A 40-year-old woman and her live-in domestic companion of 15 years both share the identical persecutory belief that neighbours are poisoning their water supply. When separated for one month, the younger dependent companion's belief fades completely while the older dominant woman retains it fully. What is the diagnosis in the dependent companion?
- A Schizophrenia, paranoid type
- B Shared psychotic disorder (folie a deux) ✓
- C Persistent delusional disorder
- D Induced brief psychotic disorder with marked stressor
Correct answer: B. Shared psychotic disorder (folie a deux)
Explanation
Folie a deux is a delusional disorder transferred from a dominant person with a genuine psychotic illness to a more passive, dependent individual in close association. The hallmark is that the induced delusion disappears when the pair is separated, exactly as described here, while the index case retains it. Because the delusion resolves entirely on separation, the companion does not meet criteria for an independent persistent delusional disorder or schizophrenia.
Reference: Kaplan and Sadock's Synopsis of Psychiatry, 12th ed.
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