A Case of Probable Shared Psychotic Disorder in a Pair of Older Identical Twins
Mrs. G, a 76-year-old woman with a history of depression and anxiety, was admitted to an inpatient psychiatric unit after reporting a new onset of psychotic symptoms for one month. She was being treated as an outpatient prior to admission with sertraline 100 mg daily and alprazolam 0.5 mg twice a day for approximately 10 years for depression with anxiety, and was at her normal state of mental well-being until one month prior to admission. At that time, her identical twin sister, Ms. T, moved in with the patient and her husband. Ms. T had a long history of over 40 years of psychosis and carried a diagnosis of schizophrenia. She was continuously paranoid about her neighbors and kept moving from one address to another. Shortly after her sister moved into her apartment, the patient began having the same delusions, as well as auditory hallucinations of people calling out her name.
Mrs. G was brought into the hospital by her daughter after her paranoid delusions worsened and she became agitated and anxious. The patient was awake at night and believed that four young men were trying to break into her home. She was not performing her activities of daily living and was afraid to leave the apartment, even during the day. Mrs. G covered all of the windows because she believed that people were trying to look into her apartment from nearby rooftops. She was evaluated in the emergency department by the psychiatrist on call and was admitted to the hospital’s inpatient geriatric psychiatric unit. Mrs. G received a 1-mg dose of lorazepam in the emergency department for anxiety and restlessness.
On admission to the hospital, she denied any worsening of her depression since she started having the psychotic symptoms. She was anxious and worried that one of the men whom she thought was trying to break into her home would try to come and harm her in the hospital. Mrs. G was restless and paced up and down the unit. She had no known history of psychotic symptoms. There was no evidence of confusion or delirium and no recent change in the dosage of sertraline or alprazolam.
Shared psychotic disorder, also known as “folie à deux” or induced psychosis, is a rare delusional disorder shared by two or more people with close emotional ties.1 In this disorder, an “inducer” (the primary case) who is the “originally” ill patient transmits his/her delusional beliefs to another patient.
Shared psychotic disorder was first described by Ernest-Charles Lasègue and Jean-Pierre Falret in 1877. Shared psychotic disorder is probably rare, as the rates of incidence and prevalence are unknown. The disorder is characterized by the transfer of delusions from one person to another. According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision, diagnostic criteria for shared psychotic disorder, the delusion in the secondary case must be similar to that of the person who already has an established delusion and cannot better be accounted for by another psychotic disorder (eg, schizophrenia) or mood disorder (eg, major depressive disorder with psychosis), a substance, or a general medical condition.2 Most commonly, the primary case or “inducer” is chronically ill and is the influential partner in the relationship over a more suggestible partner.
The two people in the psychotic dyad typically live together in relative social isolation and have an unusually close relationship. Approximately 50% of the psychotic dyads consist of mother-daughter or sister-sister pairs, although other constellations can occur less commonly. The onset is often sudden. The secondary case is often more passive, less intelligent, more gullible, and more lacking in self-esteem than the primary case or the inducer.3 There are several known risk factors for shared psychotic disorder, including old age and female gender (Table I). In addition, a genetic predisposition to idiopathic psychoses has also been suggested as a possible risk factor.4
The phenomenon of shared delusional disorder in identical twin pairs is intriguing because of its sociogenic or contagious factors in addition to its genetic factors. Psychosis of twins may represent the genetic tendency toward delusional thinking. Aside from genetic loading, there is some evidence that ego-identity confusion and failure to develop a separate identity can lead to this disorder.5 The concordance rate for schizophrenia in monozygotic twins is 50% and 17% for dizygotic twins.6 This may explain the high number of reported twin cases (especially sister-sister pairs) in the literature, as twins share many biological and psychological factors. Twins are the “perfect” couple for the development of this unusual clinical condition.7
Historically, treatment has consisted of trying to separate the pair; however, separation of the pair has been shown to be ineffective on its own.8 The differential diagnosis needs to rule out dementia, delirium, and substance abuse including alcohol, levodopa, marijuana, or sympathomimetics such as amphetamines.4 Other psychiatric disorders including mood disorders (major depressive disorder with psychosis or bipolar disorder) and primary psychotic disorders (schizophrenia or delusional disorder) must also be ruled out. In addition, malingering and factitious disorder with predominantly psychological signs and symptoms need to be included in the differential diagnosis. Presently, the standard of practice is to use antipsychotics to treat the underlying illness in combination with separation from the primary case in the dyad. In addition, adjunctive treatments can include supportive psychotherapy without direct confrontation of the delusions, involvement with community services and activities to promote socialization, assistance with housing and finances to facilitate separation, and even utilization of Adult Protective Services in cases of suspected abuse (Table II).
Outcome of the Case Patient
In the hospital, the patient was kept apart from her sister, who was being treated in the partial hospital outpatient program. Mrs. G was started on risperidone 0.5 mg twice a day. In addition, she received supportive psychotherapy in the hospital. Her sertraline and alprazolam for depression and anxiety were continued at the same doses in the hospital. She gradually began to doubt her beliefs and blamed them on Ms. T, “who put them in my head.” She felt safe to go home, as her sister had moved out of her apartment in the interim. In the hospital, all routine blood work was within normal limits. An 18-channel electroencephalogram was performed and was interpreted as normal. A contrast-enhanced magnetic resonance imaging scan of the brain was also interpreted as normal. Mrs. G returned home after two weeks of risperidone treatment in the hospital, and remained stable on the risperidone dose without any recurrence of the psychosis after a one-year follow-up period while being seen by a geriatric psychiatrist on a monthly basis. She continued to take her prior regimen of sertraline 100 mg per day and alprazolam 0.5 mg twice per day on discharge from the hospital as well as at her one-year follow-up.
In this case, one twin (Ms. T) had a history of psychosis due to schizophrenia going back several years, while the patient (Mrs. G) developed new-onset–induced psychotic disorder shortly after her sister moved into her home. The patient’s symptoms improved shortly after admission to an inpatient psychiatric unit, when she was separated from her sister and treated with an atypical antipsychotic with resolution of the symptoms. While a literature review revealed similar case reports,6,9,10 this case adds to the available known data about shared psychotic disorder in the geriatric population, and particularly in identical twins.
The authors report no relevant financial relationships.
Dr. Kohen is a Staff Geriatric Psychiatrist at The Zucker Hillside Hospital, North Shore-Long Island Jewish Health System, and an Assistant Professor of Psychiatry and Behavioral Sciences at Albert Einstein College of Medicine; and Dr. Kremen is a Unit Chief on an Inpatient Geriatric Psychiatry Unit at The Zucker Hillside Hospital, North Shore-Long Island Jewish Health System, Glen Oaks, NY, and an Assistant Professor of Psychiatry and Behavioral Sciences at Albert Einstein College of Medicine, Bronx, NY. Dr. Lantz is Chief of Geriatric Psychiatry, Beth Israel Medical Center, First Ave @ 16th Street #6K40, New York, NY 10003; (212) 420-2457; fax: (212) 844-7659; e-mail: firstname.lastname@example.org.
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