Informing DSM-5: biological boundaries between bipolar I disorder, schizoaffective disorder, and schizophrenia

The fifth version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) opted to retain existing diagnostic boundaries between bipolar I disorder, schizoaffective disorder, and schizophrenia. The debate preceding this decision focused on understanding the biologic basis of these major mental illnesses. Evidence from genetics, neuroscience, and pharmacotherapeutics informed the DSM-5 development process. The following discussion will emphasize some of the key factors at the forefront of the debate.

Family studies suggest a clear genetic link between bipolar I disorder, schizoaffective disorder, and schizophrenia. However, large-scale genome-wide association studies have not been successful in identifying susceptibility genes that make substantial etiological contributions. Boundaries between psychotic disorders are not further clarified by looking at brain morphology. The fact that symptoms of bipolar I disorder, but not schizophrenia, are often responsive to medications such as lithium and other anticonvulsants must be interpreted within a larger framework of biological research.

For DSM-5, existing nosological boundaries between bipolar I disorder and schizophrenia were retained and schizoaffective disorder preserved as an independent diagnosis since the biological data are not yet compelling enough to justify a move to a more neurodevelopmentally continuous model of psychosis.