Schizophrenia and bipolar disorder (previously manic depression) may be different manifestations of the same disease, according to a new study published in The Lancet medial journal last week. While hundreds or even thousands of genes, plus environmental factors, may contribute to the development of either condition, some genes may be involved in both, the research suggests.

Authors Dr Paul Lichtenstein and Dr Christina Hultman, from the Karolinska Institute in Stockholm, Sweden, wrote: 'We showed evidence that schizophrenia and bipolar disorder partly share a common genetic cause. These results challenge the current dichotomy between schizophrenia and bipolar disorder, and are consistent with a reappraisal of these disorders as distinct diagnostic entities.'

The research team identified 35,985 people with schizophrenia (0.40 per cent of the population) and 40,487 people with bipolar disorder (0.45 per cent of the population) and used Sweden's multi-generation register, a population database that links nearly every person born in Sweden (population: around 9 million) to link each individual to his or her parents, to ascertain which of these people were related.

The data suggested that those with a family history of either schizophrenia or bipolar disorder had an increased risk of both, suggesting an inherited link between the two conditions. Those with a parent with schizophrenia were 9.9 times more likely to have schizophrenia themselves and 5.2 times more likely to have bipolar disorder. Similarly those with a parent with bipolar disorders were 6.4 times more likely to have bipolar disorders and 2.4 times more likely to have schizophrenia. The research suggests that schizophrenia is 64 per cent inherited rather than being caused by environmental factors, while the heritability of bipolar disorder is 59 per cent. Genetic factors largely accounted for cases where people had both conditions.

The findings suggest that some mental health conditions need to be reclassified in the light of new genetic studies. 'How many distinct disorders it might be useful to recognise, or whether the functional psychoses are better conceptualised as a continuum or as a set of overlapping pathological processes, is not yet known', wrote Michael Owen and Nick Craddick of the University of Cardiff, Wales, in an accompanying commentary.