History and conceptual problems of the relationship between obsessions and hallucinations

Contemporary psychiatric nomenclature defines schizophrenia (SCZ) and obsessive-compulsive disorder (OCD) as distinct disease entities characterized by non-overlapping diagnostic criteria. Nevertheless, a complex association between SCZ and OCD exists on the psychopathological level. And although the relationship between obsessions and delusions has been widely studied and discussed, the relationship between obsessions and hallucinations has not received the same attention. This article presents an historical overview of the studies on the co-occurrence of obsessions and hallucinations. We also analyze the clinical significance of this overlap, as discussed in the early descriptions of these phenomena in the nineteenth century and continuing through the most recent, contemporary conceptualizations. In clinical practice today, we may encounter both SCZ patients with typical ego-dystonic obsessive-compulsive symptoms and SCZ patients affected by obsessions that intertwine with psychotic symptoms, generating complex psychopathological syndromes (e.g. "obsessive hallucinations"). A further complication is that some OCD patients show perceptual disturbances. Taking into consideration the possible coexistence of obsessive-compulsive symptoms and psychotic symptoms is crucial for proper diagnosis, prognosis, and treatment. Further investigations are required to fully evaluate the psychopathological interrelationships between obsessions and hallucinations.