Supine No-Retractor Method in Microvascular Decompression for Hemifacial Spasm: Results of 100 Consecutive Operations

Abstract

Objectives In microvascular decompression (MVD) for hemifacial spasm (HFS), the patient is placed in the lateral or park-bench position that is complicated and uncomfortable for anesthesiologists, nurses, and even the patient. Careless retraction of the cerebellum by a spatula could be the major cause of surgical complications. In our method, a patient is laid supine avoiding the complicated positioning. The subfloccular approach from a small cranial window sited on the more lateral and basal side of the occipital cranium enables the surgeon to reach all the segments of the facial nerve root without a spatula. We introduce our surgical procedures in detail along with our excellent results.

Methods A total of 100 consecutive patients experiencing primary HFS were operated on with MVD by a single surgeon in our institution from August 2012 to April 2014.

Results Overall, 94 patients showed the complete disappearance or a satisfactory alleviation of HFS. De novo neurologic deficits were not encountered after surgery including hearing impairment. In 47 cases, multiple offending vessels were observed in multiple possible affected sites in addition to the root entry/exit zone.

Conclusions We believe this approach is superior for the safe and precise decompression of any part of the facial nerve root.

Introduction

After Jannetta's observation, most surgeons have emphasized the importance of vascular compression on the root entry/exit zone (REZ) as the source of the hyperactive dysfunction syndrome of cranial nerves.^{1 2 3 4} The decompression of this area has been the aim of microvascular decompression (MVD) for hemifacial spasm (HFS). However, recent pathologic studies have revealed that the facial nerve root is already exposed on the pontine surface at the pontomedullary junction, several millimeters proximal to the so-called REZ.^{5 6} Accumulated operative findings and clinical results have also suggested that these superficial segments and the subsequent peripheral portion of the facial nerve root are also susceptible to vascular compression.^{6 7 8 9} Thus, for complete treatment, the facial nerve root should be thoroughly inspected and decompressed from its origin at the pontomedullary junction to the peripheral portion just a few millimeters away from the REZ.

Since Gardner and Jannetta, the traditional retrosigmoid approach has become the standard method of MVD.^{10 11 12 13} This approach enables the surgeon to reach the REZ over the retracted flocculus. It is sufficient to cover the REZ but seems insufficient to treat the entire root of the facial nerve safely and effectively. Much more cerebellar retraction should be needed especially for the management of the pontomedullary sulcus, which is one of the major causes of surgical complications.^{8 13 14 15} In our supine no-retractor method, the adequate arachnoid dissection and cerebrospinal fluid (CSF) drainage from the cistern let the cerebellum fall down by its own weight to make an appropriate working space. Time and labor for the preoperative positioning of the patient can also be eliminated. We describe the details and advantages of our surgical procedure with practical operative findings and excellent results.

Material and Methods

All patients consented to have their clinical data subsequently submitted to a journal. A hundred consecutive patients experiencing primary HFS were operated on with MVD using the present method by a single surgeon in our institution from August 2012 to April 2014. These 100 patients (24 male; 76 female) were 34 to 81 years of age with a mean age of 58.9 years. The symptom presented on the right in 41 patients and on the left in 59 patients (Table 1). All patients were diagnosed by their typical clinical symptoms before surgery with synkinesis and magnetic resonance imaging and magnetic resonance angiography. They also underwent pure tone audiometry before and after surgery. Intraoperative brainstem auditory evoked response (BAER) monitoring was applied to all patients.

Table 1. Characteristics of 100 patients.

Description	Value
Sex
Male	24
Female	76
Age, y
Mean	58.9
Range	34–81
Side
Right	41
Left	59

Surgical Procedures

Following the induction of anesthesia and intubation, the patient is placed in the supine position. After being fitted with devices for BAER monitoring, the head is placed on a horseshoe-shaped headrest. The neck is flexed and rotated contralaterally as much as possible (∼ 40–60 degrees in most cases); the vertex is slightly tilted toward the floor for better exposure of the lateral bottom of the skull. Finally, the patient's head is securely fixed with tape, and the shoulder of the operative side is taped down and out of the way to provide easier and wider access to the operative field. The patient's body is also secured on the table so the table can be rotated laterally.

