The Surgical Technique of Vagus Nerve Stimulator Implantation

Abstract

Chronic electrical stimulation of the vagus nerve is now an accepted therapeutic alternative for a number of medical conditions. The effectiveness of this therapy is lost when the device is not implanted correctly. This report is intended to review the surgical anatomy of the vagus nerve in the neck, the surgical technique for implantation of the stimulator electrode array and pulse generator, and the possible complications or pitfalls that a surgeon might encounter during the procedure. Several associated procedures are also discussed such as electrode revision or removal, pulse generator replacement, and alternative sites for electrode or pulse generator placement.

Keywords

Anatomy, Surgical technique, Vagus nerve stimulation, VNS

Outline
Introduction 551
Surgical Anatomy 551
Operative Technique 554
Replacement of the Electrode Lead 558
Replacement of the Pulse Generator 558
Complications of Surgery 558
Conclusion 559
References 559

Introduction

Vagus nerve stimulation (VNS) is a treatment option for a number of disorders. It is most commonly used to treat medically intractable seizures but has also been approved by the United States Food and Drug Administration (FDA) to treat severe depression ( ). It has been suggested (but not approved by the FDA) that VNS may also be used to treat tremor, obesity, and some inflammatory or autoimmune disorders ( ). The potential effectiveness of any of these treatments cannot be fully achieved if the device is not implanted correctly. There have been a few previous reports that describe the technique of implantation ( ). Some of these articles were written soon after the device was approved for general use. This chapter is intended to describe this surgical technique in detail and includes some ideas learned from many years of experience with this technique.

It is important to note the statement from the manufacturer (LivaNova PLC, London, UK) that accompanies the device ( http://us.livanova.cyberonics.com/en/vns-therapy-for-epilepsy/healthcare-professionals/vns-therapy/manuals-page/ ) states that the vagal nerve stimulation therapy system is indicated for use as an adjunctive therapy in reducing the frequency of seizures in adults and adolescents over 12 years of age with partial onset seizures that are refractory to antiepileptic medications. Use of this device is also indicated for the adjunctive long-term treatment of chronic or recurrent depression for patients 18 years of age or older who are experiencing a major depressive episode and have not had an adequate response to four or more adequate antidepressant treatments. It cannot be used in patients after a bilateral or left cervical vagotomy.

Surgical Anatomy

The surgeon is mostly concerned about the left vagus nerve because this is the nerve that is most commonly implanted. Stimulation of either vagus nerve has antiseizure effects, but stimulation of the right vagus nerve has a greater likelihood of adverse cardiac effects. Therefore, surgery for implantation of stimulating electrodes on the right is only very rarely performed and is done only when the left vagus nerve is no longer available ( ).

A basic understanding of the anatomy of the neck is important for successful implantation of a vagal nerve stimulator. One should understand not only the routinely encountered structures but also the less commonly encountered structures and anatomic variants of the major structures. It is helpful to discuss these structures in the order that they are encountered during a typical surgical procedure for implantation of vagal nerve stimulator electrodes, but, first, one should begin with a description of the fascial planes of the neck (see Fig. 42.1 ).

An understanding of the fascial planes in the neck is very important to a successful surgical exposure. The superficial fascia is the subcutaneous tissue containing the platysma and cutaneous nerves of the cervical plexus. This most superficial layer is also called the “investing layer” because it surrounds the sternocleidomastoid and trapezius muscles. This fascia attaches in the midline posterior to the spinous process. The next layer is the intermediate layer, which envelops the sternohyoid and sternothyroid muscles medially and the anterior and posterior bellies of the omohyoid muscle. Also included in this layer is a visceral layer that surrounds the thyroid gland, esophagus, and trachea.

The deepest layer of fascia is the prevertebral fascia. The carotid sheath is a condensation of deep fascia in which are embedded the common and internal carotid arteries, internal jugular vein, and vagus nerve. The carotid sheath blends in front with the pretracheal and investing layers of deep fascia and, behind, with the prevertebral layer of the fascia.

Most of the dissection for a vagal nerve stimulator electrode placement is in the anterior triangle of the neck. The anterior triangle is bounded anteriorly by the midline of the neck, posteriorly by the anterior border of the sternocleidomastoid, and superiorly by the lower margin of the body of the mandible. When one dissects in this area, one first encounters the skin, the subcutaneous fat, the platysma, and the investing layer of fascia. The platysma muscle fibers usually run vertically in the neck and are incomplete toward the midline. Branches of the transverse cutaneous nerve are found in the investing layer of cervical fascia, superficial to the sternocleidomastoid muscle.

