What is acute disseminated encephalomyelitis (ADEM), and how does it typically present and appear on MRI?

• A. An isolated brain tumor
• B. A chronic degenerative disease with normal MRI
• C. A mild viral syndrome with normal MRI
• D. Post-infectious inflammatory demyelinating disorder with multifocal neurologic deficits and encephalopathy; MRI shows large asymmetric white matter lesions

Answer: (D)

ADEM is a post-infectious inflammatory demyelinating disorder that typically follows a viral illness or vaccination and presents with multifocal neurologic deficits along with encephalopathy. This combination—acute/subacute onset after infection plus altered mental status or-level consciousness—is key for identification in children.

On MRI, ADEM shows large, poorly defined white matter lesions that can be asymmetric and may involve subcortical regions as well as deep gray structures. These lesions are usually T2/FLAIR hyperintense, and post-contrast enhancement can be variable. The pattern and size of these lesions, along with the acute, encephalopathic presentation after an infection, help differentiate ADEM from other conditions.

The other options don’t fit because: a brain tumor would be a focal mass lesion rather than a diffuse, inflammatory process; a chronic degenerative disease would not typically present acutely with encephalopathy and often shows progressive, long-standing changes or a normal MRI; a mild viral syndrome would lack encephalopathy and would not produce the large demyelinating lesions seen on MRI.