The relation between the clinical course and MRI findings and response to plasmapheresis were determined by a retrospective record review of 13 children with acute disseminated encephalomyelitis (ADEM) admitted to St Christopher’s Hospital for Children, Philadelphia, PA, during 1998-2003. Six of 12 children responded to initial treatment with high-dose methylprednisolone and IV immunoglobulin; one improved spontaneously. The 6 who were unresponsive had a progressive neurologic course, and 5 had delayed onset of MRI abnormalities involving deep gray matter and brainstem. These 5 recovered slowly following plasmapheresis, with residual neurologic deficits. 
COMMENT. MRI abnormalities may appear late in the course of ADEM, leading to a delay in diagnosis and treatment. In almost half the patients in the above series, a significant lag occurred, up to a few weeks, between the presentation of symptoms with fever, ataxia, paresis and mental changes, and the appearance of MRI lesions, usually involving the basal ganglia, thalami, and brainstem, as well as subcortical white matter. Late development of MRI lesions involving deep gray matter and brainstem may be predictive of a prolonged course and lack of response to corticosteroids. The absence of white-matter changes in the MRI up to 3 weeks after onset of symptoms does not exclude a diagnosis of ADEM. Plasmapheresis is recommended in patients with a severe progressive course, unresponsive to corticosteroids and IV immunoglobulin.