A new Swedish study may help settle the controversy over the relationship between bipolar I and bipolar II disorders. Despite genetic overlap between these two bipolar subtypes, the findings show that each type tends to cluster within families, suggesting that they are distinct disorders with different biological origins.
The study, published in the journal Biological Psychiatry, also found unique differences between the two conditions. For example, although bipolar I tends to show up among males and females somewhat equally, bipolar II is more prominent among females. In addition, bipolar I tends to cluster in families with schizophrenia, which is not the case for bipolar disorder II.
“Hopefully, our findings increase awareness of the need for refined distinctions between subtypes of mood disorder,” said study leader Dr. Jie Song of the department of clinical neuroscience at Karolinska Institutet in Sweden.
According to Song, the findings go against the common notion among many clinicians that bipolar II is merely a milder form of bipolar I. The proposed distinction between the subtypes has significant implications for patient treatment strategies.
“We have tended to view the two forms of bipolar disorder as variants of the same clinical condition. However, this new study highlights important differences in the heritable risk for these two disorders,” said Dr. John Krystal, editor of Biological Psychiatry.
The research is the first nationwide family study to investigate the differences between the two main subtypes of bipolar disorder. The researchers looked at the occurrence of each subtype among families from the Swedish national registers.
Although there is a strong genetic relationship between bipolar I and bipolar II disorders, the new findings suggest these conditions are not totally separate. Specifically, the family occurrence for each subtype was found to be stronger than co-occurrence between the subtypes, indicating that bipolar I and bipolar II disorders tend to run in families separately, rather than occurring together.
“Within the context of our emerging appreciation of polygenic risk, where gene variations are implicated in several disorders, the new findings point to only partial overlap in the risk mechanisms for these two forms of bipolar disorder,” said Krystal.
The research also offers some additional clues that bipolar I and II disorders have distinct origins. Only bipolar disorder II showed gender differences. For example, the proportion of females to males was higher in bipolar disorder II but not bipolar disorder I. In addition, bipolar I clustered together in families with schizophrenia, which was not apparent for bipolar disorder II.
Song says that future research is needed to characterize new biomarkers to help improve treatment and prognosis.