The most common indication for thymectomy is myasthenia gravis (MG). MG is an autoimmune disorder caused by anti-acetylcholine receptor antibodies that reduce the number of available acetylcholine receptors at the neuromuscular end plate. Clinically, this results in muscle weakness and muscle fatigability. The exact role of the thymus in the pathogenesis of this disease remains poorly understood. Nevertheless, observations made by Blalock¹ over 50 years ago suggest that surgical thymectomy can induce remission. Unfortunately, precise preoperative selection criteria for patients likely to achieve remission have not been defined. Likewise, the extent of operation required, whether radical or simple thymectomy, has not been established.

This chapter outlines the transcervical approach to thymectomy. This approach is used most frequently for patients with autoimmune, nonthymomatous MG. Recently, however, some groups have expanded the use of this technique to other anterior mediastinal abnormalities, such as intrathymic parathyroid adenomas.² This chapter emphasizes patient selection, operative technique, and the advantages of the transcervical technique over more radical transsternal thymic resection. Because the procedure is performed most commonly on patients with MG, special attention is directed toward the care and management of this challenging disease.

GENERAL PRINCIPLES

Patient Selection

The diagnosis of MG is suspected on clinical grounds. The muscle groups involved and their fatigability with repetitive exercise vary greatly from one patient to another. The diagnosis is confirmed by a neurologist on the basis of a positive Tensilon test, the presence of acetylcholine receptor antibodies, or characteristic electromyographic responses. It is important to classify patients on initial surgical evaluation to permit meaningful standardized measures of postoperative outcomes. In this regard, the Myasthenia Gravis Foundation of America Clinical Classification and Score for disease severity has been developed.³ Patients should be classified preoperatively according to the expanded quantitative MG scheme with the assistance of a neurologist.

Those considered for thymectomy must be medically optimized; surgical thymectomy is not a treatment for acute exacerbations of MG. The goal of thymectomy in MG, by any approach, is the long-term induction of remission. In this regard, close collaboration with a neurologist is required. Preoperative optimization can include the use of cholinesterase inhibitors, immunosuppressives, plasmapheresis, and IV immunoglobulin. Only patients who are medically stable can be considered for thymectomy. The practice guideline from the American Academy of Neurology states: "For patients with non-thymomatous autoimmune MG, thymectomy is recommended as an option to increase the probability of remission or improvement."⁴ Thus, once medically optimized, all patients with MG, regardless of the severity of disease, should be considered for thymectomy by a qualified thoracic surgeon.

Preoperative Assessment

All patients have a CT scan of the chest to evaluate for thymoma. In our practice, patients with nonthymomatous autoimmune MG are evaluated for transcervical thymectomy. Those with contraindications to this approach, including the presence of thymoma, are considered for transsternal resection (see Chap. 136). We obtain the preoperative forced vital capacity in all patients. This simple test can be invaluable postoperatively in determining respiratory muscle strength in patients who fail the initial extubation attempt in the OR.

Patients undergoing transcervical thymectomy must be able to flex and extend their neck throughout a wide range. Patients with limited cervical range of motion owing either to osteoarthritis or to surgical fusion are not acceptable candidates for the transcervical approach. A history of thyroid surgery or previous tracheostomy is not a contraindication, but it does make dissection significantly more difficult, and the conversion rate to a transsternal approach is higher. Prior cardiac surgery through a median sternotomy is also a contraindication.

OPERATIVE TECHNIQUE

The anesthetic plan is reviewed with the anesthesiologists. A list of drugs that exacerbate MG is generated by the pharmacist and reviewed with the anesthesiologist and all providers who will be caring for the MG patient. Depolarizing muscle relaxants are avoided. Once general anesthesia is induced, the patient is intubated with a single-lumen endotracheal tube. The operating table is turned by 90 degrees. Patient positioning is vital to successful performance of the transcervical procedure. The head must be positioned at the top edge of the bed. The patient remains in a supine position, and both arms are tucked in at the sides. An inflatable bag is place under the shoulders. The bag is inflated to provide the maximal neck extension, without allowing the head to hang free. The anterior neck, chest, and upper abdomen are prepped and draped in a sterile fashion. A 5-cm curvilinear incision is made two fingerbreadths above the sternal notch (Fig. 134-1). Electrocautery is used to raise subplatysmal flaps to the sternal notch inferiorly and to the level of the thyroid cartilage superiorly. The cleidocleido ligament is divided at the sternal notch with cautery. Self-retaining Gelpe retractors are place on each side of the incision to provide exposure.

Attention now turns to identifying the cervical poles of the thymus gland. The avascular midline plane is opened with a pair of Metzenbaum scissors. The left pole of the gland is identified first because it usually extends deeper into the neck than the right pole. The left sternothyroid strap muscle is identified and elevated with a forceps. A Kittner dissector is used to gently "sweep out" the left cervical pole of the thymus from underneath the strap muscle. The gland is well encapsulated and differentiated from the surrounding fat by its salmon pink color. The entire dissection is performed in an extracapsular plane, and the entire left cervical pole is defined to its superior margin such that no thymus tissue is left in the neck. A 0 silk ligature is tied to the left cervical pole as a handle. The left pole is dissected down into the sternal notch. The upper body of the gland is identified and used to locate the right cervical pole. The right pole is dissected free with the Kittner dissector to its most cephalad extent and tagged with a 0 silk ligature (Fig. 134-2). The tagging ligatures are retracted anteriorly by the assistant to permit dissection of the thymic veins, which drain directly into the innominate vein (Fig. 134-3). This exposes the thymic venous tributaries that empty into the innominate vein. It is important to confirm that both thymic poles descend anteriorly to the innominate vein. In up to 5% of patients, one thymic pole, usually the left, passes posterior to the innominate vein. Figure 134-4 shows a patient who was been converted to a sternotomy because of difficult anatomy. In this patient, the right pole of the gland can be seen coursing under the innominate vein, whereas the left pole is in the expected anterior position. All small thymic venous branches are completely dissected and ligated with fine silk before they are divided. Hemoclips are avoided because they have a tendency to fall off during blunt dissection and can lead to troublesome bleeding. Next, the tagging ligatures on the thymic poles are retracted in a cephalad direction, and a dissecting finger is used to develop the plane between the sternum and the anterior surface of the gland. Once this plane has been developed, the patient is ready to be suspended for completion of the mediastinal dissection.

