Neurovascular Decompression in Cranial Nerves V, VII, IX, and X

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Chapter 125 Neurovascular Decompression in Cranial Nerves V, VII, IX, and X

The compression of specific cranial nerves at their entrance to or exit from the brain stem has been linked to a number of clinical conditions. Tic douloureux or trigeminal neuralgia is the most common, and perhaps the best described and investigated, of these cranial nerve disorders. Dandy was the first to observe, in 1932, that vascular compression of the trigeminal nerve in the posterior fossa might be the cause of tic douloureux.¹ Gardner and Sava later described vascular compression of the seventh cranial nerve in more than half of the patients they operated on via the posterior fossa for hemifacial spasm, the motor equivalent of trigeminal neuralgia.² However, it was not until Jannetta applied the operating microscope to the systematic study of these problems that the truly remarkable incidence of pathologic vascular compression of cranial nerves at their entry to or exit from the brain stem was appreciated.³ Based on his findings, Jannetta devised an operative technique (microvascular decompression) to displace these vessels from the affected nerve without sacrificing neural integrity.^3,⁴ This technique has been used to relieve the specific cranial nerve syndromes successfully in the majority of patients treated. This chapter discusses the individual cranial nerve vascular compressive syndromes and the decompression techniques that can be used in treating them.

Fifth Cranial Nerve

Clinical Presentation

Trigeminal neuralgia is characterized by a stereotypical clinical syndrome. Patients suffering from trigeminal neuralgia exhibit brief, intense paroxysms of pain confined to one or more divisions of the trigeminal nerve. These attacks are usually triggered by light cutaneous stimuli within the trigeminal territory. The pain typically is described as an intense stabbing or electrical shock sensation that patients frequently describe by rapidly flinging open their hands to indicate the paroxysmal nature of the jolts of pain. The pain is often more pronounced during the day, and many patients are pain free or have markedly fewer episodes at night. The most common areas of involvement are in the second and third trigeminal divisions, specifically anteriorly and in the region of the mouth. Because the mouth is a common site of symptoms, patients may undergo unnecessary dental surgery without achieving relief before the diagnosis of trigeminal neuralgia is made. Patients classically guard their faces, refuse to be touched, and avoid shaving, washing, applying makeup, chewing, and brushing their teeth, because any of these actions may provoke an attack. Over time, patients frequently report an increase in the number of attacks, a reduction of pain-free periods, and occasionally sensory disturbances in the involved trigeminal distribution or distributions.^4,⁵

Differential Diagnosis

Trigeminal neuralgia is occasionally the presenting complaint of a patient with multiple sclerosis. Trigeminal neuralgia occurs in 1% to 3% of patients afflicted with multiple sclerosis. Similarly, 2% to 4% patients with trigeminal neuralgia eventually are found to have multiple sclerosis.⁶ The clinical picture and site of the pathologic process (the root entry zone of the nerve) appear to be identical in idiopathic trigeminal neuralgia and multiple sclerosis patients. However, in multiple sclerosis, the cause of neuralgia is intrinsic demyelination of the nerve, which is not due to extrinsic vascular compression. Subsequently, microvascular decompression does not benefit multiple sclerosis patients, and a neural destructive procedure should be used to treat medically refractory cases.

A history of sustained facial pain that is not paroxysmal must be questioned carefully. Patients sometimes describe frequent repetitive jabs of pain as one prolonged attack. This does not preclude the diagnosis of trigeminal neuralgia. However, slowly developing pain that builds in intensity, lasts for variable periods (often hours to days), and then subsides is not characteristic of trigeminal neuralgia. The most common mistake in the diagnosis of trigeminal neuralgia, in our experience, has been mislabeling patients who suffer from chronic cluster syndrome. This and other atypical facial pain syndromes do not respond to surgical decompressive procedures.

