In 1991, Kirsh and colleagues showed in a randomized, prospective, double blinded clinical trial that MG patients who receive a perioperative epidural have less postoperative pain and improved pulmonary function.11 General anesthesia can be performed safely with selective use of agents that do not potentiate the neuromuscular defect. While rarely considered, ether should not be used because it will increase the neuromuscular block. Chemical paralysis is not required for a sternotomy. Neuromuscular blocking agents, especially competitive (nondepolarizing) agents, should be avoided because of the long-lasting adverse effects on myasthenic patients. Myasthenic patients are very sensitive to the administration of nondepolarizing agents, which could result in prolonged postoperative respiratory failure. If needed for intubation, a small dose of a noncompetitive depolarizing agent (e.g., succinylcholine) may be used.
The patient is positioned on the operating table in the supine position. A transverse shoulder roll is placed behind the patient, and the neck is extended with the occiput of the head resting on a “donut” pillow. After induction of anesthesia, a urinary catheter and arterial line should be placed. The operation is carried out via a median sternotomy (Fig. 136-3). The incision can be a midline incision from the sternal notch to below the xiphoid process. Since many of these patients are young women, a more cosmetic incision might be preferable. The more cosmetic Y-shaped skin incision follows the upper contours of the breasts and converges on a point in the midline approximately 10 cm below the sternal notch and continues down the midline to the tip of the xiphoid (Fig. 136-3). A cephalad skin/subcutaneous flap is developed carefully so that cosmesis of the upper chest area is maintained. The superior limit of the subcutaneous dissection is the sternal notch. Electrocautery is used to divide the fascia overlying the sternum. The cleidocleido ligament, which is attached to the posterior surface of manubrium at the sternal notch, is divided vertically, and the sternum is separated from the upper mediastinal structures by blunt finger dissection. Inferiorly, the avascular anterior mediastinal space is entered with finger dissection just below the xiphoid and developed as far cephalad as possible. The sternum is divided longitudinally with a sternal saw.
Y-shaped skin incision with a cephalad skin/subcutaneous flap to permit access to the entire sternum for midline sternotomy. The Y-shaped incision provides superior cosmesis of the upper chest area, particularly in females.
In adults, obese individuals, and patients on long-term steroids, it is often not possible to distinguish the thymus from the mediastinal fat until the specimen is removed and examined microscopically. Gross and microscopic thymus is found outside the confines of the classic cervical mediastinal lobes in the neck in approximately 30% of specimens and in the mediastinum in approximately 98% of studies.1,3 Therefore, wide exposure and meticulous dissection are needed to remove all the potential thymic tissue completely and safely. An en bloc dissection from diaphragm to thyroid gland and from phrenic nerve to phrenic nerve is undertaken by means of a combination of cautery and sharp dissection of the mediastinal pleura and pericardium. All thymus, suspected thymus, and mediastinal fat, including both mediastinal pleural sheets, are removed.
The mediastinal dissection begins at the diaphragm. The mediastinal fat, including the anterior pericardiophrenic fat pad, is elevated bilaterally and continued superiorly with the lower lobes of the thymus. The mediastinal pleura is incised anteriorly, and once the lower lobes are mobilized bilaterally, the phrenic nerves should be clearly identified. The lymph nodes and fat that course along the nerves are separated from the phrenic nerves on either side anteriorly as part of the en bloc specimen. The thymus is elevated off the pericardium using upward traction and electrocautery (Fig. 136-4). The main arterial supply to the thymus is derived from the internal thoracic arteries, which enter the gland laterally at the level where the isthmus joins the two lobes. Once identified, the arteries and accompanying veins should be isolated and divided in continuity between ligatures. These lateral vessels tend to tether the gland in position, and their division permits the en bloc specimen to be rotated upward to expose the undersurface of the gland (Fig. 136-5). This method is the safest approach to the brachiocephalic vein and the venous drainage of the thymus. The brachiocephalic vein should be exposed and the thymic veins isolated and divided in continuity between ligatures. The thymus then can be elevated from the brachiocephalic vein, and the dissection is continued laterally anterior to the phrenic nerves until the entire specimen is tethered only by the cephalic horns of the thymus in the vicinity of the thyroid and inferior parathyroid glands. An attempt should be made to identify and spare the inferior parathyroids and their blood supply, but this step is not always possible. The cephalad extent of the thymus should be isolated carefully, and the branches of the inferior thyroid vessels entering the two horns should be clamped at the superior aspect of the thymic lobes, divided, and ligated.
After the mediastinal pleura is opened, the thymus is elevated, along with the adherent mediastinal pleura, from the pericardium and separated from the phrenic nerves bilaterally.
Branches of the internal thoracic arteries are divided to permit the en bloc specimen to be rotated upward, exposing the undersurface of the gland and the draining veins. The exposed brachiocephalic and thymic veins are isolated and divided between ligatures or clips (inset).
It is important to include in the en bloc dissection all the fatty thymic tissues that lie in the sulcus between the superior vena cava and the aorta, as well as the tissues in the region of the aortopulmonary window deep under the brachiocephalic vein. During this part of the dissection, identification and avoidance of the left phrenic and vagus (recurrent laryngeal) nerves, which are especially at risk here, are essential. Injury to either of these nerves would be catastrophic, particularly in a patient with MG. The wide mediastinal exposure obtained by opening both pleural spaces not only ensures complete thymus removal but also helps to safeguard the phrenic and vagus nerves.
Gross evidence of invasiveness is the most important prognostic sign of malignancy. Malignant thymic tumors are notorious for local extension and lack of distant metastases, as well as having a deceptively benign appearance on microscopic examination. When a thymoma is present, the same extensive removal of thymic tissue is performed en bloc with the thymoma. The initial exploration includes palpation and visualization of each hemidiaphragm for possible tumor implants. If the pericardium, lungs, brachiocephalic vein, and sternal periosteum are adherent to the tumor, they are removed en bloc because tumor invasion can be present. Unless a phrenic nerve is nonfunctional from tumor invasion, it should be preserved, if at all possible. Postoperative radiation therapy can be used as adjuvant therapy to control residual unresected tumor.
After the thymectomy and anterior mediastinal dissections are complete, two angled chest tubes are placed through separate incisions in the superior epigastrium and positioned along the diaphragms in both pleural cavities. The sternum is reapproximated using no. 5 stainless steel wire, and the fascia and skin incisions are closed.