Delusional disorder and its differentiation from schizophrenia: A narrative review

The nosological status and psychopathology of delusional disorder have been a subject of debate since Kraepelin distinguished it from schizophrenia and affective psychoses. Contemporary diagnostic manuals define delusional disorder primarily by the presence of delusions, offering limited guidance for its differentiation from schizophrenia. Notably, DSM-5 introduced a major, seemingly unexplained change by allowing bizarre delusions in delusional disorder, contrary to ICD-11, prior DSM editions, and classical descriptions. This narrative review revisits the seminal works of six classical authors (Kraepelin, Jaspers, Kretschmer, Sérieux and Capgras, and De Clérambault), who shaped the concept of delusional disorder (paranoia), and their detailed clinical cases of the disorder. All considered delusional disorder to be an independent psychotic disorder, characterized by chronic, systematized, nonbizarre delusions, preservation of personality, and minimal hallucinations, with a largely intact experiential framework outside of the delusional theme. Additional features such as delusional misinterpretations, illusions, and false memories were also emphasized in the classical literature. We examined these authors' clinical cases of delusional disorder for the presence of delusional features characteristic of schizophrenia (delusional mood, first-rank symptoms, autistic-solipsistic delusions, and double bookkeeping), which index alterations in the structure of experience rather than mere delusional content. Such delusions were rarely found in the classical clinical cases of delusional disorder. Our findings highlight psychopathological distinctions between delusional disorder and schizophrenia, suggesting that schizophrenia involves a qualitative alteration of the experiential framework that is absent in delusional disorder. These findings raise concerns about the validity of the DSM-5 change, allowing bizarre delusions in delusional disorder.