Various other conditions must be considered in the differential diagnosis of trigeminal neuralgia. These include herpes zoster (postherpetic neuralgia), dental disease, orbital disease, temporomandibular dysfunction, and temporal arteritis, as well as post-traumatic neuralgias. None of these typically presents with the classic lancinating paroxysmal pain of trigeminal neuralgia. Once the clinical diagnosis of trigeminal neuralgia is made, magnetic resonance imaging (MRI) should be performed to rule out demyelinating disease, infection, inflammatory processes, vascular malformations, and neoplasms.

Medical Therapy

After the diagnosis of trigeminal neuralgia is made, a trial of 100 mg of carbamazepine given twice a day is initiated. The dose is increased by 100 mg every other day until pain control is achieved or toxicity develops. Patients are instructed to take this medication on a full stomach and to have monthly hematologic studies performed so that toxicity can be detected early. In this manner, good control of the pain can be achieved in the vast majority of patients.^7,⁸ Patients who are allergic or intolerant to carbamazepine can be treated with phenytoin, oxcarbazepine, clonazepam, and baclofen. Although gabapentin is frequently promoted for this indication, no studies have been published showing its efficacy, and it has rarely proved useful in our clinical experience. Surgery is reserved for patients whose pain is refractory to medical treatment or who develop medication-related side effects.

Indications

If the patient meets the typical clinical picture and fails medical therapy, surgical treatment options include microvascular decompression, selective percutaneous lesioning (with radiofrequency thermal coagulation or glycerol chemoneurolysis), or stereotactic radiosurgery of the trigeminal nerve. It is our practice to explain the relative benefits and risks of the surgical treatment options and assist the patient in choosing among these procedures (Table 125-1). We believe that microvascular decompression is the procedure of choice for the treatment of trigeminal neuralgia in otherwise healthy and relatively young (generally younger than 70 years) patients.⁹

Table 125-1 Relative Benefits of Percutaneous Trigeminal Neurolysis, Radiosurgical Trigeminal Neurolysis, and Microvascular Decompression of the Trigeminal Nerve for the Treatment of Trigeminal Neuralgia

Procedure	Benefits	Drawbacks
Percutaneous trigeminal neurolysis	• Safe, well tolerated despite age or infirmity • Brief or no hospitalization • Easily repeated if necessary

• Treats symptoms, not cause

• Destructive; permanently alters facial sensation

• Risk of corneal anesthesia

• Dysesthetic sequelae can be severe

• Increased recurrence with time

Stereotactic radiosurgery

• Safe, well tolerated despite age or infirmity

• No hospitalization

• Treats symptoms, not cause

• Delayed therapeutic response

• Risk of corneal anesthesia

• Increased recurrence with time

• Limited therapeutic data

Microvascular decompression

• Nondestructive; spares nerve

• No numbness

• No dysesthesia

• No corneal anesthesia

• Treats apparent cause; may be curative

• General anesthesia required

• Craniectomy required

• Increased risk of serious, lethal complications

• Potential increased risks with advancing age or comorbidities

Preoperative Considerations

We obtain high-resolution magnetic resonance images of the posterior fossa before treatment to reveal any unrecognized neoplasms, infectious causes, inflammatory processes, demyelinating plaques, or vascular malformations. We rarely obtain cerebral angiograms, because the information obtained by a routine angiogram is not helpful enough to warrant the small but inherent risk associated with arteriography.

Operative Positioning

While we frequently use the sitting position for microvascular decompression procedures (as described in detail later), others often use a lateral or park bench position (Fig. 125-1).¹⁰ Briefly, for lateral or park bench positioning, the patient is induced, intubated, and placed under general anesthesia, and the head is secured in three-point pin fixation. The patient is then placed in the lateral decubitus position with the unaffected side down. A gel roll is placed in the axilla of the dependent side, all pressure points are padded, and the nondependent arm is supported by a padded armrest. The neck is then secured in a slightly flexed position, and the head is turned 10 to 20 degrees from the affected side to provide optimal exposure and a clear line of sight to the pathology during surgery. The vertex of the head is then positioned based on the site of vascular compression.¹⁰ For decompression of the fifth cranial nerve, the vertex of the head is maintained parallel to the floor. For decompression of the lower cranial nerves, the vertex of the head is turned 10 to 20 degrees toward the floor. The patient is secured to the operative table with 3-inch tape and/or Velcro straps, and the shoulder of the affected side can be gently taped caudally to provide additional working space. The remaining portions of the operation are similar to those described later, including incision, dural opening, intradural dissection/decompression, and closure.

