Idiopathic seizures are those whose cause is unknown. Unfortunately, about 6 out of 10 seizures are idiopathic.
In the case of focal seizures, we presume that there is an irritation to or scar on some part of the brain, but the scar is invisible to MRI. With generalized seizures, the genetic or metabolic abnormality is unidentified.
Patients and families are mystified by the absence of answers, typified by the common refrain, "The doctor’s told me everything was normal." As frustrating as this may be, two points should be reassuring. First, no tumor, stroke, infection, vascular malformation or other problem was found. Second, we do not need to know the cause to use medicines to treat the seizures.
As MRI and other forms of imaging the brain continue to improve, more and more causes of seizures will be identified.