A vertebral artery tortuous course below the posterior arch of the atlas (without passing through the transverse foramen). Anatomical report and clinical significance.

A vertebral artery (VA) coursing below the posterior arch of the atlas (C1) without passing through the transverse foramen of C1, combined with a tortuous course within the spinal canal has rarely been reported in the literature. This article describes a case encountered during an anatomical study of the far-lateral approach, and reviews its embryonic development and clinical significance. The suboccipital triangle was filled with numerous venous plexures. After exiting from the transverse foramen of C2, instead of passing upwards through the transverse foramen of C1, the VA turned directly medially towards the spinal canal. At the spinal canal, it first formed an angle downwards, then turned upwards, piercing and entering the lateral part of the dura at C1 level. The diameter of this VA seemed to be within its normal limits. The course of the contralateral (right) VA was normal but with a small caliber and mainly supplied the posterior inferior cerebellar artery (PICA); after PICA, it became much thinner and dysplastic, the basilar artery was mainly supplied by the left VA. The bilateral posterior communicating arteries were large in diameter but there was dysplasia of the P1 segment of the posterior cerebral arteries bilaterally. Marked tortuosity of the bilateral intracavernous internal carotid artery (ICA) was also found. We did not find any osseous abnormality in the occipito-axial region or of C1-C2 joint. An abnormal course of the VA should be kept in mind during exposure of the craniocervical junction, especially in the variety of lateral approaches; due to compression of the nerve roots or the spinal cord, this abnormal course of the VA could give rise to clinical symptoms, which could be resolved by microvascular decompression technique.