FIGURE 125-1 View of patient positioning laterally on the operating table for left-sided trigeminal neuralgia. The inset demonstrates the incision position for left-sided microvascular decompression for trigeminal neuralgia.

(Adapted from McLaughlin MR, Jannetta PJ, Clyde BL, et al. Microvascular decompression of cranial nerves: lessons learned after 4400 operations. J Neurosurg 90:1-8, 1999.)

Anesthetic Considerations

During the operation in the sitting position, the patient should be chemically paralyzed and under controlled ventilation for two reasons: (1) this minimizes motion in the field, which is greatly magnified by the operating microscope, and (2) it prevents the patient from developing a gasp reflex should a small amount of air embolization occur. This reflex occurs with only a tiny entrainment of air and can rapidly result in a massive air embolism.

To detect air embolization, a Doppler precordial detector and an end-tidal carbon dioxide monitor are used because they can detect even minute amounts of air. This detection allows the anesthesiologist to raise the venous pressure and prevent further entrainment of air, avoiding massive air embolization. The effectiveness of these methods of intravenous air detection is such that we abandoned the use of central venous pressure catheters for this type of surgery. The central venous pressure catheter was originally inserted to allow the aspiration of air trapped in the right atrium. Small amounts of air, however, do not sequester in the atrium but pass through the heart into the pulmonary circulation. If detected at its earliest stages, raising the venous pressure to prevent further entrainment is sufficient.

When the patient is positioned in a sitting position, positive end-expiratory pressure, although not routinely employed, can be used to raise the venous pressure almost to the level of the head, creating the physiologic equivalent of having the patient supine. This approach should prevent any air entrainment and add a further margin of safety. The level of venous pressure can be adjusted to avoid significantly raising intracranial venous pressure, with its associated increase in intracranial pressure.

If the patient is placed in a lateral decubitus position, the risk of air embolism is less but not fully obviated, so similar monitoring is probably warranted. As with all neurosurgical anesthesia, it is desirable for the patient to have as smooth an induction as possible; it is even more important that anesthesia be terminated so that the patient is allowed to awaken gradually and does not buck on the endotracheal tube.

Positioning Preferences

Although the patient can be positioned in either a sitting position or the lateral decubitus position for microvascular decompressive procedures, we use the sitting position for such procedures because it can be easily achieved in patients of any physiognomy and produces a relaxed operative field with the intracranial structures in their normal anatomic relationships. Moreover, it offers advantages for the anesthesiologist in that it avoids chest compression, allows good ventilation, and provides good access to the patient. In this position, cerebrospinal fluid and irrigant do not pool but flow out of the wound. Bleeding can be irrigated from the wound minimizing the use of suction during the operation. This aspect is critical because of the potential risk of inadvertent injury to cranial nerves, other neural structures, or small blood vessels with suctioning.

We achieve the sitting position using a pin-fixation head holder for rigid fixation of the skull. The patient’s head is rotated 15 to 30 degrees to the ipsilateral side. The head is flexed gently to provide ample room for placing one or two fingers beneath the patient’s chin (Fig. 125-2). The elevation (or angle) of the back of the table is such that the patient is actually in a semisitting or slouched position, although higher elevation of the backrest may be necessary in the older patient with a less flexible neck.

FIGURE 125-2 Lateral view of patient positioning on the operating table.

(Adapted from Apfelbaum RI: Microvascular decompression of the trigeminal nerve. In Wilson CB [ed]. Neurosurgical Procedures: Personal Approaches to Classic Operations. Baltimore: Williams & Wilkins, 1992, pp 137-155.)

