The microvascular decompression procedure has proven to be a safe and effective option in the surgical management of neurovascular compression syndromes in general and trigeminal neuralgia in particular. This article aims to serve as an overview of the decision-making process, application of the surgical technique, and clinical outcome pertaining to this procedure.
Key points
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Microvascular decompression (MVD) has proven to be a safe and effective option for patients with medically intractable trigeminal neuralgia (TN).
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MVD provides a high rate of early pain relief, with relatively low recurrence and complication rates in experienced hands, as compared with other surgical modalities such as the percutaneous techniques and stereotactic radiosurgery.
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MVD should be considered as the first line of surgical treatment in patients with TN, including carefully selected candidates in the elderly population (provided that there are no significant medical comorbidities associated), and in whom the pathophysiology of the pain is most likely to be the neurovascular compression and no other alternative mechanisms (ie, multiple sclerosis, herpetic neuralgia) are likely.
Introduction
Trigeminal neuralgia (TN) is one of the many types of facial pain syndromes, which is thought to occur as a result of the ignition of hyperexcitable axons at the trigeminal root and neuronal somata at the trigeminal ganglion. Compression of the root entry zone (REZ) of the nerve by the neighboring offending artery, which was initially proposed by Jannetta, and for which the microvascular decompression (MVD) procedure was introduced thereafter, has been shown to contribute to the demyelination process within the trigeminal nerve, a main finding in patients with TN.
In contemporary neurosurgery, the first line of management for patients suffering from this debilitating problem is medical treatment (ie, Carbamazepine, gabapentine). In cases of failed medical therapy or drug intolerance, or simply when patients do not prefer to take these medications for a long period of time, other treatment options, including MVD, should be considered :
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MVD
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Percutaneous techniques:
- ○
Glycerol rhizotomy
- ○
Balloon compression
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Radiofrequency rhizotomy
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Gamma Knife radiosurgery (GKRS)
Introduction
Trigeminal neuralgia (TN) is one of the many types of facial pain syndromes, which is thought to occur as a result of the ignition of hyperexcitable axons at the trigeminal root and neuronal somata at the trigeminal ganglion. Compression of the root entry zone (REZ) of the nerve by the neighboring offending artery, which was initially proposed by Jannetta, and for which the microvascular decompression (MVD) procedure was introduced thereafter, has been shown to contribute to the demyelination process within the trigeminal nerve, a main finding in patients with TN.
In contemporary neurosurgery, the first line of management for patients suffering from this debilitating problem is medical treatment (ie, Carbamazepine, gabapentine). In cases of failed medical therapy or drug intolerance, or simply when patients do not prefer to take these medications for a long period of time, other treatment options, including MVD, should be considered :
- •
MVD
- •
Percutaneous techniques:
- ○
Glycerol rhizotomy
- ○
Balloon compression
- ○
Radiofrequency rhizotomy
- ○
- •
Gamma Knife radiosurgery (GKRS)
Surgical technique
Patient Selection
Establishing the diagnosis accurately
One cannot overemphasize the significance of establishing accurate clinical diagnosis of TN in ensuring a favorable postoperative outcome for MVD. The patient’s description of the pain, and the neurologic examination, should leave no doubt that what is being dealt with is TN before referring to any radiologic study or intervention. Miller and colleagues reported that patients presenting with type 1 TN pain (50% episodic pain), according to the classification of Burchiel, had significantly higher chance of having a favorable outcome following MVD for TN than patients with type II pain (84% vs 64%). In this context, one also has to keep in mind the possibility of multiple sclerosis–induced neuralgia and postherpetic neuralgia before proceeding with MVD.
Age as a factor in the decision-making process
Age has been an important factor for neurosurgeons in choosing the appropriate treatment option for patients with TN, and traditionally, there has been a tendency to offer percutaneous procedures or GKRS rather than MVD to patients with advanced age. However, recent studies have shown no significant differences in complications or short-term and long-term outcome of MVD in elderly patients than in younger ones. One study concluded that although complications show a tendency to increase with advanced age, age itself does not act as a risk factor in isolation.
