I was told today by a pharmacist that two of the anticonvulsants i have been taking for 20 years can interact with each other and cause excess buildup of ammonia and potential brain swelling. i take dilantin, depakote, and topomax. thank you?

Needs to be put into perspective. Both Dilantin, and Depakote, could theoretically cause liver damage (rather uncommon event) and this could result in high ammonia. Not clear to me, if you truly require 3 anti-epileptic drugs, and you might consider modification. Talk to your neurologist about your future medication usage, as this depends upon seizure frequency, co-morbidities, etc.