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History of Thymectomy and Myasthenia Gravis

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  In 1939 Blalock and associates first reported remission of myasthenia gravis (MG) in a young woman after resection of the thymus. By 1944 more than 20 cases of MG treated by thymectomy had been reported. The relationship between MG and the thymus gland has been well established, although the mechanism for improvement of symptoms after thymectomy is less clear. Evidence supports surgery in severe disease refractory to medical treatment. Some debate remains over the indications for thymectomy as a treatment of patients with milder symptoms and no evidence of a thymoma on chest imaging.

  
  
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  Following thymectomy, up to 40% of patients with MG can be expected to have a complete response as measured by no requirement for medication. Continued resolution of symptoms can occur for up to 18 months after thymectomy. An additional 30% to 40% of patients will achieve a partial response, usually manifested by a significant reduction in the amount and type of medications required for controlling MG symptoms. In general, patients with nonthymomatous MG, of younger age, and with shorter disease duration have a better remission rate. No laboratory test or other diagnostic evaluation can predict a patient’s response to thymectomy.

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Patients with Thymoma

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  Thymoma is the most common neoplasm of the adult anterior mediastinum, accounting for 20% to 25% of all mediastinal tumors. Thirty percent of patients who have a thymoma experience symptoms suggestive of MG, and 15% of patients with MG will have a thymoma. An additional 5% of patients with thymoma will have other systemic syndromes, including red cell aplasia, dermatomyositis, systemic lupus erythematosus, Cushing syndrome, and the syndrome of inappropriate antidiuretic hormone secretion.

  
  
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  No clear histologic distinction between benign and malignant thymoma exists. The propensity of a thymoma to be malignant is determined by its invasiveness. Although considered to have an indolent growth pattern, thymoma has the ability for both local invasion and intrathoracic recurrence. Malignant thymomas can invade the vasculature, lymphatics, and adjacent structures within the mediastinum. Seemingly benign thymomas can also metastasize to the lungs and pleura. As a result, the evaluation and treatment of these tumors, particularly in locally advanced disease, require a multidisciplinary approach to improve long-term patient outcomes.

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A Multidisciplinary Approach

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  Treatment and care of the MG patient undergoing thymectomy require multiple specialties. Neurology, anesthesia, critical care, and the entire surgical team should be involved both before and after surgery.

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Preoperative Workup

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  Computed tomography (CT) scanning of the chest to assess for the presence of a thymoma is helpful. In cases with thymoma, assessment of invasion is helpful for surgical planning. The diagnosis of thymoma is usually made based on radiologic findings. Chest CT scan is the imaging procedure of choice with intravenous contrast dye to show the relationship between thymoma and surrounding vascular structures. Magnetic resonance imaging can also be useful for evaluating the invasion of mediastinal structures.

  
  
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  A measurement of pulmonary function should also be performed to ascertain whether any involvement of respiratory muscles exists. If severe disability of forced vital capacity is present, the possibility of postoperative mechanical ventilation should be discussed with the patient, anesthesia team, and critical care physicians.

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Neurology

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  Working closely with the neurologist is critical for medical optimization of the patient’s condition before and after surgery. If the patient is unable to be medically optimized or respiratory dysfunction is present, preoperative plasmapheresis should be performed. This procedure significantly reduces the level of circulating anti–acetylcholine receptor antibodies and should be done the week before the planned surgery. Significant improvement in respiratory function and muscle strength has been shown postoperatively when plasmapheresis was performed preoperatively on MG patients.

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Anesthesia

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  Anesthesia administration by someone experienced in the care of patients with MG is necessary. Patients should take their MG medication the morning of the operative procedure. The use of succinylcholine or other nondepolarizing muscle relaxants should be avoided. A single-lumen tube is usually adequate, although a double-lumen tube may be helpful if a large thymoma is present or extends toward one side. A double-lumen tube is also used if pleural or lung metastases from thymoma are present and need to be simultaneously resected. Deep anesthesia is maintained by an inhalation agent and short-acting narcotics. Monitoring of neuromuscular transmission can be performed by peripheral nerve stimulation to aid in the titration of muscle relaxants and to ensure complete reversal of neuromuscular block following the surgical procedure.

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Partial Sternal-Splitting Incision/Sternotomy