Reversible dementia with idiopathic hypereosinophilic syndrome.

A 66-year-old woman with hypereosinophilic syndrome became rapidly demented. Evaluation revealed CSF eosinophilia, background slowing on EEG, and periventricular MRI abnormalities. Following steroid therapy, there was rapid resolution of the dementia and normalization of CSF and EEG. These findings support the concept of a direct neurotoxic effect on the human CNS produced either by eosinophils or possibly by eosinophil-derived neurotoxins.