What is the Relevance of Sodium in Epilepsy Control

I'm an epileptic since birth. (I'm 46 years old now). Occasionally when I have samples taken by my doctor to check my blood levels I appear to be "Low in Sodium". When I ask my doctor what relevance Sodium has in controlling/eliminating fits he tells me bluntly "It would take too long to explain". I would like to knkow what part Sodium plays, what foods, Cod Liver Oil, Vitamins etc I might consume to offset this shortfall. (Salt is not one of these I'm sure).

When I look up websites like 'Weight Watchers" I find 'Low Sodium Diets': nobody seems to suggest 'Sodium Rich Diets'.

Can anybody answers for me please?