A 6- to 7-cm curved incision described in Fig. 1 is made behind the root of the mastoid processus where the digastric groove is palpable over the skin. This is the landmark for burr hole placement for the craniectomy. The inferior curve of the designed incision, surrounding the digastric grove, prevents the overhang of the skin edge that limits the operative view from the lateral base through the microscope in the supine position. Care is taken not to cross the medial margin of the sternocleidomastoid (SCM) muscle, preventing injury to the great auricular nerve. The minor occipital nerve is also preserved by reducing the superior curve of the skin incision.

Fig. 1.

Drawing depicting the design of the skin incision and the cranial window on the left side.

After the skin incision, a sharp dissection is made under the epicranial muscle to disclose the fascia of SCM adopting fishhook retractors and a Gelpi retractor. The SCM is sharply dissected from the mastoid bone at its attachment and partially cut off in the same shape as the reflexed skin flap by using monopolar cautery to provide a free muscle flap used for duraplasty and mastoid air cell packing at the end of surgery. Subsequently, the splenius capitis (SC) and the longissimus capitis (LC) muscles underneath are cut down thoroughly from the tip to the root of the mastoid eminence to widely expose the retromastoid space including the digastric groove and the mastoid foramen. The mastoid emissary vein is usually identified and waxed in its foramen. These procedures provide adequate bony exposure, enough for the usual suboccipital craniectomy of MVD for HFS. However, more extensive exposure is needed for our approach.

The goal of bone exposure is to identify the lateral occipital bottom where the occipital bone suddenly makes a vertical curve toward the foramen magnum behind the mastoid processus. For this purpose, the fatty layer between the lamina prevertebralis and superficialis (investing layer) of the deep cervical fascia (DCF) should be separated. This layer is composed of thick and fatty connective tissue and easily recognized caudal to the margin of the SC.¹⁶ It attaches to the mastoid processus and the occipital bone around the digastric groove under the SC and the LC, and it continues to the carotid sheath deep under the posterior center of the digastric muscle (DM). The occipital artery (OA) always appears in this layer from under the posterior vender of the DM, where the surgeon can readily preserve it. The thorough separation of this layer is important. Under it, we can find the transverse processus of the atlas and the jugular processus, the principal landmarks of the occipital bottom. Subsequently, the optimal craniectomy could be placed at the very lateral part of the suboccipital skull base. The partial dissection of the superior oblique muscle (SO) is sometimes needed to develop enough space for craniectomy in patients with a thick and short neck.

This procedure could be safely done at the attachment of the SO to the transverse processus of the atlas that is palpable just beneath the tip of the mastoid eminence. Care must be taken not to injure the vertebral artery behind the SO. A burr hole should be placed at the root of the mastoid processus just medial to the digastric groove (the incisura mastoidea). The craniectomy is extended 2.5 × 1.5 cm to the posterior margin of the jugular processus with a high-speed drill and a rongeur, exposing the medial curve of the sigmoid sinus. The posterior end of the jugular processus is the medial limit of the craniectomy that indicates the most distal part of sigmoid sinus. It is easily identified by recognizing the cancellous diploe of the bone edge at the root of the mastoid processus. The cranial window sits at the bottom of the occipital bone lateral to the condylar fossa (Fig. 2). The surgeon never ruptures the condylar emissary vein that locates more medially during the craniectomy. Mastoid air cells should be thoroughly waxed, if opened.

Fig. 2.

Left: Postoperative three-dimensional computed tomography showing the site of the cranial window with a titan plate removed; dotted white line indicates the cranial window. Right: Position of the optimal cranial window demonstrated on the dry skull; blue line indicates the transverse-sigmoid sinus; black line indicates the optimal position of a cranial window; arrow indicates the outlet of the condyle emissary vein; arrowheads indicate the posterior margin of the jugular processus.

