On admission, the patient complained of dizziness and an unsteady gait, followed by weakness of his left extremities and apathy after 4 days. Neurologic examination revealed left central facial and hypoglossus nerve palsy, visual neglect, left-sided hemiparesis with increased deep tendon reflexes, the Babinski sign, and spasticity. Decreased proprioceptive sensation was found on the left side of the body. His movements, speech, and attention were slowed, but memory and body perception were maintained; neither aphasia nor apraxia was detected. Electroencephalography showed a nonspecific diffuse slowing of background activity above the right frontotemporal regions. Cerebrospinal fluid examination revealed normal protein levels (0.25 g/L) and glucose content without cells. No intrathecal immunoglobulin synthesis or damage of blood-brain barrier were found (IgG index, 0.7) (no oligoclonal bands were detected by isoelectric focusing). Cultures were negative for bacteria and fungi. Antiviral antibodies for cytomegalovirus, varicella-zoster virus, herpes simplex virus 1, and herpes simplex virus 2 were not found; herpes simplex virus type 1 DNA could not be detected by polymerase chain reaction in the cerebrospinal fluid. Serology tests were negative for the human immunodeficiency virus (HIV). Immunoglobulin levels in the serum were measured twice. IgG levels were normal (6.9 g/L; normal range, 6-16 g/L), IgM levels were slightly increased (2.18 g/L; normal range, 0.5-2.0 g/L), and IgA levels were reduced 4 days after the patient's first symptoms (<0.2 g/L; normal range, 0.8-4.0 g/L); similar results were obtained 1 month later (IgG, 8.5 g/L; IgM, 3.16 g/L; IgA, 0.1 g/L). Serial cranial magnetic resonance imaging showed 2 large lesions with a high signal on T2-weighted images and a low signal on T1-weighted images in the white matter of the right frontal and temporal lobes. The lesions progressively extended into the right side of the brainstem and into the left hemisphere through the corpus callosum (Figure, A). None of the lesions showed contrast enhancement, and the ipsilateral third ventricle was slightly compressed.