[Hypoplasia of the left internal carotid artery associated with anterior communicating aneurysm, intercarotid anastomosis and left rete carotidis].

Hypoplasia of the internal carotid artery with cerebral aneurysm is extremely rare. The authors report a case of hypoplasia of the left internal carotid artery associated with anterior communicating aneurysm, intercarotid anastomosis and left rete carotidis. A 45-year-old man was admitted to Minamata Municipal Hospital because of severe headache and disturbance of consciousness. CT scan showed subarachnoid hemorrhage with intracerebral hematoma. Right carotid angiogram showed the anterior communicating aneurysm arising from the right A1-A2 corner and intercarotid anastomosis on the cavernous portion. The distal part from the C4 portion of the left internal carotid artery was revealed through these anastomotic vessels. Left carotid angiogram showed hypoplasia of the left internal carotid artery and unusual tortuous collateral vessels arising from the left internal maxillary artery, ascending pharyngeal artery and its branches to the left internal carotid artery. These collateral vessels are called "rete carotidis" or "rete mirabile caroticum." Both the intercarotid anastomosis and rete carotidis are unusual anastomotic vessels and are considered not to appear during normal development of the cranial circulation in humans. CT scan with skull base bone target image showed hypoplasia of the left carotid canal. In this case, the change in cranial circulation due to hypoplasia of the internal carotid artery was considered one of the main factors in the development of these unusual anastomotic vessels and cerebral aneurysm.