The transdiagnostic dimension of psychosis: implications for psychiatric nosology and research

If psychosis is a transdiagnostic dimension, the expression of which is governed by a dynamic set of contextual and emotional factors that are amenable to treatment, current approaches in psychiatric nosology and therapeutic research may need to be revised. The dominant approach to date is to clinically and conceptually situate psychotic symptoms in the construct of schizophrenia. However, schizophrenia, which has a lifetime prevalence of 1%, only represents the poor outcome fraction of a much broader spectrum of psychotic disorders which have a lifetime prevalence of 3.5%. Therefore, research findings in schizophrenia may reflect mechanisms of prognosis rather than fundamental associations with psychosis and other symptom domains per se. Similarly, the discovery that up to 30% of individuals with non-psychotic common mental disorders have subthreshold psychotic symptoms that situate them on the transdiagnostic dimension of psychosis – and which impact clinical severity and treatment response – indicates that the rigid separation between ‘psychotic’ and ‘non-psychotic’ hampers both clinical practice and research. Diagnostic manuals in psychiatry would benefit from a system of transdiagnostic dimensions, including a transdiagnostic dimension of psychosis. Introduction of transdiagnostic dimensions allows for a system combining a nomothetic (i.e., group-specific) categorical diagnosis with an idiographic (i.e., person-specific) combination of dimensional scores. The advantage of such a system is that it encourages consideration of how symptoms dynamically interact with each other in a network of psychopathology, and of how this network is impacted by the social world.