Delusions in schizophrenia spectrum disorders: diagnostic issues.

Family studies of schizophrenia frequently include relatives of schizophrenia probands with diagnoses falling within the schizophrenia spectrum. As part of an ongoing genetic linkage study of schizophrenia, the authors examined case material from 50 relatives (of schizophrenia probands) who received a DSM-III-R diagnosis of a nonaffective psychotic disorder or schizotypal or paranoid personality disorder. Eleven exhibited episodic or chronic delusions that resulted in diagnostic dilemmas, often arising from issues pertaining to the classification of delusional phenomena. Four of these cases are presented here. Unusual beliefs were often difficult to classify as odd beliefs versus full delusions, brief/transient versus persistent delusions, bizarre versus non-bizarre delusions. It is suggested that these might be considered continuous rather than dichotomous dimensions. Several possible implications for genetic studies of schizophrenia are discussed.