I. Brief description
The origin of microvascular decompression began with clinical studies on the surgical treatment of trigeminal neuralgia. Decades of research have shown that compression of the nerve roots into/out of the brainstem region by the vessels responsible for the cerebellopontine angle of the cerebellum can lead to a syndrome known as neurovascular compression syndrome.
For patients with primary TN and glossopharyngeal neuralgia who can tolerate craniotomy, MVD has become the preferred surgical treatment. It is the main direction of functional neurosurgeons to combine MVD with CPA selective partial cerebral nerve root dissection to maximize surgical efficiency and reduce the complication rate.
Diagnosis of primary TN
1. Clinical manifestations of primary TN: Primary TN is mainly characterized by recurrent paroxysmal severe pain in the trigeminal nerve distribution area. Most of the pain is unilateral, occasionally bilateral onset is seen as tear-like, electric shock-like, pinprick-like, knife-like or burning-like severe pain, which may be accompanied by lacrimation, salivation, runny nose or facial twitching.
Trigger points or trigger points exist, mostly located in the upper and lower lips, nose, nasolabial folds, gums, cheeks, and corners of the mouth, and can be triggered by actions such as chewing, eating, drinking, wind, cold, brushing teeth, washing face, and speaking. Some patients may have intermittent periods ranging from several weeks to several years.The imaging examination and significance of TN are detailed in the Expert Consensus on Facial Myoclonus Treated by Microvascular Decompression in China (2014).
Secondary TN: TN secondary to CPA tumor is difficult to differentiate from typical primary TN in terms of symptoms and signs, and the diagnosis depends on CT and MRI, and the possibility of CPA tumor increases when TN is associated with other cranial nerve disorders. In this paper, we focus on the surgical treatment of primary TN.
3. Differential diagnosis of primary TN: The multi-causal nature of TN leads to difficulties in its diagnosis and differential diagnosis. Primary TN needs to be differentiated from other painful diseases of the head and face. For example, GN, intermediate neuralgia, pterygopalatine neuralgia, atypical facial pain, cluster headache, postherpetic pain, odontogenic pain, etc. Typical clinical features of primary TN that can be differentiated include: (1) clear range of pain; (2) episodic; (3) presence of remission periods; (4) presence of triggering factors and trigger points; and (5) effective administration of carbamazepine at the initial time.
III. Diagnosis of primary GN
1. Clinical manifestations of primary GN: GN is less common. Episodes of pain are limited to the root of the tongue, tonsil area, pharynx, mandibular angle, mastoid area, and external auditory canal area on one side, and can be triggered by swallowing, chewing, talking, coughing, and yawning. A few patients may present with cardiogenic syncope, arrhythmia, and hypotension, etc. Pain relief after spraying dicaine in the pharynx is the most important feature of GN. Carbamazepine is mostly effective.
2. Differential diagnosis of primary GN: stem overgrowth, occupying lesions of CPA, malignant tumor (such as nasopharyngeal carcinoma) invading the skull base are common secondary causes, therefore, preoperative frontal and lateral plain radiographs of the stem and CT or MRI should be routinely examined.
4. Indications and contraindications for surgery
Indications for surgery: Not all primary TN and GN require surgical treatment, and carbamazepine will remain the most effective and commonly used drug for symptomatic treatment of TN and GN for a long period of time in the future.
The toxic side effects of carbamazepine should be noted, including: drowsiness, dizziness, gastrointestinal reactions, ataxia, liver damage, white blood cell reduction, and severe allergic reactions (e.g. exfoliative dermatitis). The toxic side effects of the drug make it intolerable for a significant proportion of patients to seek alternative treatments.
Clinical indications for MVD should be both avoided by blindly expanding the indications for MVD and must be made clear: for patients who can tolerate open surgery, MVD is the surgical treatment of choice, superior to other means such as gamma knife or radiofrequency.
