Psychosis, mania, and depression are unlikely to have shared hereditary causes, report researchers in Molecular Psychiatry.
They found that the three conditions aggregated within families, but the presence of one condition did not increase the likelihood for another occurring.
“This suggests that there are distinct underlying etiological pathways for the major subtypes of mood disorders that should be pursued in future research on these disorders,” say lead researcher Caroline Vandeleur (University Hospital of Lausanne, Prilly, Switzerland) and co-workers.
The team studied 62 patients with schizoaffective disorder, 100 with bipolar I disorder, 23 with bipolar II disorder, 108 with major depressive disorder (MDD), and 110 orthopedic controls, as well as 1734 of their first-degree relatives.
They found that patients with bipolar I disorder had a 12.2-fold increased chance for having a relative with the same disorder, but were no more likely than controls to have a relative with schizoaffective disorder, bipolar II disorder, or MDD.
Bipolar II disorder, however, did not aggregate within families. Instead, patients with this disorder were twofold more likely than controls to have a relative with MDD, while MDD itself predicted having relatives with MDD.
This “casts doubt on the validity” of bipolar II disorder, say Vandeleur et al. “Rather, the evidence of shared underlying diatheses between [bipolar II disorder] and MDD suggests that hypomanic mood episodes that alternate with episodes of major depression are likely to represent a particular clinical manifestation of MDD rather than a subgroup of [bipolar disorder].”
A similar pattern emerged when the team analyzed the type of mood episode, rather than the diagnostic category; mania and depression strongly and specifically aggregated within families, whereas hypomania did not.
Patients with schizoaffective disorder had an increased likelihood for having relatives with the same disorder, but also with bipolar I and II disorder. But analysis of mood episodes showed that psychosis was also strongly retained within families, and did not relate to mania or depression.
This shows “the importance of suspension of arbitrary diagnostic distinctions,” say the researchers and suggests it may be worth assessing psychosis, mania, and depression separately, rather than within the current diagnostic categories.
However, they note that other factors, such as neuroimaging and treatment response, do provide boundaries between the mood conditions, and that their etiology is probably multifactorial.
“Our findings demonstrate that continued dissection of the core features of these conditions will be essential to our future understanding of their etiology, course and treatment,” the team concludes.
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