So the grain substitutes block the absorption of taurine which is an essential amino acid for proper heart function. There are a lot of examples of dogs whose hearts went back to normal size and function - or at least close to it - after putting them on diet with corn in it. FYI, my dogs do...
Any chance you feed grain free? It potentially sounds like DCM. Or, FYI, if the dog does have seizures (dcm aside, not related), gsps so have an unmapped Gene for epilepsy in some lines OR some other non-genetic disposition to seizures. But my vote is dcm.