[Rupture of a small aneurysm originating from an infundibular dilatation--case report].

Infundibular dilatations (IDs) are funnel-shaped enlargements of the origin of cerebral arteries. Usually IDs occur at the junction between posterior communicating artery and the internal carotid artery. Progression from an ID of the posterior communicating artery to an aneurysm has previously been described, but it is unclear whether an ID is a pre-aneurysmal state or a normal anatomical variant. In this study, we describe the successful treatment of a small aneurysm originating from a pre-aneurysmal ID. A 63-year-old man suddenly developed severe headache and consciousness disturbance and was admitted to our hospital. CT scans revealed subarachnoid hemorrhage (SAH) mainly in the left sylvian fissure. Three-dimensional computed tomographic angiography (3D-CTA) revealed an ID of the left posterior communicating artery. A small aneurysm of 1.5 mm in diameter was also observerd to extend posterolaterally from the wall of the ID. In addition the left posterior communicating artery was well developed, and the angle between the left internal carotid artery and left posterior communicating artery was large. On the bosis of the CT and 3D-CTA findings, the small aneurysm originating from the ID was considered to be the cause of SAH. On day 18, left pterional craniotomy was performed, and the patient underwent clipping of the small aneurysm. The postoperative course was uneventful. An ID of the posterior communicating artery can develop into an aneurysm and subsequently rupture. The development of an aneurysm from the ID may be influenced by hemodynamic stress and hypertension. Thus patients with the pre-aneurysmal ID should be carefully followed up for a long time.