Significance of radiographic findings
The significance of demonstrating a close anatomic relationship or contact between the vessels and the trigeminal nerve preoperatively has been debated in the literature, because this finding has also been reported quite commonly in patients without TN. However, when preoperative assessment of the neurovascular relationships of the region is desired, 3D constructive interference in steady state magnetic resonance (MR) ( Fig. 1 ), which is a heavily T2-weighted sequence with very high resolution of the cerebrospinal fluid (CSF) tissue contrast and high-resolution 3D time-of-flight MR angiography may provide adequate information to the surgeon in most cases.
The other important detail when reviewing preoperative radiological studies would be the tentorial alignment and angle. The authors’ group has shown previously that a steeper tentorial angle would result in a decreased distance between the trigeminal nerve and the acoustico-facial nerve complex, limiting the operative field between these cranial nerves.
Surgical Approach and Patient Positioning
In the authors’ practice, they use the “simplified retrosigmoid approach” for MVD procedures. Following induction of general anesthesia, the patient is placed in the supine position with the head rotated to the contralateral side and slightly flexed, and it is verified that the jugular vein is free of compression ( Fig. 2 A). The ipsilateral shoulder may be elevated and supported by a shoulder roll.
Procedure
Incision
A 4 to 5 cm curvilinear skin incision is placed using the asterion as the main landmark (see Fig. 2 B). Approximately one-third of the incision remains above, and two-thirds are below the asterion.
Craniectomy and dural opening
A triangular craniectomy is performed using a cutting drill and Kerrison rongeurs so that the upper lateral corner of the triangle sits at the junction of the transverse and sigmoid sinuses (see Fig. 2 C). During this stage, one needs to be prepared for the possibility of brisk bleeding from the emissary veins or sinuses themselves. In individuals with generously aerated mastoid bones, one can enter wide air cells, which could potentially result in CSF rhinorrhea. Therefore, when encountered, these should be appropriately plugged and sealed.
A Y-shaped dural opening is performed creating 2 triangular flaps based on the sinuses. Following this, the cerebellum is retracted gently and the lateral cerebellomedullary cistern is entered to drain CSF (see Fig. 2 D); this should be done with care so as to minimize cerebellar trauma, which can potentially result in serious swelling.
Intradural dissection and retraction
The microscope is tilted upwards. It cannot be overemphasized that the cerebellar retraction remains very mild and gentle throughout the surgery. In fact, with adequate CSF drainage and arachnoidal opening, the cerebellum falls out of the operative field without necessitating any significant retraction for the most part of the procedure. Also, one has to keep in mind that excessive retraction of the petrosal surface of the cerebellum can result in injury to the cochlear nerve. Therefore, a more petrotentorial retraction would not only yield a better access to the trigeminal nerve but also lower the risk of this potential complication.
Intraoperative neuromonitoring
Routine use of intraoperative monitoring of the facial and cochlear nerves has been described and advocated by some groups in the literature. However, in the authors’ practice, they do not feel the need to do so on a routine basis, as the core of the surgery takes place relatively far from the acoustico-facial nerve complex and, with appropriate retraction, the risk of nerve damage is reduced.
Petrosal veins
The issue of whether to preserve or sacrifice the superior petrosal vein/complex (SPV) is probably one of the most controversial aspects of MVD ( Fig. 3 ). SPV drains the anterior aspect of the brain stem and cerebellum and empties into the superior petrosal sinus. Its most common tributaries are the vein of the cerebellopontine fissure, vein of the middle cerebellar peduncle, transverse pontine vein, pontotrigeminal vein, and the veins draining the lateral surface of the cerebellum.
Related posts:
Introduction to Neuropathic Pain Syndromes
Intracranial Ablative Procedures for the Treatment of Chronic Pain
Percutaneous Treatments for Trigeminal Neuralgia
Neurolysis, Neurectomy, and Nerve Repair/Reconstruction for Chronic Pain
Nerve Blocks for Chronic Pain
Transcranial Magnetic Stimulation for Chronic Pain
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