The carotid triangle is bounded superiorly by the posterior belly of the digastric, inferiorly by the superior belly of the omohyoid, and posteriorly by the anterior border of the sternocleidomastoid muscle. This triangle is important for surgical access to the carotid sheath. The carotid sheath lies just deep to the inferior corner of the carotid triangle.

Within the carotid sheath lies the common carotid artery, the internal jugular vein, and the vagus nerve. Also within the carotid sheath are the nerves of the ansa cervicalis and the sympathetic plexus.

The internal jugular vein is usually first encountered during a dissection into the carotid sheath at this level. Occasionally, one will find a branch from the jugular vein in this region. This is usually a branch that crosses medially over the carotid artery and is a superior thyroid vein or middle thyroid vein. The common facial vein usually branches from the jugular vein at a more cephalad level, near the thyroid cartilage. The internal jugular vein is bluish and has a much thinner wall than the carotid artery. It is the largest structure in the carotid sheath and enlarges when the patient is in the supine position for surgery.

The common carotid artery is found deep and medial to the jugular vein. The carotid artery bifurcates into the internal carotid artery and external carotid artery, usually at the level of the upper border of the thyroid cartilage. In approximately 70% of cases, the common carotid artery divides into the internal and external carotid arteries at the level of the fourth cervical vertebra (C4). In approximately 20% of cases, the division is at the C3 level, and in 10%, it is at the C5 level ( ). There are usually no branches from the carotid artery inferior to the bifurcation, but a descending branch of the external carotid, the superior thyroid artery is often seen in a dissection of the common carotid. The common carotid artery is white and pulsates easily. It is distinguished by vasa vasorum, which mark the surface of the artery. The artery was also covered with loose areolar tissue. Often, the vagus nerve is adherent to the loose areolar tissue around the artery.

The vagus nerve is the largest nerve in the carotid sheath and at the C5-6 level should be the only nerve without any branches. It is composed of both motor and sensory fibers. It originates in the medulla oblongata and leaves the skull through the middle of the jugular foramen in the company with the 9th and 11th cranial nerves. The vagus nerve possesses two sensory ganglia, a rounded superior ganglion that is situated on the nerve within the jugular foramen and a cylindrical inferior ganglion that lies on the nerve just below the foramen. Below the inferior ganglion, the cranial root of the accessory nerve joins the vagus nerve and is distributed mainly in its pharyngeal and recurrent laryngeal branches.

The vagus nerve passes vertically down the neck within the carotid sheath, lying at first between the internal jugular vein and the internal carotid artery and then between the vein and the common carotid artery. Its position is usually deep between the internal jugular vein and the carotid artery. The vagus nerve is separated from the sympathetic trunk by the prevertebral layer of the cervical fascia.

There are several branches of the vagus nerve that are of surgical importance in the dissection in the neck. The pharyngeal branch arises from the inferior ganglion of the vagus nerve and passes between the internal and external carotid arteries to reach the pharyngeal wall. The superior laryngeal nerve arises from the inferior ganglion and runs downward and medially behind the internal carotid artery. It divides into internal and external laryngeal nerves. There are also two or three cardiac branches arising from the vagus nerve. These branches usually arise from the nerve, superior to the thyroid cartilage. On the left side, the recurrent laryngeal nerve arises from the vagus nerve as the vagus nerve crosses the arch of the aorta in the thorax. This nerve hooks around and beneath the arch behind the ligamentum arteriosum and ascends into the neck in the groove between the trachea and the esophagus. A nonrecurrent laryngeal nerve occurs in approximately 1% of patients on the right but is extremely rare on the left ( ).

The ansa cervicalis is a plexus of nerves that arises from the cervical plexus, formed by the anterior rami of the first four cervical nerves. The ansa cervicalis also lies within the carotid sheath but usually is anterior to the carotid artery and jugular vein. It is important to distinguish this nerve from the vagus nerve so as to avoid placing the stimulating electrodes on the wrong nerve. The ansa cervicalis supplies the sternohyoid muscle, the sternothyroid muscle, the omohyoid muscle, and the sternocleidomastoid muscle.

Deep to the carotid sheath is the prevertebral layer of fascia. This fascia covers not only the vertebral body and the longus coli muscles on the anterior vertebral bodies but also the scalenus anterior muscle. Within the prevertebral fascia and deep to the carotid sheath is the phrenic nerve.

