Advances in Microsurgical Treatment of Facial Myasthenia Gravis
Abstract: Microvascular decompression surgery has become the treatment of choice for facial myasthenia because of its small trauma, good safety, high cure rate and low complication rate. It is important to master the microsurgical skills and local anatomical knowledge, and to correctly perform the local exposure, identification of responsible vessels, selection and placement of cushion, determination of treatment effect and prevention of surgical complications during the microvascular decompression surgery to ensure the success of the surgery.
Keywords: facial nerve; facial muscle spasm; microvascular decompression
Hemifaeialspasm (HFS) generally refers to primary or idiopathic cases of paroxysmal hemifacial muscle involuntary twitching, which progresses slowly and does not improve spontaneously. The incidence of HFS is l1/1 million in foreign epidemiological surveys. Microvascular decompression (microvasculardecompression, MVD) has become the preferred treatment for HFS after decades of development and improvement.
1.Etiology
In 1966, Jannetta pioneered MVD, believing that HFS was caused by demyelination of the facial nerve root in the CPA area due to compression of the responsible vessel and short-circuiting of impulses between afferent and efferent nerve fibers, and MVD was performed by pushing the responsible vessel away from the facial nerve root with a pad. In 1982, the results of 229 cases of HFS treated by MVD were reported: 98% of the patients were found to have intraoperative vascular compression, and only 2.2% of them were ineffective after MVD. After the mid-1970s, MVD was rapidly promoted in clinical practice because of its safety and effectiveness in treating HFS. It was found that the responsible vessels for compression were mostly dilated, lengthened, tortuous, and sclerotic vessels of the vertebral basilar artery system, and that the pulsatile impact compression of the vascular collaterals on the root exit zoon (REZ) of the facial nerve caused HFS. The abnormal increase in excitability of facial nerve nucleus due to vascular compression is also one of the causes of HFS.
2. Indications for surgery
The indications for surgery for HFS treated with MVD are
① Primary HFS;
② No history of facial nerve injury;
(3) No serious systemic diseases. The diagnosis of primary HFS must be differentiated from the following diseases: habitual ophthalmoplegia, painful facial twitching, hysterical ophthalmoplegia, limited motor epilepsy, post-facial nerve paralysis spasm, facial twitching associated with chorea and tardive dyskinesia, facial spasm caused by motor neuron disease, etc.
3.Surgical techniques
3.l Surgical incision design
Jannetta used a vertical incision 0,5 cm inside the hairline behind the ear parallel to the hairline, 3 5 cm long, and the length of the incision depended on the length and thickness of the patient’s neck.
3.2 Surgical position
The patient’s head was placed in a downward lateral position on the healthy side, with the head dropping l50. and rotated 100 to the healthy side, and the neck was slightly flexed forward so that the affected mastoid was in the highest position of the head.
3.3 Exploration of CPA
The diameter of the bone window was 1.5 cm to 2.0 cm, and its anterior and inferior edges were close to the level of the sigmoid sinus and skull base. After dural incision, cerebrospinal fluid is slowly discharged under the operating microscope. The cerebral pressure plate should be gradually retracted and deepened, with a retraction range of lcm, and the retraction should be intermittent to avoid damage to the auditory nerve due to high tension for a long time. The linguopharyngeal and vagus nerves should be revealed in turn, and the nerve roots and the arachnoid membrane covering the dorsolateral surface of the pontine brain should be cut open. 2 mm wide brain pressure plate at the front end should be placed on the surface of the cerebellar vermis and retracted, while the patient’s head should be rotated backward by l50. and the optical axis of the operating microscope should be adjusted to clearly reveal the facial nerve REZ.
3.4 Treatment of the responsible vessels
3.4.1 Identification of the responsible vessel The responsible vessel passes through the facial nerve REZ in the form of a collaterals and causes compression. Be careful not to mistake free vessels located in the distal segment of the facial nerve, within the lateral pontine pool, especially those that are only in contact with or parallel to the facial nerve trunk, for the responsible vessel. When multiple vessels are present in the REZ, the responsible vessel is often located on the deeper side of the vascular plexus. The following factors can influence the identification of the responsible vessel.
① Change in patient position ;
(ii) Poor exposure of the facial nerve REZ;
③ Displacement of the responsible vessel stroke by stretching of the cerebellar hemispheres, excessive and rapid discharge of cerebrospinal fluid, or extensive dissection of the arachnoid. The common responsible vessels reported by several scholars are: anterior inferior cerebellar artery trunk and/or branches (38.6% to 65%) > posterior inferior cerebellar artery trunk and/or branches (15.3% to 50%) > vertebral artery (17% to 25%) > multiple arteries in common compression (4.2% to 9%). knodo did not find any vein alone constituting compression in more than 1000 cases of HFS, while Levy et al. showed a 0.2% incidence of venous compression.
3.4.2 Selecting a suitable cushion By cushion, we mean a material that separates the responsible vessel from the facial nerve REZ and prevents its repositioning. Teflon (polytetrafluoroethylene) cushion and Ivalon sponge are the two most commonly used cushions [14,15 J. When using cushions, they can be torn into small clusters to make them soft and flexible, and then made into cigar-shaped 8 to make them easy to fix.