Indications for surgery for TN and GN include: (1) primary TN and GN, excluding secondary lesions; (2) severe symptoms affecting the patient’s daily life; (3) poor effect of conservative treatment or serious side effects; (4) patient’s requirement for active surgical treatment.
2. Contraindications to surgery: (1) the same as other contraindications to general anesthesia craniotomy, such as the presence of serious systemic diseases with poor control; (2) insufficient understanding and preparation of the patient for the efficacy of surgery and possible complications.
V. Surgical techniques
1. Treatment strategy of supraclavicular vein: (1) Principles of treatment of supraclavicular vein in CPA exploration: subclavicular vein branches located at the skull base can be cut off directly after electrocoagulation if they obstruct the surgical approach, while supraclavicular vein branches near the cerebellar curtain should not be cut off as far as possible to avoid serious consequences such as venous infarction or even hemorrhage. In TNMVD, if the supratentorial vein obstructs the surgical access and the gap between the trigeminal nerve root and the cerebellar curtain cannot be penetrated without disconnecting the vein, the access can be made from above the auditory nerve.
(2) Electrocoagulation and hemostasis of supratentorial vein: When the supratentorial vein is short and thick and free, it is sometimes futile and dangerous to try to reveal the REZ between trigeminal nerve root and cerebellar curtain by dissecting the arachnoid membrane or through the access above the auditory nerve, and forceful pulling of cerebellar hemisphere can tear the trunk of supratentorial vein from the supratentorial sinus and cause accidental hemorrhage, so it is also appropriate to cut the vein.
The vein should be repeatedly cauterized with less power close to its cerebellar side when electrocoagulated, and the thicker branches of the genus sometimes need to be cut completely in fractions. The arachnoid membrane around the vein should be cut free as much as possible before electrocoagulation to avoid contraction of the arachnoid membrane due to electrocautery, which may pull the vein and cause tearing and bleeding at the supratentorial sinus. Occasionally, the vein may rupture and bleed during traction or electrocoagulation, which often takes the operator by surprise, and patient compression after aspiration is the only way to stop the bleeding.
(3) Circumstances in which it is not recommended to cut the supraclavicular vein: extreme caution should be exercised in cutting the supraclavicular vein in the following cases: ① the proposed supraclavicular vein mainly drains venous blood from the brainstem.
(ii) The supraclavicular branch to be cut is closer in color to an arterial appearance than other branches, i.e., the vein is arterialized, and it is estimated that the blood flow in it is more rapid, and cutting it may cause acute congruent flow disorders.
(3) There are few branches of supraclinoid veins within the visual field, and the proposed branch of supraclinoid veins is abnormally large, so it is expected that it is more difficult to compensate for other branches after cutting.
In the above three cases, even if the trigeminal nerve root cannot be fully revealed without dealing with the supraclavicular vein, or even if the supraclavicular vein genus branch itself is the responsible vessel, it is not recommended to cut it by electrocoagulation, and at this time the trigeminal sensory root PR is feasible to ensure the therapeutic effect.
2. Vascular decompression techniques: (1) TN pain decompression technique: The main responsible vessels in TNMVD are the superior cerebellar artery and its branches, the anterior inferior cerebellar artery and its branches, the branch of supraclavicular vein, and the basilar artery in that order. Veins alone or involved in compression are often seen in TN, but rarely in other cerebral neurological disorders, and the responsible vein should be free and padded, and not cut off as much as possible.
Since arachnoid thickening adhesion itself may be an important pathogenic factor in TN, the sensory roots of the trigeminal nerve should be fully dissected from the brainstem to the bursa of Meyer, so that they can be completely released in the axial position, and then vascular decompression should be performed. Teflon cotton or polyester spacer (polyethylene terephthalate fiber) is used for decompression material.
(2) GN decompression technique: The main responsible vessels in GNMVD are the posterior inferior cerebellar artery and its branches, the anterior inferior cerebellar artery and its branches, the vertebral artery, and the branches of the inferior rock vein, in that order.
