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Background: With similarities in heritability, neurobiology and symptomatology, the question has been raised whether schizophrenia and bipolar disorder are truly distinctive disorders or belong to a continuum. This narrative review summarizes common and distinctive findings from genetics, neuroimaging, cognition and clinical course that may help to solve this ethiopathogenetic puzzle. Methods: The authors conducted a literature search for papers listed in PubMed and Google Scholar, using the search terms "schizophrenia" and "bipolar disorder" combined with different terms such as "genes", "neuroimaging studies", "phenomenology differences", "cognition", "epidemiology". Articles were considered for inclusion if they were written in English or Spanish, published as full articles, if they compared subjects with schizophrenia and bipolar disorder, or subjects with either disorder with healthy controls, addressing differences between groups. Results: Several findings support the hypothesis that schizophrenia and bipolar disorder are discrete disorders, yet some overlapping of findings exists. The evidence for heritability of both SZ and BD is obvious, as well as the environmental impact on individual manifestations of both disorders. Neuroimaging studies support subtle differences between disorders, it appears to be rather a pattern of irregularities than an unequivocally unique finding distinguishing schizophrenia from bipolar disorder. The cognitive profile displays differences between disorders in certain domains, such as premorbid intellectual functioning and executive functions. Finally, the timing and trajectory of cognitive impairment in both disorders also differs. Conclusion: The question whether SZ and BD belong to a continuum or are separate disorders remains a challenge for further research. Currently, our research tools may be not precise enough to carve out distinctive, unique and undisputable differences between SZ and BD, but current evidence favors separate disorders. Given that differences are subtle, a way to overcome diagnostic uncertainties in the future could be the application of artificial intelligence based on BigData. Limitations: Despite the detailed search, this article is not a full and complete review of all available studies on the topic. The search and selection of papers was also limited to articles in English and Spanish. Selection of papers and conclusions may be biased by the personal view and clinical experience of the authors.
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OBJECTIVE: We examined age of onset of bipolar disorder as a potential course-of-illness modifier with the hypothesis that early onset will engender more severe illness. STUDY DESIGN: A total of 480 carefully diagnosed adult outpatients with bipolar disorder (mean age, 42.5 +/- 11.6 years) were retrospectively rated for age of illness onset, time to first pharmacotherapy, and course of illness. Clinicians prospectively rated daily mood fluctuations over 1 year. RESULTS: Of the 480 patients, 14% experienced onset in childhood (12 years or younger); 36% in adolescence (13 to 18 years); 32% in early adulthood (19 to 29 years); and 19% in late adulthood (after 30 years). Childhood-onset bipolar illness was associated with long delays to first treatment, averaging more than 16 years. The patients with childhood or adolescent onset reported more episodes, more comorbidities, and rapid cycling retrospectively; prospectively, they demonstrated more severe mania, depression, and fewer days well. CONCLUSIONS: This study demonstrates that childhood onset of bipolar disorder is common and is associated with long delays to first treatment. Physicians and clinicians should be alert to a possible bipolar diagnosis in children in hopes of shortening the time to initiating treatment and perhaps ameliorating the otherwise adverse course of illness.