I. Preface
In 1966, Jannetta pioneered micmvascular decompression (MVD) for the treatment of neurological disorders, based on the theory that compression of the responsible vessels in the cerebellopontine angle (CPA) in the root entry/exit zoon (REZ) of different cerebral nerve roots can lead to the corresponding syndrome. MVD was introduced to China around 1985 because of its safety and efficacy, and was rapidly promoted in clinical practice. Nearly half a century has passed since the creation of MVD, but MVD, which is the best treatment effect in the field of functional neurosurgery, is still not popular in China, and there is still a great imbalance in the development of the region, and the treatment level needs to be improved. MVD is a type of functional neurosurgery with a high degree of delicacy, and its standardized operation technique still needs to be further promoted in order to improve the efficiency of the operation and avoid serious complications that are difficult for patients to accept. For these reasons, it is important to organize experts and scholars in China to discuss and write an expert consensus on the treatment of neurological disorders in the brain for MVD, in order to improve the overall treatment level of MVD in China.
The brain neurological disorders that can be treated by MVD mainly include idiopathic hemifacial spasm (HFS), primary trigeminal neuralgia (TN), primary glossopharyngeal neuralgia (GN) and so on. Drug treatment for HFS is always ineffective. Botulinum toxin injection therapy always recurs, and repeated injections may lead to irreversible facial paralysis, myasthenia, and even facial deformation. Therefore, once HFS is diagnosed, MVD is the only radical treatment option. Studies on the etiology of HFS, delayed cure after MVD, and delayed facial palsy will further deepen our understanding of the nature of HFS disease and provide useful guidance for clinical work.
II. Diagnosis of idiopathic hemiplegic HFS
1. Clinical manifestations of idiopathic hemiplegic HFS: HFS manifests as paroxysmal hemiplegic involuntary facial muscle twitches, mostly starting after middle age, and very rarely with bilateral sequential attacks. The onset of HFS starts in the upper and lower eyelids, and gradually expands slowly to the cheeks and all the muscles of the face, and in severe cases, the neck muscles may be involved. There are intervals between convulsive attacks. There are no positive signs on neurological examination. The disease progresses slowly and rarely heals spontaneously.
2. Secondary hemiplegic HFS: Secondary HFS is very rare, mostly caused by epidermoid cysts, meningiomas or nerve sheath tumors in the cerebellopontine area, with typical symptoms, and mostly combined with ipsilateral TN or tinnitus, vertigo, hearing loss and other vestibular cochlear nerve compression symptoms.
3. Differential diagnosis of idiopathic lateral HFS.
Definitive diagnosis of idiopathic HFS must be differentiated from the following diseases: habitual oculomotor spasm, hysterical oculomotor spasm, limited motor epilepsy, post-facial nerve paralysis spasm, oculomotor syndrome, chorea and facial twitching associated with tardive dyskinesia. Because of its unique clinical signs, the diagnosis of typical HFS is not difficult to confirm. When the clinical examination is not sufficient to establish the diagnosis, facial electrophysiological examination is essential for the differential diagnosis of HFS, and the diagnosis of HFS can be established when typical abnormal muscle response (AMR) waves are monitored.
Preoperative evaluation
1.Imaging assessment.
(1) Significance of imaging evaluation: Before MVD treatment, accurate imaging evaluation is important for excluding secondary lesions, screening of patients for surgery, identification of responsible vessels intraoperatively, and prediction of surgical difficulty. The significance of pre- and postoperative thin-section CT scans of the cranial fossa is to identify tumors, significant vascular disease, and to detect grossly responsible arteries and bony malformations of the skull base, but not to show the cerebral nerves and their surrounding fine vessels. High-field intensity conventional serial MRI scans can show the brain parenchyma, brain nerves and blood vessels in the posterior cranial fossa and are superior to CT in detecting CPA tumors or vascular diseases, but it is more difficult to clearly show the fine blood vessels. In recent years, the application of MRI imaging techniques such as FISP, FLASH, FFE, SPGRj MP-RAGE, 3D-TOF, T2W FSE, bFFE, CISS, FIESTA, 3D-FIESTA+C, MPR, MRTA greatly improved the level of observation and identification of vascular neural structures in CPA.
(2) Diagnostic imaging criteria for cerebral neurovascular compression: if neurovascular compression or contact is seen at more than two levels, NVC is diagnosed; if neurovascular contact can be shown only at one level, NVC is suspected. They are not sufficient to confirm or exclude the diagnosis, and cannot be used as indications or contraindications for MVD surgery.
