It is now well-established that there is a partial overlap of genetic influences between bipolar disorder and schizophrenia, with a genetic correlation of around 0.6.While the symptoms of these two chronic mental health disorders may appear similar, they are in fact distinct. Bipolar disorder is characterized by extreme mood swings, while schizophrenia is characterized by delusions and hallucinations. In addition to the differences in symptoms, research has also shown that patients with either disorder experience a decline in cognitive function after the onset of the disease. However, this decline is more pronounced in schizophrenia than in bipolar disorder.
Bipolar II disorder is diagnosed when a person has experienced at least one episode of major depression and at least one episode of hypomania (a milder form of mania). The debate over whether bipolar disorder and schizophrenia are separate disorders or part of the same spectrum continues to be a topic of discussion among researchers. Some believe that shared polygenes with psychotic bipolar disorder influence liability to schizophrenia and predispose to the development of dopamine dysregulation following environmental stressors. On the other hand, some argue that both disorders cause the same general cognitive deficits.
A study involving 350 coordinates (184 for bipolar disorder, 166 for schizophrenia) from selected studies found that schizophrenia and bipolar disorders are not completely dichotomous entities, at least at the level of the neuroanatomical phenotype. Further analysis of data from registries and hospitals (1973 to 200) involving 9 million Swedes from 2 million families revealed that there is an increased risk of schizophrenia and bipolar disorder among biological and non-biological relatives of patients with one or both disorders. In addition, research has shown that caudate head injuries can affect the motivational and learning abilities present in both schizophrenia and bipolar disorder. A further objective was to understand how the overall genetic risk of each disease is related to the presence of symptoms that a person with either disorder will have.
Overall, it appears that gray matter deficits identified in bipolar disorder are more restricted in extent compared to schizophrenia. While a major depressive episode is not required for the diagnosis of bipolar disorder, depression often alternates with manic episodes and tends to occur more frequently than mania in many people. In conclusion, it is clear that there is a partial overlap between bipolar disorder and schizophrenia, both in terms of genetic influences and symptoms. However, further research is needed to understand how these two disorders are related and how they differ.