Correlation between idiopathic hemifacial spasm and the MRI characteristics of the vertebral artery.

We studied the correlation between the anatomy of the vertebral artery (VA) and the symptoms of primary hemifacial spasm (HFS). Between June and October 2009, 88 patients with primary HFS underwent a preoperative MRI and subsequent microvascular decompression surgery. Each patient's VA was categorized into one of three types according to their MRI findings. In Type I, the bilateral VA that join to form the basilar artery in the mid line appear symmetrical. In Type II, the VA distribute asymmetrically and deviate to one side (Type IIa deviate to the symptomatic side; Type IIb to the contralateral side). In Type III, both VA occur on the same side (Type IIIa occur on the symptomatic side; Type IIIb on the contralateral side). The correlation between deviation of the VA and the symptomatic side of primary HFS was analyzed statistically. Of the 88 patients, the number of patients with each type of VA, as identified by three-dimensional time-of-flight MRI was: Type I=3, Type II=48 (Type IIa=40, Type IIb=8) and Type III=37 (Type IIIa=36, Type IIIb=1). The anatomical characteristics of the VA (lateral deviation) were significantly related to the symptomatic side of the primary HFS (chi-squared [χ(2)]=102.14; p<0.01; relative risk=8.44). The likelihood that the VA deviated to the symptomatic side was 86.4%, while the likelihood of deviation to the asymptomatic side was 10.2%. Thus, anatomical variation of the VA (lateral deviation) is one of the risk factors for primary HFS.