Cindi,
I had a Harlequin Great Dane that I bought to breed to my female. I drove to Tuscon, Arizona for both of these dogs. My bitch was $1200, and I paid $2000 for the male. He was around six or eight months old when she went into heat at a year and a half. I didn't want her bred at that age, and kept them separate.
He had a seizure and did a complete back flip, and when he was coming out of it, as he stood swaying, he barked at me as though he didn't know who I was. It was scary. Colliepup was only about a year and a half, and I thought OMG- if he flips like that onto my baby, he will be crushed.
We took him to the vet, and decided to monitor him, with the understanding that if it continued we would put him down, because we couldn't risk our son. Well, it did not continue... until the next heat cycle, and that time he twisted a gut and got bloat. On a Sunday. Of course.
He spent the night at the emergency clinic, where they were able to tube him and flip his stomach. Another couple of days at our regular vet to stabilize him, and then he was neutered and his stomach stapled to his abdominal wall. So much for raising and showing Great Danes!
I never used the Phenobarb that was sent home, and we soon confined both dogs to a special yard. It was years before he had another seizure that we witnessed (but since he was not a house dog we may have missed them), but they were more frequent as he aged. Still only a couple a year though. But the kids were warned that if he ever acted strange that they were not to enter the "dog yard".
One other thing- he was never the same after those initial seizures- we used to say that Viktor was a few cans short of a six-pack.
I hope Marble will be okay... I know that my dear departed Chase would have been very traumatized at receiving a rectal injection since even a nail trim or ear cleaning was stressful.
((Hugs))