3.4.3 Vascular decompression After the responsible vessel is fully free by sharp dissection, it is pushed away from the REZ in the direction of the skull base, and the decompression pad is placed between the responsible vessel and the brainstem, and if necessary, the second and third pads are used to further push away the vessel for adequate decompression. The pad should not be too large to avoid forming new compression. After the responsible vessel is padded, the artery should not be twisted into an angle, otherwise the blood supply to the brainstem may be affected. When the responsible vessel is a thick, tortuous and sclerotic vertebral artery, it is difficult to push the vessel and place the pad, so some authors use the method of decompression by making the pad into a band around the vessel and then fixing it on the dura with medical adhesive. When the responsible vessel emits multiple short brainstem penetrating arteries or when the penetrating artery is passing between the facial and auditory nerve roots, it is very difficult to place the pads because of the small operating space and the need to preserve all the penetrating arteries. When a vein is alone or involved in compression, it can be cut by electrocoagulation.
4.Surgical efficacy
4.1 Efficacy
The cure rate of MVD for HFS is 70% to 94.7%, and the total effective rate is 87.5% to 99.3%. Fukushima et al. applied MVD to treat 2890 cases between 1978 and 1993, with an average follow-up of l0 years, and the cure rate is 85%, and the total effective rate is 96%. Based on the long-term follow-up of 751 patients, Kondo concluded that the operator’s skill in microsurgery, familiarity with the microscopic anatomy of the CPA, and responsibility for the operation had no effect on the outcome. The results of this study are based on the long-term follow-up of 751 patients. With recent advances in neuroendoscopic techniques, successful use in MVD surgery has been reported. The local magnification and good illumination of the endoscope can make up for the deficiency of the tubular field of view of the operating microscope, and it can clearly reveal the facial nerve REZ and place the pad cotton without excessive pulling and stripping the nerve tissue, which is clinically significant to improve the efficacy of MVD.
4.2 Delayed healing after surgery
Many scholars found that about l3% to 50% of HFS patients’ symptoms did not disappear immediately after MVD surgery, but gradually disappeared after 1 week to 3 months or even 6 months, which is considered as delayed cure. For example, Ishikawa et al. found that about 50% of HFS patients reappeared after an average of 4 days of curing performance after MVD treatment, and the cure was delayed after an average of 28 days. Shin et al. found that 37.4% of patients had delayed cure for an average of 73 days, and this time was significantly and positively correlated with the duration of disease (delayed cure time [days] = 0.014 × duration of disease [years] + 7.83). Therefore, most scholars suggest that postoperative patients should be continuously followed up for at least 6 months. As for the reason for delayed cure, some scholars believe that the cause of HFS immediately after MVD is because the etiology of HFS in such patients lies in the direct pulsatile percussive compression of the facial nerve REZ by the responsible vessel, and the symptoms disappear immediately after vascular decompression, while in another group of patients with a longer course of treatment, the long-term compression of the REZ by the responsible vessel causes localized heavier demyelinating lesions and overexcitation of the motor nucleus of the facial nerve in the REZ, and although after MVD Although the vascular compression was released, regenerative repair of the facial nerve root demyelinating lesion and/or stabilization of facial nerve motor nucleus excitability took some time to complete, so it led to delayed healing.
4.3 Postoperative ineffectiveness
MVD treatment for HFS is ineffective in 2.2%-6% of patients after surgery. The main reasons for ineffectiveness are.
(i) poor exposure of the facial nerve REZ affects the correct identification of the responsible vessel;
(ii) Difficulty in identifying the responsible vessel due to displacement during surgical exploration;
③Mistaking the responsible vessel for a vessel that is only in simple contact with the facial nerve or parallel to it;
④Difficulty in placing pads when the responsible vessel is a thick and sclerotic vertebral artery or has multiple short and small penetrating arteries, and failure to adequately decompress;
(⑤) The placement of too many pads or the placement of the facial nerve REZ constitutes a new compression. In principle, secondary MVD should be effective in patients who are ineffective after surgery. Due to the existence of delayed healing in some patients, it is recommended that secondary surgery should not be performed urgently in patients with ineffective results, but at least 6 months of follow-up.
4.4 Postoperative recurrence
Postoperative recurrence of MVD for I-IFS is defined as a period of complete healing after surgery followed by a return of symptoms, with the recurrence of spasticity that can be less severe, more severe, or the same as before surgery.
The recurrence of spasticity can be milder, heavier or the same as before surgery. Some authors define the recurrence of symptoms within 1 year as not cured, and the recurrence of symptoms after 1 year as relapse.
The recurrence rate of HFS after MVD surgery is 3.3% to 20%. Payner et al. reviewed the literature and found that among 571 patients who were followed up for more than 2 years, the average recurrence rate of MVD after surgery was 7.0%, of which 86% occurred within 2 years after surgery, and the recurrence rate after 2 years was only 1%. In contrast, Fukushin followed up 2890 cases of MVD for an average of lO years and found 96 cases (3.3%) of recurrence, of which 79 cases were cured by secondary MVD, suggesting that recurrence of HFS can occur at any time from 1 to 8 years after surgery and not only concentrated in 2 years. The main causes of postoperative recurrence are.
① Dislodgement or displacement of the cushion insert;
(ii) Compression by a new responsible vessel;
(3) Absorption of the gelatin sponge or muscle block;
④The padding is too thin or thinned, which can still transmit the pulsatile shock compression of the responsible vessel to the facial nerve REZ;
(5) Local arachnoid adhesions form an encapsulating compression on the facial nerve root;
⑥Improper placement and/or inappropriate size of the cushion insertion may cause the pulsatile shock of the responsible vessel to re-compress the REZ through the cushion insertion. Therefore, choosing the appropriate cushion and cushion placement at the first MVD can effectively prevent recurrence, and once the recurrence occurs, reoperation is still effective.