Description

• Shared delusional disorder occurs when a delusional belief held by one person (the “primary”) becomes shared by one other (the “secondary”) or several other people associated with that person. In most cases, a second person is dependent on, or has a passive relationship with, the primarily affected person. It is sometimes difficult to determine who is the primary and who is the secondary. The individuals involved are usually close and may be isolated from others.
• By definition, the shared belief, in order to be considered delusional, is one that is not generally accepted within the person’s culture. As a result, this illness exists along a spectrum: the least severe form being a shared “overvalued idea” (those holding it will relinquish it if given evidence to the contrary), the most severe form being a shared psychosis (attempts to disprove or cure patients of their delusions feel threatening to them).
• Shared delusional disorder is also known as shared psychotic disorder, folie a deux (if two people are involved), or folie a plusieurs (if several are involved). It was first described by two 19th-century French doctors, Charles Lasegue and Jean-Pierre Falret, who believed that understanding a patient’s family relationships was key to the diagnosis and treatment of maladies mentales. Per the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5), which was published in 2013, all people involved would be diagnosed with a “delusional disorder”—so for diagnostic labeling purposes, there is no differentiating between the first person to have the belief and the subsequent ones. ICD-10 criteria also do not differentiate among the primary and secondary. For treatment purposes, however, the differentiation could be important (see “Treatment” in the following discussion). Although “shared paranoid disorder” is no longer a stand-alone diagnosis, it can be a useful definition clinically and can be coded as paranoid disorder, shared (as a modifier).
• Concealment within the dyad (of the person with primary delusions and the one who comes to share it) is very common in this illness, and so, it is difficult for clinicians to recognize it. The classic work by Elvin Semrad, MD, and Max Day, MD, “Paranoia and Paranoid States” (in the 1978 edition of The Harvard Guide to Modern Psychiatry) suggests that if health care providers can create a safe emotional space for patients, they may be able to reveal their delusions. Semrad and Day suggest that providers who take care of mentally ill patients thoroughly and painstakingly examine patients’ familial relationships to get clues about the losses in their lives that could be fueling delusions. The authors suggest that helping patients recognize their losses and supporting them through this process may help them break free of the delusions.

Epidemiology

Prevalence

• “Shared ideas” are more common than “shared psychoses.”
• Recent WHO reports estimate the world prevalence of delusional disorders to be approximately 0.1% of the population and shared delusional disorders to be a fraction of that number.
• However, researchers have found that these phenomena may occur more often than is thought. First, a literature review of 20 years of case reports showed that the secondary often had a personal or family history of mental illness himself/herself. It concluded that Lasegue’s and Falret’s definition of the disease as a relationship between a “mentally ill” person and a “mentally well” person is probably too narrow a definition. It makes the further point that an exacerbation of symptoms between two (or more) mentally ill people is not uncommon (1). Second, the experience of paranoia includes the fear of being discovered, so patients affected by the illness would be less likely than those with other mental disorders to discuss it. Third, clinicians often see only one person of the ill dyad (or triad), which might prevent clinicians from discovering that illness also exists in someone close to the patient they are already treating.
• Case reports out of India (2) and Japan (3) argue for a further expansion in the consideration of who might be involved—citing reportedly new onset delusions among the primary (who had undergone a trauma) which come to influence family members around them. The set of case reports seemed to show that the primary could have been (or at least seemed) mentally well.

Etiology and Pathophysiology

• ≥1 person may come to share in an individual’s delusions or paranoia, most commonly in cases in which those involved are isolated and emotionally close, and the secondary person(s) is (are) psychologically “vulnerable”—and may have personal or family histories of mental illness (diagnosed or not yet diagnosed).
• A group of people may adopt a delusional belief that those outside of the group are a threat or “evil,” and so, violence against those outside the group can be condoned. Psychohistorian Robert Jay Lifton, MD, says these “atrocity-producing situations” occur when the group is encouraged to adopt a leader’s ideals to combat their own sense of powerlessness.

Risk Factors

• A major risk factor for a second person (or several people) to develop a delusional disorder is to have a close family member (the primary person affected) with a psychiatric diagnosis of schizophrenia, mood disorders, or delusions. The primary family member is usually a spouse or sibling and lives with the person who will come to share the delusions (4). Sleep deprivation and overuse/“unquestioning” use of the internet can increase susceptibility.
• Social isolation was found to be a risk factor in a majority of cases, per a review of 50 years of case reports (5).

General Prevention

Adequate treatment for the primary person with mental illness, family therapy for those close to the primary, and bringing such families out of their isolation can help address major risk factors for the development of shared delusional/paranoid disorder.

Commonly Associated Conditions

• Familial mental illness
• Familial neurologic conditions (dementia, mental retardation) (5)
• Immigration/displacement from home
• Massive trauma
• Hearing (or other sensory) loss, which can lead to misinterpretation of outside phenomena
• Shared use of medications, drugs, or herbs that can stimulate hallucinations or changes in perception
• The secondary often suffers from dependent personality disorder (4), an all-consuming need to be taken care of that leads to submissive behavior and fear of separation (6).