People with a history of seizure disorder are at increased risk for developing schizophrenia or schizophrenia-like psychosis, according to a study published online June 16th by the British Medical Journal. The authors suggest that the two conditions may share common genetic and/or environmental causes.

The Danish study involved 2.27 million people who were born in Denmark between 1950 and 1987 and who were identified from national registers. Personal and family histories of epilepsy/ seizure disorder and psychosis were obtained, and individuals were monitored for up to 25 years.

The team found that people with a history of seizure disorder had nearly two and a half times the risk of developing schizophrenia and nearly three times the risk of developing a schizophrenia-like psychosis compared with the general population. The risk was the same for men and women but increased with age.

Both a family history of seizure disorder and a family history of psychosis were also significant risk factors for schizophrenia and schizophrenia-like psychosis. For seizure disorder, however, the increased risk was more pronounced for people with no family history of psychosis.

The increased risk did not differ by type of seizure, but was significantly greater the older people were when they were first admitted to hospital for epilepsy.

“We think that this study is the first, on a population level, to show that a family history of epilepsy increases the risk of schizophrenia or schizophrenia-like psychosis even after adjusting for the effects of personal history of epilepsy and other factors,” wrote the authors.

“This finding suggests that genetic or environmental factors shared by family members may have an important role.”