This is way over my head, but you can read it and see what you think. Have you discussed idiopathic encephalitis? I found this info:
What is idiopathic encephalitis?
Idiopathic encephalitis is a general term for cases in which an infectious cause cannot be found. In many cases of idiopathic encephalitis, an underlying immune-mediated cause is suspected because these animals improve after suppression of the immune system. Immune-mediated diseases occur when the body's immune system (white blood cells) mistakenly attacks normal tissue, in this case brain or spinal cord. There have been a number of different types of idiopathic encephalitis described in dogs. A few examples of idiopathic encephalitis are listed below along with characteristics of each disease:
•Granulomatous meningocencephalomyelitis (GME)
•Synonyms: inflammatory reticulosis
•Breed/Age: any breed but usually small breed dogs (Terriers and Miniature Poodles over represented), predominantly middle aged (3 to 6 years-old)
•Clinical features: may affect any area of the brain or spinal cord, may be focal or multifocal
Veterinary Teaching Hospital :: North Carolina State University College of Veterinary Medicine