The cervical part of the sympathetic trunk extends upward to the base of the skull and below to the neck of the first rib, where it becomes continuous with the thoracic part of the sympathetic trunk. It lies directly behind the internal and common carotid arteries (i.e., medial to the vagus) and is embedded in deep fascia between the carotid sheath and the prevertebral layer of deep fascia. There is a superior cardiac branch of the sympathetic trunk, which descends in the neck behind the common carotid artery. It ends in the cardiac plexus in the thorax.

At the most inferior end of the anterior neck exposure and deep to the prevertebral fascia is the thoracic duct. In the lowest part of the neck, the thoracic duct passes upward along the left margin of the esophagus. As it reaches the level of the transverse process of the seventh cervical vertebra, it bends laterally behind the carotid sheath and in front of the vertebral vessels. On reaching the medial border of the scalenus anterior, it turns downward in front of the left phrenic nerve and the first part of the subclavian artery.

Operative Technique

The following is a description of the surgical technique of vagus nerve stimulator implantation used at our hospital. This technique has evolved over the past 20 years and about 500 implants and is still evolving as we learn better ways to do things. This description is not intended to be an authoritative and complete account, because acceptable variations occur with certain operative situations and surgeon preference.

One must be sure that all of the implantable devices and materials are available before beginning the procedure. The surgeon should interrogate the generator while it is still in the sterile package using the programming device to ensure the function of the device. As with all surgical implants, we usually do not open the sterile package until the implant is actually needed in surgery.

Infection is a very serious complication and in most instances requires removal of the device. For this reason, prophylactic antibiotics should be given to the patient before surgery. In our institution, we usually give the patient an infusion of gentamicin and vancomycin before surgery (except when the patient is allergic to these medications). We also give the patient 10 days of oral antibiotics after surgery.

The surgery is usually performed with the patient under general anesthesia. There have been reports of this procedure being done under local anesthesia, but this is not widely performed ( ). Patients who are taking antiepileptic medication may have increased metabolism of anesthetic agents. Anesthetic drugs that trigger seizures such as ketamine should be avoided. The anesthesiologist should be aware that there is a risk of bradycardia or asystole during intraoperative testing of the device and appropriate drugs to counteract this phenomenon should be readily available ( ). The patient should be placed on the operating table in the supine position. The anesthesiologist is usually positioned at the head of the table and the surgeon is on the left side.

We always use two incisions: one in the neck for implantation of the electrodes and one in the chest for implantation of the pulse generator. There are some surgeons who will implant both the electrodes and the pulse generator through a single incision low in the neck ( ). We have elected not to do this because we believe that subsequent replacement of a low-battery pulse generator would be more difficult, but we admittedly have no experience with this. When the incision for the generator is made along the anterior border of the axilla, then the left arm is extended on an arm board. We no longer do this because the left arm often gets in the way of the surgeon and there is no improvement in the cosmetic result. We now make the incision for the generator in one of Langer’s lines ( ) extending from the superior edge of the axilla and the arm can be tucked at the side. Great care must be taken to prevent any unusual pressure on the neurovascular structures in the arm including the ulnar nerve. A small roll of a towel is usually placed behind the neck to allow gentle extension of the neck. The patient’s head can usually be in the midline or turned slightly to the right (see Fig. 42.2 ).

In our institution, the skin is usually cleaned and prepared three times. We first clean all the skin surfaces with isopropyl alcohol. We then wash the skin with iodinated soap and then iodinated solution. This is dried, and then the skin is finally prepared with a solution of iodine povacrylex (0.7% available iodine) and isopropyl alcohol (DuraPrep, 3M Health Care, St. Paul, MN). This is allowed to air dry. The incisions are marked with a sterile pen. The midline is identified as well as the anterior border of the left sternocleidomastoid muscle. The cricoid cartilage is palpated, and a line is drawn following a skin crease, lateral to the cricoid cartilage and centered on the anterior border of the sternocleidomastoid muscle. This usually allows the incision to be centered at about the level of C5-6. The incision should be about 3–4 cm in length to allow for adequate vertical exposure of the vagus nerve. In the axilla, the incision line is usually marked to follow one of Langer’s lines extending from the superior edge of the axilla. The surgical area is draped off with towels and then covered with an iodinated plastic adhesive drape. The skin at the site of incisions and is then infiltrated with 1% lidocaine and 0.5% bupivacaine mixed together in a 50:50 solution.

