A 54-year-old man with a 9-year history of Parkinson's disease, treated with levodopa/carbidopa 200/25 mg t.i.d., piribedil 50 mg q.i.d., and amantadine 100 mg b.i.d., presented with subacute onset confusion. Symptoms started 10 days after oral prednisone therapy. Corticoid treatment, in treatment for lumbar disk herniation, consisted of 60 mg o.d. for 5 days (without progressive tapering). No recent changes in medication for Parkinson's disease were made. There was no history of alcohol abuse, and no recent infection was reported. Clinical examination showed drowsiness and reduced spontaneous speech, in the absence of fever. Blood pressure was 135/76 mmHg. Blood count, C-reactive protein (CRP), renal and liver function tests, osmolality, levels of glucose, sodium, folate, vitamin B1, B6 and B12 were normal. Serology for Mycoplasma, Epstein-Barr virus (EBV), cytomegalovirus (CMV), herpes simplex virus (HSV), and varicella zoster virus (VZV) was negative. Brain magnetic resonance imaging (MRI) revealed a lesion in the central part of the splenium of the corpus callosum (SCC), hyperintense on T2, fluid attenuated inversion recovery (FLAIR), and diffusion-weighted images (DWI), and hypointense on T1-weighted images and apparent diffusion coefficient (ADC) map, in absence of gadolinium contrast enhancement (Fig.