Depression is more common among people with epilepsy than in the general population. Just as there are many types of epilepsy with different causes, the causes of depression also vary from person to person.

The psychological effects of having recurrent seizures are thought to be responsible for depression in many people with epilepsy, because of the uncertainty and stress of having seizures, and the social burden of stigma associated with epilepsy.

Side effects of anti-epileptic drugs (AEDs) may also be responsible for depression. This has recently been highlighted in an analysis of data from 199 studies on 11 AEDs. Some of these studies were in epilepsy, but others were in migraine or bipolar disorder, where AEDs are also prescribed. Twice as many people taking AEDs as taking placebo (dummy pills) experienced suicidal thoughts. The rate of suicide itself was also higher among the people taking AEDs.

Because this effect was seen in a number of medical conditions, it can't be due to their underlying causes, and must be due to the medications. The medicines regulatory agency in the USA, the Food and Drug Administration, advised all doctors to be alert to these risks when they prescribe AEDs. Though the study did not include all available AEDs, they are all expected to have similar effects in this regard. Read more here

Biological factors are also thought to be responsible for depression in some people with epilepsy. In some syndromes, the changes in cell development and structure that cause seizures may also cause mood disorders.

Researchers from the University of Melbourne recently investigated this by comparing patterns of behaviour in rats with absence seizures (caused by an inherited gene mutation) with behaviour of rats without epilepsy. Depression behaviour was measured using the Sucrose Preference Test, where a rat has to choose between two sources of fluid, one harder to reach than the other. Anxiety was measured using the Elevated Plus Maze, where a rat has to choose between spending time in a covered, dark place or in an open, exposed one, and the Open Field Arena, where the pattern and amount of a rat's movements is measured.

The rats with absence seizures consumed less sucrose, spent less time in the open arms of the Maze and did less exploring in the Arena than the rats which did not have epilepsy. These factors all indicate higher levels of depression and anxiety in these rats, which occurred both before and after seizures developed. This suggests that the depression here is not caused by the seizures, but is instead due to an underlying biological factor. This study was published in the journal Experimental Neurology in October 2007.

In addition to depression being more common among people with epilepsy, it appears that epilepsy is more common among people with depression. Can each condition cause the other? Or do they have common causes? An intriguing study exploring this was published in 2006 in the Annals of Neurology.

There are other possible causes for depression associated with epilepsy. There is some evidence that continually experiencing seizures may cause the brain to age more quickly than normal, which may also affect a person's tendency to depression. Clearly, there's a lot of research to be done on this topic.