From: http://psychiatricresourceforum.blogs.com/my_weblog/2006/05/index.html

It is now almost a century since the German psychiatrist Emil Kraepelin  first tried to separate major mental illness into dementia praecox (schizophrenia) and manic depression (bipolar disorder), primarily on the basis of longitudinal course: he claimed that the people with dementia praecox became worse over time and that the people with manic depression did not. There were supposed to be some other differences, but over time, it became clear that things were not so simple. In fact, when he died in 1926, he was working on a whole re-visioning of major mental illness, and returning to the idea that bipolar disorder and schizophrenia lie on a spectrum, which may also include some of the Cluster A Personality Disorder (Schizoid, schizotypal and paranoid) and Major Depressive Disorder. This idea of a continuum of different psychiatric disorders has informed a great deal of clinical practice and research, particularly in Europe.

This is an important question for clinicians who want to understand why the same patients so often present with quite different symptoms, how to explain this and the genetic risks to family members, and why it is that some medicines are effective in both schizophrenia and bipolar disorder.

There is an excellent new review article on the topic by some colleagues at the University of Bonn in Germany.

We agree with their primary conclusion, that it is very difficult – if not impossible – to separate schizophrenia and bipolar disorder. That does not mean that they are the same disorder: but it does mean that there is a great deal of biochemical, biological and genetic overlap. There are some genes that are common to both disorders, though there are others e.g. neuregulin, dysbindin, Catechol-O-methyltransferase, proline dehydrogenase and α-7 nicotinic acetylcholine receptors - that appear to be more commonly found in people with schizophrenia. Problems with bipolar disorder have some very reproducible problems in identifying and responding to emotions, while people with the schizophrenia diagnosis tend to have no reactions. It is also much easier to identify children – particularly boys - who may develop schizophrenia later life, than it is to identify children and adolescents at risk of later developing bipolar disorder.

This research into the differences and similarities of schizophrenia and bipolar disorder is continuing and attracting some first rate investigators. We shall keep you updated with new findings as they appear, and discuss the practical implications for practicing clinicians.