2. Neurophysiological evaluation: AMR is mostly used for the differential diagnosis of HFS: monitoring AMR means that the diagnosis of HFS can be established. Brainstem auditory evoked potentials (BAEPs) are used to check the function of auditory pathways before MVD surgery.
3. Indications for surgery: (1) Idiopathic HFS, excluding secondary lesions. (2) The symptoms are severe and affect the patient’s daily life. (3) The patient has the requirement of active surgical treatment.
4. Contraindications to surgery: (1) The same as other contraindications to general anesthesia craniotomy, such as the presence of serious systemic diseases and poor control, etc. (2) Insufficient understanding and preparation of the patient for surgical efficacy and possible complications.
IV. Surgical techniques
1. Preoperative preparation: 1 day before surgery the affected side of the ear is shaved behind the occipital area, with the upper border to the level of the upper edge of the auricle, posteriorly to the midline of the occipital area, and below to the hairline.
2. Anesthesia and position: general anesthesia with endotracheal intubation. when AMR monitoring is required during HFS, only short-acting inotropes are used during general anesthesia induction intubation. The affected mastoid was positioned roughly parallel to the operating table and in the highest position to facilitate the alignment of the microscope optical axis with the surgical approach.
The size of the incision depends on the length and thickness of the patient’s neck, the thickness of the local muscles, the possible deformities of the skull base such as bone depression, and the estimated difficulty of the operation before surgery. An oblique transverse incision can also be made near the turn of the occipital bone toward the skull base in the hairline behind the ear. Depending on the cerebral nerve disorder being treated, the upper edge of the bone window may be exposed below the transverse sinus, the anterior edge must reach behind the sigmoid sinus, and the lower edge may reach the skull base. The mastoid airspace is closed tightly with bone wax before opening the dura.
4.Exploration of the cerebellopontine horn region: open the lateral pool of the cerebellar medulla and slowly discharge cerebrospinal fluid (CSF). Excessive and rapid release of CSF should be avoided because it may lead to expenditure of blood in the skull base and the rock vein near the cerebellar curtain, and even bleeding in the distal part of the supratentorial septum may occur. The cerebral pressure plate should be gradually retracted and deepened, and the retraction range should not be greater than 1 cm, and the retraction should be intermittent. The subclavian vein at the base of the skull may be cut off by direct electrocoagulation if it interferes with the surgical access. The arachnoid membrane around the cerebral nerve should be sharply dissected and separated.
5.Vascular decompression techniques.
(1) Importance of cerebral nerve roots into/out of brainstem area: to judge the responsible vessel must first clarify the importance of cerebral nerve REZ in MVD, that is, vascular decompression is only for the vessel that constitutes compression of cerebral nerve REZ. The range of REZ of different types of brain nerves is different, so the range of decompression of MVD should also be different. Inadequate decompression range may lead to poor outcome, while blindly expanding the decompression range may increase the risk of postoperative complications, and the efficiency of surgery cannot be improved accordingly. Generally speaking, the range of nerve roots of sensory brain nerves into the brainstem area is much larger than that of motor brain nerves out of the brainstem area, such as the nerve roots of trigeminal nerve, linguopharyngeal nerve, and vestibulocochlear nerve into the brain 10 area can involve the whole length of the brain pool segment, while the facial nerve root REZ is limited to the vicinity of the brainstem, therefore, the MVD of trigeminal nerve, linguopharyngeal nerve, and vestibulocochlear nerve should be decompressed in the whole brain pool segment, while the decompression range of facial nerve The decompression of the facial nerve should be limited to the nerve root REZ only. When repeated exploration of the facial nerve REZ during HFS MVD does not reveal blood vessels, the facial nerve trunk can be further explored slightly distal to the REZ. Vessels located in the distal segment of the facial nerve, near the internal auditory meatus, in the lateral pool of the pons, in contact with or parallel to the facial nerve trunk only, and between the facial and auditory nerves.
(2) Judgment of the responsible vessel: the responsible vessel mostly passes through the REZ in a collaterals and causes compression. When there are multiple vessels in the REZ, the responsible vessels are often located in the deep side of the vascular plexus. the main responsible vessels in the order of HFS MVD are: the anterior inferior cerebellar artery and its branches, the posterior inferior cerebellar artery and its branches, the vertebral artery, and the branches of the inferior rock vein. Venous compression of the facial nerve REZ alone is rare. The following factors may affect the identification of the main responsible vessel.
(i) The departure of the responsible vessel from the REZ in the lateral position.
(ii) Failure to visualize the REZ well and omission of the vessel.
(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.
