[A case of Weber's syndrome due to gradual expansion of a basilar bifurcation aneurysm].

We report an 82-year-old female who developed Weber's syndrome following gradual expansion of a basilar bifurcation aneurysm. The patient first developed subarachnoid hemorrhage at the age of 78. Cerebral angiography showed an intracranial aneurysm of 1 cm diameter at the bifurcation of the basilar artery. Three years later, the patient was readmitted because both CT and MRI at that time revealed that the lumen of the aneurysm was filled with an organizing thrombus. On her third admission at the age of 82, the diameter of the aneurysm was expanded to 2.5 cm and Weber's syndrome had developed. On T2-weighted MRI, a central low-intensity area surrounded by a high-intensity zone were evident in the aneurysm, suggesting the formation of an organized or organizing thrombus. Repeated MRI examinations are useful for following the expansion of a thrombosed aneurysm.