A case series of shared delusional infestation: folie à deux revisited

Delusional infestation describes the unshakeable belief that one's skin is infected or infested with an external organism or inanimate material, in the absence of supportive medical evidence. It is one of the most challenging psychodermatological conditions to manage, given the rigidity of patients' physically focused health beliefs, and the competing need to introduce antipsychotic therapy to bring about resolution. This is rendered exponentially more complex when partners or family members are similarly afflicted. This situation is known as shared delusional infestation, shared psychotic disorder (SPD), or folie à deux. We present a series of three couples with SPD who were referred to our tertiary psychodermatology service during the same year. On examining the literature we were intrigued to discover that subtly different subtypes of SPD have been recognized since the late 1800s. These include folie simultanée, imposée, communiquée and induite. Our cases neatly demonstrate three of these variants, and highlight the difficulties in facilitating effective treatment.