Under an operative microscope, an inversed U-shaped incision is made in the dura mater. The dural flap is reflexed back and the margin of the incised dura pulled up with several 4–0 Surgitron (Surgilon, Covidien, Dublin) threads and needles to expand the dural window laterally as much as possible. These threads are also used to fix the fascia excised at the beginning to close the dural defect at the end of surgery. A subarachnoid space at the lateral part of the cerebellomedullary cistern is opened to drain CSF. In the supine no-retractor method, the surgeon should look at the surgical field in a horizontal direction searching for an appropriate subarachnoid space until the first rupture of an arachnoid membrane. Then the cerebellum hemisphere gradually sinks down by draining CSF, and the surgeon can gradually look down the operative field as usual. The CSF drainage from the cisterna magna is not at all necessary. After the wide opening of the cerebellomedullary cistern and adequate CSF drainage, the sharp dissection of the arachnoid membrane is advanced over the lower cranial nerves, choroid plexus, and the caudal part of the flocculus. The arachnoid dissection should be advanced over the choroid plexus until the origins of the lower cranial nerves are exposed. This procedure induces an additional sink down of the cerebellum by its own weight. The surgeon should keep in mind that the main purpose of this procedure is to bring the cerebellum up off the lower cranial nerves and toward the surgeon to make the subfloccular corridor to the facial nerve root.

The key point is the full split between the choroid plexus of the lateral recess of the fourth ventricle and the origin of the ninth cranial nerve root. Consequently, the gentle cephalad elevation of the flocculus with a suction tip allows the surgeon a widely opened subfloccular view of the outlet of the facial nerve root at the pontomedullary junction just behind the root of the ninth cranial nerve without a spatula. Our view from the lateral bottom also enables the surgeon to observe the origin of the facial nerve root even from the caudoventral side of the lower cranial nerves, if necessary. Using the up-down and rotation of the operative table to flip the microscope in the appropriate angle, the surgeon can inspect and treat all the concerned area of the facial nerve root including its peripheral portion without a spatula.

The facial nerve decompression is performed with the insertion of an polyvinyl alcohol prosthesis with the aid of fibrin glue, transforming the shape of the loops of the offending vessels. Care is taken not to place prosthetic materials between the compressing vessels and the affected area. To prevent neural damage, the arachnoid membrane over the cisternal portion of the eighth cranial nerve usually remains as it is except if the peripheral portion should be examined according to the preoperative diagnosis and the operative findings. During all the procedures, no spatula is needed. Intraoperative auditory brainstem response monitoring is used routinely.

After ensuring the intradural hemostasis, the fascia of the SCM flap cut off at the beginning of the surgery is used for dural closure. Intermittent stitches using the threads adopted to pull up the dural margin during durotomy and an additional running suture ensures watertight closure. The opened mastoid air cells are packed with muscle pieces from the SO flap and shielded by fibrin glue. Cranioplasty is performed with a bony flap made from bone particles confirmed with fibrin glue. A titanium plate is fixed over it with screws at the mastoid bone. The SC and the LC are sutured together to the remaining fascia at the attachment of the mastoid bone covering the entire titanium plate with 3–0 absorbable sutures, and then the wound is closed in layers.

