Characteristic anatomical conformation of the vertebral artery causing vascular compression against the root exit zone of the facial nerve in patients with hemifacial spasm

Background Hemifacial spasm (HFS) is caused by tortuous offending vessels near the facial nerve root exit zone. However, the definitive mechanism of offending vessel formation remains unclear. We hypothesized that vascular angulation and tortuosity, probably caused by uneven vertebral artery blood flow, result in vascular compression of the facial nerve root exit zone. Methods The authors observed two anatomical characteristics of the vertebrobasilar arterial system in 120 subjects in the surgical group and 188 controls. The presence of the dominant vertebral artery (DVA) and laterality of the vertebrobasilar junction (VBJ) were observed. We also analyzed the morphological characteristics of the surgical group showing the presence of DVA. The morphological characteristics were classified into three types: type I had the VBJ and DVA on the same side, type II had the VBJ within 2 mm of the midline, and type III had the VBJ opposite the DVA. Result The DVA was more prevalent in the surgical group than in the control group (71 % versus 54 %, P < 0.05). The surgical group patients with HFS on the left were more likely to have a DVA on the left ( P < 0.05) and with HFS on the right were more likely to have a DVA on the right ( P < 0.01) compared with controls. The direction of the VBJ was more common on the same side as the DVA, which corresponds with the laterality of the HFS. In the surgical group with the DVA and HFS on the same side, type I was predominant, but in the surgical group with a contralateral DVA and HFS, type III was predominant. Conclusion The presence of a DVA and shifting of the VBJ on the same side plays a role in the angulation and tortuosity of vessels in the perivertebrobasilar junction, resulting in neurovascular compression of the facial nerve root exit zone and thereby causing HFS.