It wasn't a great dane, but my boyfriend's dog is somewhere in the 10-12 y/o range and had a spell where she was having seizures, walking in strange directions, and was generally unwell. they didn't think she would make it past a day or two, but it turned out what she had was an extremely severe inner ear infection. they put her on medicine for it and she still lives to wreak havoc, though she wasn't exactly the same dog because of the seizures. I don't know if thats useful or fit the bill for your situation, but i thought it sounded similar