A 6-year-old girl is brought to the emergency department with a 3-day history of fever, headache, and progressive lethargy, culminating in a generalized seizure. A brain MRI reveals large, bilateral, poorly demarcated T2 hyperintense lesions in the cerebral white matter, thalami, and basal ganglia. What feature is essential for diagnosing this patient with acute disseminated encephalomyelitis (ADEM) and distinguishing it from a first attack of multiple sclerosis (MS)?