(3) Decompression of the responsible vessel: After the responsible vessel is fully free, it is pushed away from the REZ in the direction of the cerebellar curtain, skull base or ventral side, and the cushioned openings are placed between the responsible vessel and the brainstem. Teflon (polytetrafluoroethylene) is used for the cushion opening. Emphasis is placed on “insulating” the responsible vessel away from the REZ rather than simply between the vessel and the REZ. The pad should not be too large to avoid new compressions. After placement, ensure that the pad is secured to prevent slippage. Care should be taken not to twist the artery at an angle after the responsible vessel is cushioned. When there is a branch of the subclavian vein alone or involved in the compression, it can be fully dissected free and pushed away from the REZ with the pad cotton, or cut off after electrocoagulation when it is difficult to dissect and free.
(4) Suspension of the responsible artery: when encountering a part of difficult decompression, the suspension of the responsible artery can be used: Teflon cotton is wrapped around the responsible artery and then pushed to the dura of the cranial wall, first the local dura is roughened by electrocoagulation, and a small amount of medical adhesive is applied between the responsible artery or the Teflon cotton wrapped around the artery and the dura to fix it, so that the responsible artery can be suspended away from the REZ and achieve satisfactory decompression effect.
(5) Application of neuroendoscopy: The application of neuroendoscopy during MVD helps to judge the responsible vessel, evaluate the decompression of the nerve root and the size and placement of the pad cotton, which has certain clinical significance to improve the effect of surgical treatment and reduce the recurrence of symptoms and complications. However, under the current technical conditions, it is not advocated to fully promote the all-endoscopic MVD.
6. Intraoperative neurophysiological monitoring.
(1) Abnormal muscle response monitoring: AMR, also known as lateral spread response (LSR), is an objective electrophysiological index unique to HFS. Intraoperative monitoring of AMR is recommended in conditional units, which is helpful in judging the responsible vessel intraoperatively, improving the efficacy and reducing complications. It is generally believed that the degree of AMR wave amplitude disappearance is positively correlated with postoperative efficacy, but it is often seen clinically that cases with AMR not disappeared also have complete remission of symptoms after surgery, while there are other patients with complete disappearance of AMR, but not complete remission of symptoms or even no remission after surgery, i.e., there are false positive and false negative rates. Recommendation: The decompression operation can be terminated if the operator confirms that the decompression is complete and the AMR disappears. If the AMR does not disappear, the REZ should be thoroughly explored again, and the decompression operation should be terminated even if the AMR still exists after confirming that the responsible vessel is not missed.
(2) Brainstem auditory evoked potential monitoring: intraoperative monitoring of BAEPs is recommended in units where available. progressive prolongation of latency and/or decrease in wave amplitude should be taken seriously regardless of the circumstances. Although there are no uniform, absolute alarm criteria, generally any prolongation of latency greater than 1,0-1,5 ms from baseline or a change in amplitude greater than 50% (especially abrupt changes) should immediately stop the surgical operation and look for the cause. However, due to the delayed nature of waveform superimposition, BAEPs cannot monitor snail nerve function in real time, and when intraoperative changes in BAEPs are detected, sometimes nerve function cannot be recovered.
V. Efficacy evaluation
1.Efficacy evaluation criteria.
(1) Cure: complete disappearance of symptoms.
(2) Apparent remission: symptoms basically disappear and only occasionally appear under specific circumstances such as emotional stress.
(3) Partial remission: symptom reduction, but still daily episodes.
(4) Ineffective: no change or worsening of symptoms. Both of the above (1)(2) are considered effective.
2.Delayed resolution and efficacy evaluation time: About 20%-25% of HFS patients’ symptoms cannot disappear completely immediately after MVD surgery, or reappear after a few days of remission, and the symptoms can be similar to those before surgery, slightly reduced or significantly reduced, and gradually disappear completely only after a period of time (1 week to 1 year).
This phenomenon is called delayed resolution.) In view of the existence of delayed resolution, it is recommended that patients with HFS after MVD should be followed up for at least 1 year before evaluating the efficacy. A second MVD should not be performed for patients with persistent symptoms within a short period of time after MVD.
3. Treatment of ineffectiveness or recurrence: Secondary MVD can be performed after the first MVD for HFS is ineffective or recurrence, but the difficulty and risk of surgery increases, the efficacy decreases and complications increase.
[Illustration: Arterial vascular compression of the facial auditory nerve root (REZ) zone with abnormal alignment is seen at the root of the facial nerve (VII) during surgery].
[ILLUSTRATION: The Tefflon pad was placed between the responsible arterial vessel and the brainstem, away from the facial nerve root, to achieve microsurgical decompression].