Bipolar disorder and schizophrenia: distinct illnesses or a continuum?

Bipolar disorder continues to present complex diagnostic and therapeutic challenges. Originally considered 2 separate diseases (mania and depression), bipolar disorder is now recognized to be a single disorder characterized by different subtypes and degrees of severity. Despite the availability of official guidelines, such as the DSM-IV and ICD-10, diagnosis is still problematic. Traditionally, bipolar disorder has been considered a clinical entity distinct from schizophrenia, although that assumption is being increasingly challenged. Proponents of a bipolar continuum theory support the concept of an expanded psychiatric continuum ranging from unipolar to bipolar disorders all the way to schizophrenia. This notion is supported by various independent findings. Both bipolar disorder and schizophrenia demonstrate a high degree of genetic transmissibility. Some data reported in family and twin studies suggest hereditary overlap between the 2 disorders. Gene mapping for both diseases is in its early stages, but certain susceptibility markers appear to be located on the same chromosomes. Bipolar disorder and schizophrenia also demonstrate some similarities in neurotransmitter dysfunction. As further indirect evidence of a possible association, many newer atypical antipsychotic agents approved for the treatment of schizophrenia are also proving useful for bipolar disorder. Ongoing research should aid in the understanding of bipolar disorder and foster the development of more effective treatment.