Results

As of July 2014, 94 patients (94%) showed complete disappearance or satisfactory alleviation of HFS. Six patients (6%) remained unchanged (Table 2). The most common culprit vessel was the anterior inferior cerebellar artery (AICA) (Table 3), and multiple (two or three) possible culprit vessels were identified on one or more affected areas in 36 cases (36%). The variations of the common trunk of the AICA and posterior inferior cerebellar artery (PICA) were also found as the culprit vessels in 28 cases (28%), whose higher rate in the patients with HFS was suggested in the previous study.^{17 18} The distribution of the affected area is also shown in Table 3. We divided the concerning areas of the facial nerve root into the following four zones for convenience: the root exit zone emerging from the pontomedullary junction, the attached zone adhering to the pontine surface, the root detachment zone (RDZ) separating from the pons correlated to the histologic transitional area, and the peripheral zone in the cistern merely covered the peripheral myelin. The RDZ, corresponding to the REZ, was the most common affected area in our series. There were 29 cases (29%) with multiple potentially affected sites. A culprit vessel was identified in the peripheral zone in 8 cases (8%). Five of them were accompanied with additional possible offending vessels on other proximal zones.

Table 2. Results and complications.

Results and complications	No. (%)
Results
Complete/satisfactory regression	94 (94)
Unchanged	6 (6)
Complications
Delayed facial palsy	6 (6)
Wound infection	2 (2)
Cerebrospinal fluid leak	1 (1)
Meningitis	1 (1)

Table 3. Operative findings.

Offending vessels	Number	Affected site	Number
AICA	23	RDZ	77 (53)a
AICA-PICA	17	AZ	29 (7)
VA + AICA	11	RExZ	20 (8)
VA + PICA	12	PZ	8 (3)
PICA-AICA	11
PICA	9
AICA + PICA	7
VA + AICA + PICA	6
VA	4

Abbreviations: AICA, anterior inferior cerebellar artery; AZ, attached zone; PICA, posterior inferior cerebellar artery; PZ, peripheral zone; RDZ, root detachment zone; RExZ, root exit zone; VA, vertebral artery.

^aNumbers in parentheses indicate the number of cases with the single affected site.

Note: AICA-PICA indicates the well-developed common trunk from the basilar artery feeding both the original AICA and PICA territory with the absence of normal PICA; PICA-AICA indicates the well-developed common trunk from the VA feeding both the original PICA and AICA territory with the absence of normal AICA.

Six patients (6%) experienced delayed transient facial weakness > 1 week after surgery that completely resolved in a month. In one patient, meningitis developed but was resolved by the transvenous application of antibiotics for a week. Two patients (2%) had a wound infection, one of which needed the removal of the titanium plate. Another early operated patient who received the Gore-Tex dural plasty (W. L. Gore and Associates, Arizona, United States) developed CSF rhinorrhea that required an operative intervention to reinforce the dural closure on postoperative day 14 (Table 2). Thereafter, the SCM muscle flap has been applied for the dural plasty, and no CSF leakage has occurred. Our patients had no postoperative hearing impairment.

Discussion

Recent studies have all showed that the exact histologically transitional zone of the facial nerve is the only 2- to 3-mm area separating the surface of the pons where the central myelin merges into the peripheral myelin.^{5 19 20} Current histologic and anatomical studies have also described the characteristic tract of the facial nerve root.^{5 6} The facial nerve root already exits the brainstem at the upper edge of the supraolivary fossette in the pontomedullary sulcus, and it firmly adheres to the pontine surface for 8 to 10 mm before detaching from the pons. Susceptibility of this central myelin segment has also been reported.⁷ Thus it is more likely that a spasm could be caused by the compression of any part of the facial nerve root in addition to the relatively short RDZ that corresponds to the traditional REZ. In the present study, we also divided the facial nerve tract into four zones according to previous reports.^{5 6 7} In only 53 cases, a possible single or multiple offending vessels were identified at the RDZ. In the other 47 cases, the facial nerve roots were compressed at three alternative zones. A single or multiple possible culprit vessels were remedied at multiple zones in 29 cases. These findings are compatible with many recent articles.^{6 9 10 21} In their previous paper, Campos-Benitez et al identified a single primary offending vessel in an affected zone according to the severity of compression. However, in many cases, the facial nerve root seemed to be compressed by the arterial complex of the posterior circulation composed of the vertebral artery, AICA, and PICA on one or more affected sites. It was practically difficult to discriminate a single vessel as a primary culprit out of the arterial complex. Accordingly, we categorized possible offending vessels in the possible affected zone as shown in Table 3. All potential culprit vessels should be treated at any possibly affected zone. A simple decompression of the most prominent culprit vessel in direct touch with facial nerve root is not effective. We believe that the concept of the transformation of the entire arterial complex in the posterior fossa is important.

