic twins. This article presents a case of folie à deux in monozygotic female twins. One was pregnant. The first part of the paper reports the case and recalls the different definitions of the disorder. A second part reviews published cases of the illness in twins. The third part treats about twin studies and schizophrenia. A fourth part reports data about delusional disorders in pregnancy. Eventually we discuss our reported case. Folie à deux was first described by Lasègue and Falret in 1877. They reported cases of shared delusional disorder, involving two persons or more. The first subject presented a delusional disorder. The second one, influenced by the first subject, was often most vulnerable. The treatment was based on separating the two delusioned persons. This nosological concept has been early criticized. Separation did not always improve symptoms. CIM-10 classification describes an “Induced delusional disorder”, and DSM-IV a “shared delusional disorder”. Literature shows many clinical forms of the disease. Folie simultanée should be the simultaneous occurrence of two genetically driven delusional disorders, as schizophrenia. Folie imposée involves a vulnerable person in contact with another one who suffers a delusional disorder. Folie communiquée should have a genetic origin but there are less data on it. Many articles discuss those clinical forms, and some of the reported cases of folie à deux could actually have been folie simultanée. The occurrence of a simultaneous delusional disorder in twins questions about the genetic or environmental etiology of folie à deux. Scarcity of the disorder and lack of epidemiological data leads to biases and limits the studies. First episodes of delusional disorders in pregnant women are rare. Affective disorders occur more frequently during and there are few data on delusional disorder in pregnancy. Pregnancy has been described as a low-risk period for schizophrenia, but on the other hand, it is a specific time leading to personal and familial changing, which could be considered as a period of vulnerability. This example of shared delusional disorder in monozygotic twins is a good illustration of the interactions between common genetic background, common and individual environmental histories, and a particular twin relationship, at the junction of nature and nurture. We consider the clinical history reported as a case of “folie induite”, or induced delusional disorder, because of the vulnerability induced by pregnancy.