MRI - Arnold-Chiari malformation and hydrocephalus: This 47-year-old woman presented with headaches and clumsiness of gait. Examination was significant for diminished upgaze, truncal instability and bilateral hyperreflexia. The T1-weighted axial MR image on the left shows symmetrical marked enlargement of the lateral ventricles with slight effacement of the cortical sulci. The T1-weighted sagittal image on the right demonstrates an inferior displacement of the cerebellar tonsils and a fourth ventricle that does not appear enlarged.