What Causes Absence Seizures While Taking Dilantin For Grand Mal?

For Absence seizures that last a few seconds.every few days... Keppra ? I am already taking DILANTIN 400mg a day for "grand mal"...now under control. but,now,like I said...absence seizure every once in a while.I suppose it can be called also : "PETIT MAL" ! An answer will be appreciated. Thanks. Edgard.