A Polytrac retractor is assembled and secured to the table. The Cooper thymectomy retractor blade (Pilling Company, Fort Washington, PA) is key to providing adequate exposure (Fig. 134-5). The Cooper thymectomy retractor blade is placed under the sternum, and the circulating nurse deflates the bag underneath the patient's shoulders until the patient is suspended by the retractor. Note, however, that the patient's head should not be hanging in midair. Parker retractors are used to provide lateral retraction and are secured to the table rails by Penrose drains. A headlight is mandatory for illumination.

We continue to develop the substernal, prethymic plan after the anatomy around the innominate vein has been confirmed. This is done bluntly with the aid of tonsil balls secured by a ring clamp. The gland is mobilized from the pleural reflections laterally and the pericardium posteriorly. This maneuver is performed bluntly by using two ring clamps with tonsil ball sponges attached to the ends. One is used for retraction, whereas the other is used for dissection. The pleural reflections are gently swept away from the lateral extent of the gland; care is taken to avoid violation of the capsule of the gland. The recognition, identification, and removal of aberrant thymic tissue also can be accomplished through the transcervical approach. It does not require conversion to a transmanubrial or transsternal approach. Once the entire gland has been mobilized, it is brought out through the cervical incision (Fig. 134-6) and oriented for the pathologist (Fig. 134-7). Hemostasis is confirmed visually, and the superior mediastinum is irrigated with saline. The anesthesiologist is asked to give and hold a large breath to permit inspection of the pleura. If the pleural membrane has been violated, it is opened widely, and the incision is closed over a red rubber catheter to evacuate the pleural space (Fig. 134-8). There is rarely a need for formal chest tube placement. The patient is allowed to emerge from anesthesia and extubated in the OR under close supervision. If there is any doubt as to the appropriateness of extubation, a forced vital capacity determination is done. Patients with a forced vital capacity of more than 1 L usually can be extubated without difficulty.

Currently, patients are admitted to the short-stay-procedure unit on the morning of the procedure. After the procedure, a chest radiograph is obtained. If it is normal, the patient is permitted to recover for 8 hours and then discharged. Patients remain on their preoperative medications without dosage alterations. A surgical follow-up is scheduled in 3 weeks for inspection of the surgical site, and patients then are referred back to their neurologist to consider drug weaning. It may be 2 years or more before the benefit of thymectomy is realized. It is important for patients to understand and accept this fact before undergoing this procedure.

EXPECTED OUTCOMES FOR TRANSCERVICAL THYMECTOMY

The ideal outcome for thymectomy in MG by any approach is complete remission. This goal is realized when there is complete resolution of symptoms, enabling discontinuation of all medication. However, there remains a cohort of patients that experiences substantial reduction in symptoms after thymectomy but never achieves complete remission. There is a considerable controversy concerning the extent of thymectomy that is required to induce remission. Masaoka and colleagues performed detailed anatomic studies of the distribution of thymic tissue within the anterior mediastinum and discovered a high prevalence of ectopic thymic tissue.⁵ Jaretzki and colleagues confirmed this observation by noting a prevalence of ectopic thymus, either microscopic or macroscopic, in the neck in 32% and in the mediastinum in 98% of specimens resected by their technique of transcervical-transsternal maximum thymectomy.⁶ Unfortunately, the role of ectopic thymic deposits in the persistence of symptoms after thymectomy for MG is unknown, although the topic has fueled heated debates. Opponents of the extended transcervical approach argue that residual extracapsular rests of thymus lead to a failure of remission and suggest that maximal thymectomy should be performed in all patients with MG. Justification for this view exists in sporadic cases in which transcervical thymectomy was performed, symptoms persisted, and on transsternal exploration, residual thymus was identified and removed.^7,8 However, if 98% of patients have ectopic mediastinal thymic deposits and these are missed by the transcervical approach, the procedure should yield terrible outcomes, which is not the case. In fact, complete remission rates among recipients of the transcervical approach, transsternal approach, transsternal-transcervical approach, and thoracoscopic approach appear to be equivalent (Table 134-1).

SUMMARY

The operative approach and extent of dissection for thymectomy in MG have remained controversial. In our opinion, the outpatient transcervical thymectomy procedure described in this chapter is less morbid and less costly than transsternal and extended maximal thymectomy but offers similar outcomes. At our institution, the transcervical thymectomy procedure is the approach of choice for patients with nonthymomatous MG.

EDITORIAL COMMENT

Transcervical thymectomy is a useful surgical option for patients with thymic hyperplasia; typically, patients with myasthenia gravis. Transcervical thymectomy, however, is not an option for patients with large glands and thymomas. The transcervical dissection is limited by the brachocephalic vein posteriorly and the sternum anteriorly (the thymus gland is the only normal anatomic structure between these structures). Although video optics have improved visualization and broadened surgical indications, the risk of inadequate resection or local seeding precludes the use of transcervical thymectomy in the vast majority of patients with thymomas.

–SJM

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