Operative Procedure

After the patient is satisfactorily anesthetized and positioned, the operative table is angled an additional 15 to 20 degrees to the ipsilateral side so that the surgeon approaches the patient at an angle of approximately 45 degrees from the midline. The hair is shaved only from the posterior quadrant of the head on the affected side, and sterile preparation and draping are performed. The incision is vertical and located 3 to 5 mm medial to the mastoid notch (Fig. 125-3). We use a linear incision approximately 8 cm long that is centered two thirds above and one third below the mastoid notch.

FIGURE 125-3 Placement of the incision for microvascular decompression of the fifth, seventh, and ninth cranial nerves. For decompression of the fifth cranial nerve, the incision (dotted line) is positioned as shown so that two thirds of the length is above the level of the mastoid notch. For decompression of cranial nerves VII and IX, the incision is placed slightly lower so that half of the length is above the level of the mastoid notch.

To reduce scalp bleeding, the scalp is routinely infiltrated with 0.5% lidocaine with epinephrine (1:200,000). After incision of the skin, hemostasis is achieved with Dandy clamps secured to the drapes with elastic bands. Electrocautery is used to divide the occipital muscle mass down to the occipital bone. The anastomosis between the occipital and the posterior auricular branches of the external carotid artery is often encountered with this exposure and divided. The muscle mass is then stripped from the posterior surface of the occipital bone. Care must be taken not to strip too far laterally, because this determines the exposure once the self-retaining retractor is placed. Bridging emissary veins are coagulated, and their openings in the bone are sealed with bone wax. A Valsalva maneuver at this point can confirm hemostasis.

A modified Weitlaner retractor (Apfelbaum retractor) that serves as the base for a self-retaining brain retractor (Codman & Shurtleff, Raynham, MA) is then placed within the wound (Fig. 125-4). The retractor is secured by placing a gauze sponge through the loops of the handle and clipping it to the drape above the patient’s head to provide good three-point fixation and achieve adequate stability for the retractor arm. Several bur holes are made and enlarged into a circular craniectomy (2.5-3 cm) (Fig. 125-5). The craniectomy should extend superiorly to the transverse sinus and laterally to expose the sigmoid sinus. The lateral extension of the bony opening often carries the craniectomy over mastoid air cells, which are thoroughly waxed at the completion of the craniectomy. Care is taken to displace the dura as the rongeuring proceeds to avoid entering the dura or venous sinuses. Bridging veins may also be encountered and have to be coagulated.

FIGURE 125-4 Position of the self-retaining retractor. Fixation of the retractor base to the drapes via an encircling gauze sponge provides firm, three-point fixation. The notched blade of the retractor protects cranial nerves VII and VIII.

FIGURE 125-5 A, Placement of initial bur holes and rongeuring of the base to create a circular craniectomy. B, Dural incision for a left-sided approach showing the L-shaped dural incision. The exposure afforded by the dural opening can be expanded in a T shape at the corner of the opening. The transverse and sigmoid sinuses are exposed by the craniectomy.

The dura then is opened in an inverted L-shaped manner 3 to 5 mm parallel to the sigmoid and transverse sinuses (Fig. 125-5). The dura can be further opened in a T shape at the superior corner if increased exposure is needed. The dura is then secured with tenting sutures superiorly and laterally to retract the sinuses slightly and complete the exposure.

Occasionally, adhesions or bridging vessels are encountered along the superior posterior margin of the cerebellum along the transverse sinus. These must be divided sharply to free the cerebellum. A flexible retractor arm is then placed on the retractor base. This retractor arm should be positioned so that a gentle arch is formed and sharp kinks and bends are avoided (Fig. 125-4). Its tension is adjusted so that it remains in any position in which it is placed but can be readily moved without undue force. A specially shaped retractor blade that has an elongated finger at its superior margin (Codman & Shurtleff) is used (Fig. 125-4). The purpose of the finger is to allow deeper retraction in the vicinity of the trigeminal nerve while avoiding deep retraction and potential injury to the seventh and eighth cranial nerves. A narrow retractor blade could achieve the same depth of exposure superiorly but might penetrate into the cerebellum. Once the retractor blade is in place, the superior lateral margin of the cerebellum is retracted in a medial-to-inferomedial direction, and the operating microscope is brought into use.