Because the reliability of the intraoperative monitoring is still limited to the diagnosis of a complete decompression, the MVD surgery has been modified to cover the entire facial nerve root to improve the cure rate.^{22 23} However, in the lateral or park-bench position, the cerebellar hemisphere covers the operative field even after the arachnoid dissection and CSF drainage. The surgeon needs to compress the cerebellar hemisphere medially by a spatula placed near the root of the eighth cranial nerve behind the flocculus to secure the view of the facial nerve root over the retracted flocculus. The constant surveillance of a spatula is imperative during the manipulation, especially at the deep-seated origin of the facial nerve root in the pontomedullary sulcus and pontine surface.^{14 24} In a recent report using the refined retrosigmoid over floccular approach, it is suggested that the surgeon needs to remove part of the flocculus for an adequate view of the facial nerve root, avoiding overretraction by a spatula in the case with hypertrophy of the flocculus. This never happened in our method.⁸ The origin of the facial nerve root is recognized in the pontomedullary junction just medially behind the choroid plexus and the outlet of the ninth nerve from the medulla. On the contrary, the outlet of the eighth cranial nerve from the pons is observed lateral to the origin of the ninth cranial nerve covered by the flocculus. Thus the eighth cranial nerve injury by retraction is possibly caused by the careless placement of a spatula on the flocculus over the origin of the eighth cranial nerve.²⁰ Bleeding from bridging veins is encountered by careless traction with a spatula but never happens in our method.¹⁵

The exact combination of our design of the skin incision, procedures of the muscle separation, and position of the craniectomy allows the surgeon the most basal view of the facial nerve root ever observed without the aid of a spatula. This subfloccular corridor could be obtained only through the cranial window situated on the more lateral and basal side of the cranium ever described, which enables the surgeon to treat all zones of the facial nerve tract by a gentle cephalad elevation of the flocculus with a suction tip. In our method, the surgeon can readily reach the more proximal segments of the facial nerve root around the pontomedullary junction whose importance was recently indicated without the cerebellar retraction by a spatula.^{5 6 7 8 22 23} Adequate split of the fatty layer between the two laminas of the DCF after the clear separation of the perivertebral is critical to expose the lateral skull base. The recognition of this space, which has never been mentioned by a neurosurgeon, is also useful for other surgical approaches to the lateral skull base because this space is safe with no important structure encountered except for the OA.

We have operated on > 200 cases, and no difficulty has ever been encountered with the up-down and rotation of the operating table. The horseshoe-shaped head rest is applied instead of the three-point head fixation device, therefore freeing patients from neck or brachial plexus injury caused by overrotation. Even the patient with a history of cervical cord injury could be treated safely by our method. To perform the precise peeling off of the suboccipital muscles in the present method, the surgeon should be acquainted with the disposition of the structures of the craniocervical junction. It is also helpful for other skull base surgeries such as cerebellopontine angle tumors including acoustic neuromas.

In the consecutive cases operated in the last 21 months, we have demonstrated an excellent cure rate and never had a major surgical complication such as a hearing disturbance or cerebellar injury. Although a long-term follow-up study is awaited, we believe the subfloccular approach from the more lateral and basal side of the occipital cranium in the supine no-retractor method is valuable and contributes to an excellent result in MVD for HFS.

Conclusions

In our previous long-term follow-up studies,^{25 26} we indicated acceptable results in the supine no-retractor method performed by more than one surgeon. In this report, we fully described the details of our subfloccular lateral basal approach. We hope this article has helped convey the nuances of our method and will contribute to safe and effective MVD surgery.