Necropsy results came back,
Final diagnosis is Granulomatous Meningoencephalitis.
Vet's comments: "The histology features of the non-suppurative Meningoencephalitis in this case are most consistent with the entity "Granulomatous Meningoencephalitis" (GME). GME is a relatively common, progressive, invariably fatal neurologic disease of unknown origin that occurs most commonly in dogs of small breeds (e.g. Terriers and toy breeds). The reported age range is 6 months to 12 years."