Select appropriate surgical position according to the surgeon’s preference. Usually select lateral position with head fixed by frame. Head is up 15–20° and flexed. The chin is about two fingers from the sternum. The upper shoulder is pulled to the caudal end with a strap to keep the head hyperextended while avoiding excessive traction-induced brachial plexus injury. The mastoid root should be at the highest point. In case the head frame is not available, a ring pillow can be used to fix the head.

Electrophysiological monitoring plays a very important role in the operations on cranial nerves. It not only makes the surgeries safer, but also makes the surgeries more effective. Electrophysiological monitoring mentioned here refers to monitoring of the brainstem and cranial nerves. Specific methods include SEP, BAEP, EMG, AMR, LSR, and ZLR.

Surgeries of different cranial nerve diseases have their specific methods of monitoring. For instance, in a MVD surgery for HFS, SEP, BAEP, and EMG of the facial nerve and abduction nerve, AMR and ZLR should be applied simultaneously, while for TN, SEP, BAEP, and EMG of oculomotor nerve and trochlear nerve, BTEP of the trigeminal nerve should be conducted. Brachial plexus may be injured by excessive traction on the operated side, so it is routinely monitored during all MVD surgeries. We would like to point out that the value of monitoring requires that the monitoring physician closely observe and identify various types of waves and keep in close coordination with the surgeon. Usually it is considered a sign of nerve injury when the amplitude decreased more than 20 % or more than 1 ms in latency. Otherwise, changes within this range can fully recover in most cases. The consultation and cooperation of the monitoring physician and the surgeon is important.

Intravenous anesthesia with endotracheal intubation is the most commonly used method. Except for the induction phase of anesthesia, muscle relaxant drugs are restricted so as not to interfere with neurophysiological monitoring. During the surgery, the amount of infusion should be strictly controlled, and the carbon dioxide partial pressure should be around 26 mmHg. When the cerebrospinal fluid fluctuates significantly and affects surgical manipulation, ß-blockers may help. Usually Exelon 20–40 mg is given intravenously. At the same time observe changes in heart rate and blood pressure.

There are no stringent requirements for the scalp incision in MVD surgeries for HFS. For aesthetic purpose, oblique incision in the hairline or transverse incision behind the ear is usually used. The incision is 4–6 cm long, with the middle point at 1 cm below the mastoid root. With a burr, rongeur, or milling drill, a craniectomy of about 2.5 cm in diameter is made. When making the craniectomy, the mastoid air cell should be closely sealed to prevent fluid and blood from flowing in which affects hearing. It is unnecessary to expose transverse sinus or its junction with sigmoid sinus, but the bone window must be low enough, and the sigmoid sinus must be visualized from the outer edge of the bone window. There are many ways to cut the dura, including semicircular, triangular, cross, T-shaped, etc. The purpose is to facilitate the surgical procedure and reduce stretch of the brain tissue. We recommend a V-shape incision with the base adjacent to the sigmoid sinus (Fig. 7.2). This incision not only exposes the area between the auditory nerve and vagus nerve, but also facilitates exposing the zone 1–4 of the facial nerve from the caudal end to the rostral end. In this way, traction of the nerve is minimized. At the end of surgery, dural closure is convenient, and the incidence of cerebrospinal fluid leakage is low.

There are two approaches of MVD for HFS: the lateral cerebellum approach and the lateral retro-cerebellum approach. In the lateral cerebellum approach, after opening the dura, the cerebellar hemispheres are pulled inward directly to expose the facial and acoustic nerve. The arachnoid is dissected further to reveal zone 2 and the surrounding blood vessels, and then decompression is executed (Fig. 7.3). The advantage of this surgical approach is to expose the facial-auditory nerve directly so as to expose zone 2 and the surrounding blood vessels. When the offending vessels are located in zone 2, this approach can shorten the operation time. However, this approach also has obvious shortcomings. It often requires excessive traction of the cerebellar hemisphere. Because the arachnoid has not been dissected, this stretch may injure the vestibular-cochlear nerve, glossopharyngeal nerve, and vagus nerve. It can also lead to cerebellar contusion. In case there are petrous veins on the dorsal side of the vagus nerve, it may lead to venous bleeding and even petrous vein tear. In addition, although this approach can expose zone 2, it is difficult to expose zone 1, 3, and 4. In case the offending vessels are located between the facial nerve and the acoustic nerve, vascular decompression will be very difficult. So this surgical approach is now rarely used.

Currently we recommend lateral retro-cerebellum approach (Zhong et al. 2010, 2014; Zhu et al. 2012a; Liang et al. 2012; Li et al. 2010, 2013a) (Fig. 7.4). The key of this approach is that, after dura opening, the cerebellum is pulled forward and medially, so as to expose the arachnoid at the bottom of the cerebello-medullar fissure. The arachnoid is first opened with a pointed blade, which allows slow release of cerebrospinal fluid. Then separate the arachnoid from the caudal end to the rostral end with spring scissors until the vagus nerve, glossopharyngeal nerve, and vestibular-cochlear nerve are completely released. In some patients there are petrous vein going on the dorsal side of vagal nerve. We suggest that this vein cannot be coagulated or cut. We recommend preserving this vein by carefully separating the adhesions surrounding the petrous vein. In case the AICA or PICA goes on the dorsal side of the vagal nerve, use the same method to preserve the arteries.