The following factors determine that during the decompression of the linguopharyngeal nerve and vagus nerve REZ, it is easy to encounter the situation that the responsible artery cannot be satisfactorily nudged: (1) the linguopharyngeal nerve root and vagus nerve root are adjacent to the skull base in anatomical position, so the local operation space is small and the REZ cannot be fully exposed; in some cases of severe skull base depression and small volume of the posterior cranial fossa, the REZ cannot even be exposed; (2) the responsible vessels are mostly tortuous (ii) the responsible vessels are mostly tortuous posterior inferior cerebellar artery trunks and vertebral arteries, and there are more penetrating arteries.
③The responsible vessels are mostly hidden in the posterior lateral sulcus of the medulla oblongata; ④The posterior group of cerebral nerves are more slender, closely arranged and more vulnerable to injury, and when even the call for spraying dicaine cannot accurately distinguish CN or TN, the MVD surgical towel may be the only sensible choice to explore both the trigeminal nerve root and the linguopharyngeal and vagus nerve roots.
3. Trigeminal sensory root PR: When the responsible vessels are not found in TN MVD, trigeminal sensory root PR is feasible and it is important to distinguish typical from atypical TN.
For ineffective or recurrent TN cases, the choice of procedure during secondary surgery should be based on PR. To ensure efficacy, only MVD should be considered when the following conditions coexist: (1) younger patients; (2) minor adhesions are found during secondary exploration; (3) clear arterial vascular compression exists; and (4) satisfactory vascular decompression.
Elderly TN patients often have serious diseases of important organs and generally have difficulty in tolerating secondary anesthesia and surgical trauma, and the risk of reluctant surgery is greater. Therefore, the indications for PR can be relaxed when selecting the procedure, so as to avoid facing a dilemma in case of ineffective surgery or recurrence.
4. linguopharyngeal nerve root dissection and vagus nerve root PR: linguopharyngeal nerve root dissection and vagus nerve root PR, MVD and the combination of the two are effective methods for treating CN, and the choice of surgical fighting style should be based on the specific situation of intraoperative exploration.
(1) MVD should be performed if there is clear responsible vascular compression of the REZ; (2) PR should be performed if there is no responsible vascular compression of the REZ; (3) MVD+PR should be performed if the responsible vascular compression is small and clear or if there is clear vascular compression but satisfactory allowable decompression cannot be achieved due to various reasons.
The pain of atypical GN may involve the anterior and posterior external ear, mastoid area, and the skin of the pharynx below the anterior angle of the mandible, and the surgery often requires cutting the upper l~3 filaments of the vagus nerve to be effective.
Evaluation of efficacy
1. Criteria for evaluation of efficacy after TN surgery: (1) cure: complete disappearance of symptoms; (2) significant remission: symptoms basically disappear, occasional attacks but no drug treatment is needed; (3) partial remission: symptoms are reduced, but drug control is still needed; (4) invalid: no change or aggravation of symptoms. Both of the above (1)(2) cases are considered effective.
2. Evaluation time of efficacy after TN surgery: delayed cure after TN and MVD can be seen occasionally, generally not tripping over 3 months.
3. Treatment of ineffectiveness or recurrence after TN: Patients with ineffectiveness or recurrence can be considered for secondary MVD, PR, radiofrequency destruction, balloon compression or stereotactic radiosurgery according to the specific conditions of the first surgery and the current physical condition of the patient.
4. Evaluation criteria for postoperative efficacy of GN: (1) good efficacy: complete disappearance of pain or greater than 95% relief without medication; (2) fair efficacy: greater than 50% pain relief with or without medication; (3) poor efficacy: no pain relief.
5. Time of GN postoperative efficacy evaluation: delayed cure after CNMVD is rare, so efficacy evaluation can be performed immediately after surgery.
6. management of ineffective or recurrent patients after GN: patients with ineffective or recurrent patients can be considered for secondary MVD. it is recommended to perform PR of the superior root filaments of the linguopharyngeal nerve root (and) vagus nerve root while releasing adhesions and vascular decompression.