Spiral twisting of distal (V4) vertebral arteries in an asymptomatic female and its embryological basis.

Intracranial vessels develop early in embryonic life through mutual interaction and integration of aortic arches giving the carotid arteries and intrinsic longitudinal neural system forming the posterior circulation. Due to the relatively short time and faster pace of completion of development, minor absent/persistent segments may give rise to either asymptomatic anatomical variants or hemodynamically significant changes. Intracranial vessels - both anterior carotid and posterior vertebrobasilar systems - are known to have many common anatomical variations like hypoplastic segments (A1 or P1 segments of anterior or posterior cerebral arteries) and persistent carotid-vertebrobasilar anastomotic channels. These variants may be displayed using MRI and MRA which represent useful non-invasive investigative modality of the neck and Willis's circle. We report an interesting hitherto rarely reported anatomical variation in the form of spiral twisting of distal vertebral (V4 segments) and vertebrobasilar junction in a 36-year-old female and discuss the underlying embryological possibilities based on the embryological development of the vertebrobasilar system.