Before the microscope is placed, an armrest is brought into position. This is a Mayo stand, modified as suggested by Malis, with the top removed and replaced with a 6 × 18–inch piece of metal. This metal sheet is padded, covered with a plastic sheet, and then covered with sterile drapes. Its height can be independently adjusted from the operating table to provide adequate support for the surgeon’s arms.

Our standard procedure is to set up the operating microscope with the left-sided stereoscopic binocular observer tube for the surgical assistant. The scrub nurse is positioned on the surgeon’s right. This positioning is used for all cases regardless of whether the exposure is on the right or left side of the patient. It allows for a standardized operative setup with excellent access between the surgeon and the nurse.

With the microscope in place, the cerebellum is gently retracted further medially. The approach should be angled as sharply as necessary to visualize the cerebellar surface. Adhesions and small bridging vessels are lysed as the cerebellum is retracted. Once the lateral extent of the cerebellum is visualized, the angle of approach is changed to follow the petrous bone anteriorly. McLaughlin et al.¹⁰ refer to this change as “turning the corner” and emphasize the necessity of doing this under direct vision. At this point, the petrosal vein can be identified. This vein frequently is encountered two thirds of the way from the dura to the trigeminal nerve, but great variability exists, and it may be absent or positioned close to the nerve. It often consists of two channels that form a Y-shaped bifurcation just before entering the dura. To permit adequate retraction and deeper dissection, the petrosal vein may need to be coagulated and divided sharply.

Once the petrosal vein has been identified and addressed, the retractor blade can be advanced. This is done while keeping the retractor blade close to the tentorium, which should be relatively horizontal in the operative field (variations in this view can be corrected with the use of the Trendelenburg control of the operating table). By advancing over the superior surface of the cerebellum, the surgeon can avoid the seventh and eighth nerves. It is not necessary to open the arachnoid around them. This approach exposes the arachnoid overlying the trigeminal nerve. If any bleeding is encountered while retracting the cerebellum, a dorsal bridging vein from the cerebellum to the tentorium may have been torn. Removing the retractor and depressing the cerebellum slightly allows inspection of this area and control of this problem before proceeding. This tear is an infrequent occurrence, but the surgeon must be aware of its possibility. When possible, we first open the arachnoid about the petrosal vein to inspect the trigeminal nerve. This approach allows detection of compressing veins whose removal by petrosal vein resection would lead to the erroneous assumption of a negative exploration. Such inspection also provides the possibility of preserving a portion of the petrosal system while still allowing adequate access to the trigeminal nerve. Preservation of part or all of the petrosal venous system is desirable in terms of reducing risk of venous cerebellar infarction.

The arachnoid overlying the fifth nerve is then opened sharply and widely to expose this area. This opening may be difficult because of the depth at which the work is being done and because of the narrow exposure. In some cases, the arachnoid is quite thin and translucent and can be easily punctured and teased free; in other cases, it is thick and opaque and must be sharply dissected. A great deal of care must be taken to avoid tugging on underlying structures. The trigeminal nerve is usually easily identified at this point, and the neurovascular relationships at the brain stem then can be identified.

The site of pathology is typically at the brain stem, and vessels impinging distally on the nerve are not usually the cause of the problem. The typical situation involves the superior cerebellar artery looping down in front of the nerve and emerging from the nerve dorsally at the point where the nerve exits the brain stem (Fig. 125-6). Opening the arachnoid widely allows full inspection of the entire circumference of the nerve at the brain stem. The first vessel seen may not be the only vascular channel involved in the neurovascular compression, because in a significant number of cases, multiple vessels have been encountered. It is also necessary to open the arachnoid anterior to the nerve to allow proper placement of the prosthesis.