In some patients, the flocculus cerebelli is abnormally hypertrophied. In such case, it is very difficult to expose the whole facial nerve only by dissection of the arachnoid. Excessive traction of the flocculus cerebelli is not recommended. Instead, we suggest resection of the flocculus cerebelli. Then, we can easily expose the facial nerve and the offending vessels, and perform MVD safely.

The success of MVD for HFS relies on identifying all the offending vessels and full decompression. Therefore, we need to know common offending vessels and their distribution. Our experiences show that the offending vessel can be a big artery like the vertebral artery or small vessels like the perforating arteries. It can be a common artery, or vein, or mixed compression consisting of arteries and veins. In fact, multiple vascular compressions are very common.

Common offending vessels include AICA, PICA, vertebral artery, and perforating artery (Zhu et al. 2012b; Feng et al. 2011; Zheng et al. 2011; Dou et al. 2014). It has been reported that in more than 50 % of patients, bilateral vertebral arteries shift to the affected side (Wang et al. 2014; Zhong et al. 2011a). The contact point of compression varies among different patients. It is reported that the contact point can be distributed in the full length of the facial nerve, from the pontine-medullar sulcus to the inner ear meatus. That is to say, there may be offending vessels on any part of the facial nerve.

In order to guide surgical manipulation and avoid omissions of offending vessels, we suggest that the facial nerve be divided into 4 zones (Li et al. 2013b; Zhong et al. 2011b) (Fig. 7.5). Zone 1, also known as REZ, is where the nerve emerges to the brainstem surface from the parenchyma and goes through the pontomedullary sulcus; zone 2 is where the root attaches to the surface of the pons; zone 3, the shortest segment among the four zones, is where the nerve gradually transits to the subarachnoidal segment; and zone 4, the longest segment, is where the nerve fibrins separate from the brainstem and extend to the internal meatus.

It is reported that the location of the offending vessels are as follows: 15 % in zone 1, 36 % in zone 2, 37 % in zone 3, and 12 % in zone 4 (Li et al. 2013b; Zhong et al. 2011b). We have similar findings in our clinical practice. Therefore, zone 4 must be inspected thoroughly during MVD for HFS. By this way we are able to find all vessels that are in contact with the facial nerve and then separate and properly fix them and ensure that all existing and potential vascular compression are entirely treated.

Because vascular compression is the main cause of HFS, anatomical contact of blood vessels with facial nerve is a necessary condition for the occurrence of hemifacial spasm. Therefore, the core mission of MVD is to separate the offending vessels from the facial nerve and then properly remove and fix the vessels to avoid recurrence. Various decompression methods can be selected according to the thickness, elasticity, tortuosity, and length of the offending vessels.

There are three most commonly used decompression methods: isolation, suspension, and biological glue adhesion. The so-called isolation is to separate the offending vessels from the facial nerve and then implant Teflon cotton in between to ensure the vessels are no longer touching the facial nerve (Zhong et al. 2012, 2014). It is currently the most commonly used method. The advantage of this method includes simplicity of manipulation, high effectiveness and univeral applicability. However, its biggest drawback is that implanted Teflon cotton may adhere to the vessels and facial nerve, which is the leading cause of recurrence (Kureshi and Wilkins 1998; Payner and Tew 1996).

The so-called suspension is to separate the offending vessels from the facial nerve and then use muscle, fascia, or artificial straps to wrap around the offending vessels and fix them to the sidewall of the posterior fossa or tentorium (Swiątnicki et al. 2014; Khoo et al. 2012). In this type of method, Teflon cotton is not required. Therefore, the biggest advantage of this type of method is that it is suitable for long and tortuous offending vessels, and perineural adhesions do not occur, which makes reoperation relatively easy. But this method has its drawbacks, which is that the surgical procedure is very difficult and the surgical risk is significantly increased. Most clinicians are reluctant to use this method.

Similarly, biological glue adhesion method is to separate the offending vessels from the facial nerve and stick the vessels to the sidewall of the posterior fossa or tentorium with biological glue (Kurokawa et al. 2004; Ichikawa et al. 2011). There is no Teflon cotton implanted between the facial nerve and the vessels. Biological glue adhesion method is mainly used for long and tortuous vessels, as well as complementary measures for thick vessels. The advantage of this method is similar to suspension, but the drawback is that reoperation will be very difficult due to glue-induced adhesion. In addition, in some patients, the glue may be unstable and lead to recurrence, and the glue itself can cause cranial nerve injuries. In fact, similar to a suspension, glue adhesion method is rarely used in clinical practice.