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(1) Background: The normal A1 segment of the anterior cerebral artery (ACA) has a supraoptic course. The proximal infraoptic course of an A1 segment leaving the internal carotid artery (ICA) near the origin of the ophthalmic artery is a rare possibility. This study aimed to determine the prevalence and detailed anatomy of infraoptic A1 segments. (2) Methods: We retrospectively studied 145 computed tomography angiograms from 92 male and 53 female cases, with ages varying from 61 to 78 y.o. (3) Results: In 21/145 cases, infraoptic or infrachiasmatic horizontal-medial courses of A1 segments that emerged distally from the ICA were found. Distal infraoptic A1 segments were bilateral in 16/145 cases and unilateral in 3/145 cases. Infrachiasmatic A1 segments were found bilaterally in 2/145 male cases. All the infraoptic/infrachiasmatic A1 segments left long ICAs with low bifurcations. In 7/34 sides with distal infraoptic or infrachiasmatic A1 segments, supracarotid courses were present. In one female, the right A1 segment had an anterior supraclinoid, supracarotid and infraoptic course. In two female cases with a bilateral distal infraoptic A1, the segment was almost contacting the respective posterior cerebral artery. (4) Conclusions: In cases with dolicho(ectatic) ICAs, the A1 segments could have infraoptic and supracarotid courses the neurosurgeons should be aware of.

Background: Fenestrations, such as those of the vertebral artery (VA), are generally regarded as arterial slits. To the authors' knowledge, large fenestrations of the VA were reported twice previously, those morphological patterns being different. We hereby report a third pattern of such large fenestration of the VA. Methods: It was found serendipitously on the computed tomography angiograms of a 63 year-old male which were documented retrospectively. Results: The findings were as follows:1.the right VA entered the posterior fossa, crossed to the left side and joined the opposite VA to form the basilar artery;2.the right posterior inferior cerebellar artery was absent;3.the left VA entered the posterior fossa then it had a huge fenestration with 2 limbs: an anterior one, appearing as the main trunk of the VA coursing to form the BA, and a posterior, coiled one, inserted at both ends into the VA;4.that posterior limb of the left VA fenestration sent off 2 posterior inferior cerebellar branches;5.a slit-like fenestration of the basilar artery was also found. Conclusions: Large fenestrations of the VA could result if, during embryogenesis, the proximal segment of the lateral basivertebral anastomosis and the distal segment of the proatlantal artery persist. Large fenestrations of the VA bring into a narrow neurosurgical field an additional arterial trunk that should be carefully documented preoperatively in order to avoid hemorrhagic events.