If you do a slow wean off the Keppra, shouldn't be a problem. Jon was on Keppra as a baby, then weaned off and put on Topomax, and no seizures during wean.
Depakote is a good all-round medication good for lots of seizures, probably best for tonic-clonics. Just make sure your son gets his liver enzymes checked about once a month, and also that he has a CBC about every 2 or 3 months. Depakote can cause issues with both liver and bone marrow (it did both with our son). And it does tend to cause cognitive dulling, but that's probably better than frequent tonic clonics.
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Our most recent experience with Keppra -- Jon ended up in hospital, had 18 tonic seizures in 2 days. Doctor thought it was because diarrhea was keeping him from absorbing his meds (which was my theory as well). So, they put him on Keppra by IV, and increased his oral dose of Zonegran, and got the diarrhea stopped, and eventually got the seizures stopped.
But now we're not sure if it was the increase in the Zonegran or the Keppra that worked. At first, it seemed like Keppra was causing seizures, because after the first IV dose, he had a seizure 30 minutes later, and after the second dose, he had a seizure 30 minutes later. But then he had several other doses of Keppra with no more seizures. So...hard to figure out -- were they causing the seizures? Or just not stopping them?
My guess is that it is the Zonegran that worked because:
1) They had previously stopped his strong tonic-clonics -- he hasn't had a tonic-clonic since he started Zonegran 2 months ago.
2) He was started on the higher IV dose of Keppra on the first day in the hospital, but continued to have a seizure every 2 to 4 hours until the next morning when the dosage of Zonegran was doubled. That's when the seizures eventually stopped.
However, the doctor doesn't want to increase Jon's Zonegran for a prolonged time, because of side effects. Apparently you can be on pretty high doses of Keppra without significant issues. So I think the doctor has convinced himself that it's the Keppra, because he can increase the Keppra, but can't increase teh Zonegran.