How often do you have epilepsy at the same time as mental retardation?

It is not unusual. I do not have the statistics at my fingertips, but there is an increased risk of seizures among people with mental retardation, compared to the general population.
Common cause. You can think of mr as coming from a brain that isn't "put together right, " so it shouldn't be too surprising that that brain might also be more a risk for having seizures.
2010 study. In a recent prospective study in epilepsia, 51(12) by sogawa and masur, 74% of children with epilepsy had average overall cognitive function ( iq); 24% had lower cognitive function.