The Relationship Between Bipolar Disorder and Schizophrenia: Psychopathological Mechanisms Underlying the Blurred Boundary

This thesis comprises six chapters, including four empirical studies (Chapters 2-5), aimed at deepening the understanding of a long-standing clinical dilemma: whether schizophrenia and bipolar spectrum disorders (BSD) represent distinct disease entities or a single disorder with differing manifestations.
The thesis utilises various empirical approaches to examine psychopathological patterns, including dimensional and structural explorations of symptoms and at-risk states for mood and psychosis. Methodologically, this includes a systematic review of the prevalence of psychosis in BSD, alongside network analyses and dimensional comparisons. These studies utilised existing questionnaires and structured interviews across diverse cohorts: patients with psychotic disorders, the general population, and young people assessed for newly developed at-risk states.
Findings from the systematic review (covering 285 articles) indicate that psychosis is highly prevalent in BSD and identified a close relationship between Bipolar I disorder and psychosis. Consequently, network analysis revealed a robust link between mania and psychosis. This relationship remained consistent across patient and healthy populations and demonstrated longitudinal validity. Furthermore, network modelling uncovered distinctive patterns: mania and positive psychosis were linked by behavioural activation and thought disorganisation, whereas depression and negative psychosis were bridged by reduced emotional expression and social coldness. Additionally, the network of at-risk states revealed that identity confusion and suspiciousness were significant symptoms potentially driving the network.
In conclusion, this thesis highlights that BSD and schizophrenia are connected not only by shared risk factors but also at the symptom level. They likely share a latent psychopathological process that manifests differently, offering strong support for a transdiagnostic approach to psychiatric practice. Future research should adopt extensive longitudinal designs capable of inferring robust causal relationships to further improve the understanding of these mental disorders.