The same exact thing happened to my Silver Appleyard.
It was a genetic issue, sometimes there can be an acidental breeding and they can inherent the genes from a Crested duck which, because of a defect causing a hole in the skull, makes them prone to seizures.
We took her into a aviary veterinarian were they purscribed Phenobarbital which is used to control seizures in dogs, it also works for ducks.
It helps control the seizures until they can grow out of it.
It took a lot of work, full of late nights and early mornings, but she grew out of it and got better. She was very lucky.
A vitamin B complex may help as well.
We kept her inside so we could keep an eye on her. When she had a seizure we held her wings down and kept her still so she didn't hurt herself. After it was over we held her in our arms until she could stand, because she was to weak to stand from the seizures.