A patient presented with the clinical features of a brain stem infarct, MRI
confirmed changes of infarction but also revealed an unusual lesion in the
left cerebellopontine angle cistern. The evolution of this lesion on MRI,
together with angiographic changes, indicated a dissecting aneurysm of the
left anterior inferior cerebellar artery, a previously unreported phenomeno
n. The presentation, causes and diagnosis of intracranial dissection are di
scussed.