Chihuahua Seizures, Flea Treatments

Let me just begin this post by saying that chihuahuas are prone to having seizures at times, especially if they are on the small side, however, I have to tell you what I have discovered, at least about my own chis. My Shorty is about 2 pounds and as in some of my past postings, I have reported him having seizures. Well, I used to use that spot on treatment for fleas, and found that it only worked a few weeks, which was very expensive for me.. I also noticed that since I stopped the spot on treatment, he has NOT had any more seizures so now I am wondering if that was the cause of his. Yes, I do have to battle fleas and I think I have found that food grade Diamitous Earth works, just as a combo of salt/baking soda on the carpets swept in. That also includes treating the yard for fleas off of other animals who pass through. Do I dare wonder if the spot on treatments ( I have used several of them) could have contributed to the seizures? I have done some reading on the internet and I hear alot of nasty things. What do you think? Do you have an opinion on this subject? If so, I would love to hear it.