A 85 year old woman presented with vertigo, nausea and vomiting.  Examination showed a left sided Horner's, left facial numbness; numbness over the right body; left sided ataxia, slurred speech, and weakness of the left soft palate.

Diffusion weighted MRI of the lower medulla: Note the high signal in the lateral medulla and inferior cerebellum on the left. The abnormal diffusion weighted images indicate that the lesion is acute.