1.Definition and treatment status
Facial muscle spasm is characterized by involuntary paroxysmal painless twitching of the muscles innervated by the facial nerve. It starts from the orbicularis oculi muscle and gradually expands to the facial expression muscle and orbicularis oris muscle, and is easily triggered by mental tension, emotional fluctuations, overexertion and speech. There are no other positive neurological signs, and the EEG is normal, while the EMG shows muscle fiber tremor and muscle bundle tremor waves.
The incidence of the disease is 11 per 1 million. In recent years, with the development of microscopic neurosurgery, the etiology of HFS has been further understood, and significant progress has been made in its treatment. The treatment methods include surgical and non-surgical treatments. For those who have poor results in non-surgical treatment, surgical treatment has become the natural choice.
2.Theoretical basis of MVD pathology
The facial nerve is different from the facial nerve near the beginning of the brainstem and the inner ear door, with a length of 0.5-1.0 cm. It has not yet migrated to the peripheral nerve and is still a central nerve, and its myelin sheath is not composed of Schwannn cells but of glial cells. Facial myospasm is thought to be caused by the compression of the facial nerve roots by the responsible blood vessels, which leads to demyelination and short-circuiting of impulses between the afferent and efferent nerve fibers.
The ultrastructural changes of facial nerve filaments in facial myospasm were observed by electron microscopy, which showed swelling and shrinkage of axons, granular disintegration of microfilaments and microtubules, swelling of mitochondria, and degeneration of internal vacuoles. The nuclei of Schwann cells were swollen and autolysed, and the cytosol was unclear. The results of domestic and foreign literature suggest that the responsible vessels for HFS mainly originate from the anterior inferior cerebellar artery and posterior inferior cerebellar artery, followed by the superior cerebellar artery, while the variant vertebral artery and basilar artery may also cause HFS.
3.Pre-surgery preparation
Magnetic resonance tomography angiography was performed on the basis of T1 brainstem median sagittal scan with 3D time leap sequence, axial imaging was first performed on the root of the pontine facet auditory nerve, and then oblique sagittal and coronal imaging was performed bilaterally along the long axis of the nerve according to the axial image. The relationship between the nerve and the peripheral vessels on the symptomatic side was determined by the following criteria:
①Nudging: the nerve is clearly indented or displaced by pressure at the contact point between the nerve and the adjacent vessels;
(ii) clear contact: no clear gap between the nerve and the adjacent vessels at the point of contact, but the facial nerve travels normally;
③Suspicious contact: a small gap between the nerve and the adjacent vessel is seen at the point of suspicious contact, but it is smaller than the diameter of the vessel;
The shortest distance between the nerve and the adjacent vessel is greater than the diameter of the vessel or there is no vessel adjacent to the nerve.
4.Surgical procedure
The posterior approach to the inferior occipital sigmoid sinus is used for craniotomy with general anesthesia, and the anterior and inferior borders are close to the sigmoid sinus and occipital condyle, respectively. After the dural incision, the operation was performed under the operating microscope. The cerebellar hemispheres are lifted inward and upward during exposure of the facial nerve, and the cerebrospinal fluid is slowly discharged to sharply dissect the arachnoid membrane lateral to the cerebellar medullary pool.
The surface of the cerebellar vermis is retracted and the arachnoid membrane between the cerebellar vermis and the nerve is sharply dissected. After revealing the facial nerve, the facial nerve REZ is further revealed by rotating the surgical bed, and the responsible vessels are separated and dissected by a combination of blunt and sharp methods, taking care not to damage the perforating arteries emanating from the arteries to the brainstem and the internal auditory artery towards the internal auditory canal. The responsible vessels mostly pass through the facial nerve REZ in the form of collaterals and cause compression. When multiple vessels are present in the REZ, the responsible vessel is often located in the deeper part of the vascular plexus.
After identification of the responsible vessel, the vessel is freed and pushed away, and an appropriate size of Teflon cotton is placed between the responsible vessel and the brainstem, which is first torn into small clusters to make it soft and flexible, and then made into a cigar shape to make it easy to fix. In cases where the responsible vessel is located deep in the vascular plexus or even hidden on the dorsal surface of the nerve root or the ventral side of the brainstem, and its structure is not well exposed by the operating microscope, the operation is performed with the aid of a neuroendoscope. Real-time brainstem auditory evoked potential monitoring was performed intraoperatively.
5.The efficiency of MVD for HFS is about 92-97%, and the main reasons for ineffectiveness are:
(1) No obvious responsible vessels were found intraoperatively;
(2) Improper placement of the cushion insert and the surrounding adhesions led to the repositioning of the responsible vessel and the re-formation of compression on the facial nerve out of the brainstem area;
(3) The cushion is too thin and can still conduct the pulsatile impact compression of the responsible vessel;
(4) The cushion is dislodged or displaced;
(5) When the compressed vessel is a thick and sclerotic vertebral artery or a short penetrating artery and the facial nerve root is wrapped by the artery, it is difficult to separate the septum and cannot be fully decompressed;
(6) The new responsible vessel constitutes the compression;
(7) Multiple causative factors exist in some HFS patients.
Reliable preoperative assessment of the responsible vessel, familiarity with the anatomical basis of application, skillful microsurgical techniques, ideal decompression materials and accurate identification of the responsible vessel are indispensable factors to ensure the efficacy of MVD for HFS.