Surgical treatment of internal carotid artery aneurysms around the carotid siphon is discussed. The surgical approach to the aneurysms in this region, is as follows: 1. A fronto-temporal approach with the patient in a 45° semi-sitting position to decrease venous pressure. 2. A Dolenc approach cutting a part of the dura mater of the superior orbital fissure to facilitate removal of the anterior clinoid process and unroofing of the optic canal. 3. Opening the medial triangle followed by transection of the optic canal dural sheath. Carotid siphon aneurysms can be divided into three groups anatomically; aneurysms of the ophthalmic segment (C2), those of the clinoid segment (C3), and those of the horizontal segment (C4). We present 29 cases of aneurysms arising from the C2 or C2/3 segment, 14 cases arising from the C3 or C3/4 segment, and 11 cases arising from the C4 segment. Anatomic localization of the aneurysms was established preoperatively by angiography and three-dimensional CT imaging. Small aneurysms of the ophthalmic segment projecting infero-medially can be clipped using a contralateral approach via the prechiasmatic root. Aneurysms of the ophthalmic segment projecting superiorly can be clipped following resection of the anterior clinoid process. The clinoid process should be resected intradurally with direct visualization of the aneurysms. Straight side-angled clips are suitable for these aneurysms. Carotid cave aneurysms, which include aneurysms of the ophthalmic segment oriented infero-medially and of the clinoid segment projecting postero-medially, can be clipped using curved fenestrated clips via Dolenc's extradural approach. For accurate clipping, opening of the medial triangle and full mobilization of the IC at the clinoid segment and optic nerve by unroofing the optic canal are required. Aneurysms of the horizontal portion are clipped after full exposure of the artery in the cavernous sinus only when the aneurysms are large and symptomatic. We used the fronto-temporal and Dolenc approaches and applied fenestrated clips to aneurysms oriented or postero-medially and straight or oblique clips to aneurysms projecting antero-laterally. Out of 40 aneurysms which underwent surgical clipping, 37 resulted in good post-operative recovery. There were three deaths secondary to complications of vasospasm and three cases with post operative visual loss. The classification of these aneurysms and the surgical techniques we employed are discussed in detail.
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