The pathophysiologic basis of hemifacial spasm is cross-transmission between different branches of the facial nerve. The hypothesis is that the cross-transmission is mediated by the sympathetic nerve fibers on the surface of the offending artery. The abnormal potential from the offending vessel to the facial muscles will be detected on hemifacial spasm patients, so as to determine if the sympathetic nerve fibers take part in the circuit of cross-transmission.

It's hypothesized that at the location where the facial nerve is compressed by an artery, ephaptic cross-transmission takes place between the nude facial nerve fibers and the sympathetic nerve fibers on the arterial surface. Thus action potentials could spread indirectly from one facial nerve fiber to several other facial nerve fibers (via the "bridge" of sympathetic fibers), or directly from the sympathetic fibers to facial nerve fibers. The investigators assume this is the electrophysiological basis of hemifacial spasm and its characteristic sign abnormal muscle response (AMR). If an abnormal potential from the offending artery to the facial muscles similar to AMR can be recorded, the hypothesis will be verified.

Inclusion Criteria:

1. the age of 18-65 years old;
2. with typical hemifacial spasm symptoms;
3. with typical abnormal muscle response;
4. preoperative magnetic resonance imaging revealed obvious paraneural artery;
5. with informed consent.

Exclusion Criteria:

1. pregnant or lactating women;
2. accompanied by other serious diseases, including liver, kidney, cardiovascular diseases;
3. accompanied by mental illness or severe neurosis;
4. unable to express the subjective symptoms.

This study will enroll 20 cases of hemifacial spasm patients who want to undergo microvascular decompression.