Schizophrenia and bipolar disorder: similarities in pathogenic mechanisms but differences in neurodevelopment

Over the past 100 years, the Kraepelinian classification of psychoses has dominated our approach to schizophrenia and bipolar disorder. However, controversy as to the nature of the illnesses--whether they can be viewed as completely distinct, essentially the same, or occupying different points along a psychosis spectrum--has intensified in recent years. This paper reviews the evidence for these differing opinions, examining both the commonalities between the two diseases and the distinctions. A genetic propensity towards psychotic disorders is widely acknowledged; more recent studies suggest a considerable overlap in genetic susceptibility to schizophrenia and bipolar disorder. The influence of early environmental effects, such as obstetric complications, on schizophrenia is also established but little such evidence exists for bipolar disorder. Structural abnormalities of the brain of developmental origin as well as neuropsychological deficits have been clearly identified in schizophrenia but less evidence has been found in bipolar disorder. The most plausible explanation is that one or more susceptible genes are shared between schizophrenia and bipolar illness, and can be thought of as predisposing individuals to psychosis, perhaps by producing a dysregulation of the dopaminergic response to stress. Other genes and environmental factors are likely to have more specific effects and contribute to producing the patterns that psychiatrists recognize as 'classical' schizophrenia and mania. In particular, genes involved in early cortical development and early neurodevelopmental insults causing developmental impairment may put individuals on a trajectory towards schizophrenia rather than bipolar illness.