CHIZOPHRENIA and bipolar disorder share common genetic causes and the two conditions may simply be different manifestations of the same disease, a study of the entire Swedish population suggests.
The question of whether the two mental illnesses are distinct entities or represent a disease continuum is widely debated in psychiatry and the report, published in The Lancet, calls for a rethink on the way schizophrenia and bipolar disorder are classified.
But the executive director of the Black Dog Institute, Professor Gordon Parker, said he would be "immensely worried" if the two conditions were reappraised into the one all-encompassing illness.
Some drugs, such as Seroquel and Zyprexa, are already used to treat both illnesses. But Professor Parker, a leading bipolar expert, said while all people with schizophrenia by definition, have psychosis, not all people with bipolar disorder do.
"If we argue that they're the same condition, we could end up putting all bipolar patients on anti-pyschotic medication, but why would you want to do that when there's known risks and side-effects?" he said.
Researchers from the Karolinska Institutet in Stockholm analysed the 9 million-strong population of Sweden over a 30-year period and found close relatives of people with schizophrenia or bipolar disorder had an increased risk of both conditions, compared to the general population.
Evidence from adoptive and half-siblings suggested the effect was mainly due to genetic factors.
"These results challenge the current nosological (disease classification) dichotomy between schizophrenia and bipolar disorder, and are consistent with a reappraisal of these disorders as distinct diagnostic entities," lead author Paul Lichtenstein and colleagues wrote.
The researchers said the finding could be used to improve treatment options and develop new medication.