The neck incision is then made through the skin and subcutaneous fat to the platysma. The platysma was separated from the subcutaneous fat with sharp dissection so as to expose 3–4 cm of the platysma from superior to inferior. A self-retaining Wietlander retractor usually helps to expose this area. An incision is then made in the platysma, following the underlying anterior border of the sternocleidomastoid muscle. A dissection is then made through the fascia to the plane between the sternocleidomastoid and the strap muscles (sternohyoid and sternothyroid muscles). This dissection should be made in the loose areolar tissue in the fascial plane between the sternocleidomastoid muscle and the strap muscles so as to minimize bleeding. Small branches of the external jugular vein can sometimes be identified in this region and usually can be ligated and cut without consequence. Small muscular branches of the nerves of the ansa cervicalis are also seen in this region. Every effort should be made to preserve as many of these branches as possible, but some of them may need to be cut to allow sufficient access to the deeper structures.

The omohyoid muscle is usually seen in the middle of the exposure. Its orientation is almost perpendicular to the sternocleidomastoid muscle, and a small triangle containing fatty tissue is found at the superior junction of the omohyoid muscle and the sternocleidomastoid muscle. The jugular vein is usually directly underneath this triangle. Great care is taken to separate the omohyoid muscle from the sternocleidomastoid, and then the omohyoid muscle is mobilized and retracted medially and inferiorly.

Except for a small Wietlander self-retaining retractor for the skin, we have not found any self-retaining retractors that are reliable and safe this for the deep neck dissection. We usually therefore place sutures in the fascia surrounding structures that need to be retracted. Therefore, to retract the omohyoid muscle inferiorly and medially, we place a suture in the fascia surrounding the muscle and then clamp the suture to the drapes with a hemostat. This technique also works for other structures including the sternocleidomastoid muscle, which is retracted laterally.

The jugular vein is then exposed by careful dissection into the fascial plane found deep to the omohyoid muscle. Dissection into this fascial plane allows entry into the carotid sheath. Inside the carotid sheath is loose areolar tissue and often several nerves are seen. If these nerves are superficial in the carotid sheath, they are most likely to be laryngeal branches of the vagus nerve or branches of the ansa cervicalis. These nerves are usually no larger than 1–1.5 mm in diameter and will have many branches. It is important not to confuse these nerves with the vagus nerve, which is much larger and deeper in the carotid sheath. There is sometimes seen a small branch of the jugular vein (most likely a medial thyroid branch) that crosses anteriorly and medially from the antereomedial surface of the jugular vein. This vein often must be ligated and cut to allow adequate exposure of the vagus nerve. The common facial vein is rarely seen entering the jugular vein because it is usually found more cephalad to this exposure at C5-6. If encountered, the facial vein can usually be mobilized and retracted cephalad.

The common carotid artery is medial and deep to the jugular vein. Further dissection of the carotid sheath medial to the jugular vein is made with blunt tipped dissecting scissors until the surface of the common carotid artery is seen. If one is at the proper level of dissection (about C5-6), then one should not see any branches from the carotid artery. If one sees branches from the carotid artery, then the dissection is too high in the neck at the level of the external carotid artery and one therefore needs to dissect farther caudal to be at the level of the common carotid artery. The jugular vein can usually be retracted laterally by carefully placing a suture in the loose adventitia surrounding the jugular vein and clamping the suture to the drapes.

The vagus nerve is found deep within the carotid sheath, between and deep to the jugular vein and common carotid artery (see Fig. 42.3 ). The vagus nerve usually cannot be found without lateral retraction of the jugular vein, but retraction of the common carotid artery is rarely necessary. The vagus nerve lies within the fascia of the carotid sheath, deep and lateral to the common carotid artery. It is adherent to the fascia and mobilization of the nerve requires careful sharp dissection. At this point, use of DeBakey forceps is very important to minimize damage to the nerve and surrounding structures. DeBakey forceps can usually hold the adventitia of the nerve, and the nerve can be elevated from the deep fascia of the carotid sheath to allow dissection around the nerve. Great care must be taken when one is dissecting the vagus nerve because the nerve can be easily damaged by excessive retraction or manipulation. The use of vessel loops around the nerve can sometimes lead to inadvertent excessive traction on the nerve and we therefore avoid their use. One must also be careful to preserve the vasa nervosum in the adventitia around the nerve so as to preserve its function.