Objective It is known that people with epilepsy are at an increased risk of depression, anxiety, completed suicide, bipolar symptoms and personality disorders. What is not known is what effect comorbid epilepsy has on the natural history of bipolar disorder.

Method 1242 adults with bipolar disorder were recruited into a genetic cohort study and undertook a structured interview and a number of scaled tests. Data were also collected for self-declared comorbid conditions including epilepsy.

Results 39 people had co-existing epilepsy (3.14%) representing a RR of 4.2 for epilepsy in bipolar disorder. This increased to an RR of 38 if there was a family history of epilepsy. People with epilepsy and bipolar disorder are more likely to have more mixed episodes, worse depression, as measured by BDI, BADDS and GAS, worse current manic symptoms, as measured by AMS and less likely to have rapid cycling of attacks.

Conclusion Epilepsy does not affect the onset of the first bipolar episode, nor the frequency of affective events. However, depressive symptoms are more severe during a relapse and ongoing mood has many hypomanic characteristics. The origins of hypomanic and depressive symptoms may underpin the shared genetic causes of bipolar disorder and some epilepsies. They represent an attractive tool for endophenotyping in both bipolar disorder and epilepsy.