11

Thymectomy

Larry R. Kaiser

HISTORY

Over the past few decades, complete removal of the thymus gland has been shown to improve the clinical course of patients with myasthenia gravis (MG), though many neurologists continue to be hesitant about referring patients for thymectomy. However, the precise relationship between the thymus gland and the onset of MG has not been completely elucidated. Alfred Blalock performed a thymectomy via median sternotomy in 1936 for a woman with thymoma and MG and noted an improvement in her myasthenic symptoms. He subsequently reported on a series of patients without thymoma who underwent thymectomy, noting similar improvement in the clinical course of the disease. In this report of 20 thymectomies, he observed improvement in 13 of 17 survivors. ¹ Recently a prospective randomized trial to assess the role of surgery on the clinical course of MG has been reported that demonstrated superiority of thymectomy over medical management with respect to clinical outcome over a 3 year period. ² This study confirmed what already has been known based on a number of carefully controlled cohort studies comparing thymectomy with standard medical management. Essentially, all of these studies have shown a significantly greater incidence of remission in the operated group versus in those treated with medication alone. ³ , ⁴ , ⁵

PRINCIPLES AND JUSTIFICATION

The presence of MG constitutes the most common indication for the performance of elective thymectomy. The other main indication is the presence of a mass within the thymus gland. Approximately 15% of patients with MG have thymoma, while approximately 35% of patients with thymoma have MG. Patients presenting with a thymoma should be thoroughly evaluated for symptoms of MG, and, likewise, those presenting with MG should have a computed tomography (CT) scan of the chest to evaluate the anterior mediastinum. The relationship between MG and the thymus gland has been well established, though the precise mechanism for the improvement of symptoms following thymectomy has not been established. Ten percent of patients with autoimmune MG are found to have a thymoma, a situation that may play a role in disease initiation through multiple mechanisms including, but not limited to, expression of self-antigens by thymoma cells and impaired negative selection of autoreactive T lymphocytes. ⁶ Approximately 90% of patients with generalized MG have measurable circulating acetylcholine receptor (AChr) antibodies. It has been well established that MG is an autoimmune disorder of the neuromuscular junction, with inflammatory destruction of the neuromuscular endplate membrane as the acute event. On a chronic basis, the membrane appears to reform, though with decreased amounts of AChr. The effect at the neuromuscular junction appears to be due solely to the autoantibodies that primarily target the 67 to 76 portion of the alpha subunit of the AChr, known as the “main immunogenic region” (MIR).

Following thymectomy, up to 40% of patients with MG can be expected to achieve a complete response, as measured by no requirement for medication. The time course of the improvement may vary, and continued resolution of symptoms may occur for up to 18 months following thymectomy. Further improvement would not be expected to occur after this time period. An additional 30%-40% of patients will achieve a partial response, usually manifested by a significant reduction in the amount and type of medication required for symptom control. A small percentage of patients fail to achieve any symptomatic relief from their disease. Patients should understand the time to achieving maximal response may vary, and there is no immediate resolution of symptoms. They should be aware of the response rate, so that an informed decision regarding thymectomy may be made. With the development and refinement of minimally invasive approaches to thymectomy, the risk-benefit ratio seems to be tilted toward the performance of thymectomy, even in the older patient or those with minimal symptoms. In the past, when a median sternotomy was required for thymectomy, many neurologists were hesitant about referring patients for such an extensive operation. However, especially with the transcervical approach, such hesitation is no longer warranted.

PREOPERATIVE ASSESSMENT AND PREPARATION

Put simply, any patient with MG is a candidate for thymectomy, but this principle certainly does not imply that all patients with the disease are referred for resection. No laboratory test or other diagnostic maneuver exists that will predict the response to thymectomy—this only can be assessed following the procedure. This unpredictability is probably one of the reasons that many neurologists refer only the occasional patient for thymectomy, despite the recognition that many patients will have a beneficial response. Much of this reluctance is predicated on the “size” and perceived magnitude of the operation—namely, a median sternotomy, which is the procedure most commonly employed for thymectomy. Less invasive approaches to thymectomy—specifically, the transcervical approach as presented in this chapter—should go a long way toward addressing many of these concerns.

Specific preoperative testing prior to thymectomy is limited to CT scanning of the chest and measurement of pulmonary function. The CT scan of the chest is used to assess for the presence of a thymoma, a finding that could dictate the operative approach. The key finding, in addition to whether or not a thymoma is present, is an assessment of size and whether the mass appears to be encapsulated or invasive. The definitive assessment of invasion can only be made at the time of operation, but often a fairly reliable assumption regarding encapsulation can be made. A CT scan of the chest should be obtained in all patients following a diagnosis of MG to specifically exclude a thymoma, a finding present in up to 30%-40% of these patients. A measurement of forced vital capacity (as a minimum) should be made to ascertain whether any involvement of respiratory muscles exists, a finding that could have implications following general anesthesia and the operation. Respiratory muscle involvement, if severe, could be predictive of a postoperative mechanical ventilation requirement and would clearly mandate the need for more intensive preoperative preparation.

Prior to thymectomy, patients should be managed with optimal medical therapy following establishment of the specific diagnosis of MG. First-line therapy in MG usually consists of pyridostigmine, an anticholinesterase inhibitor, given as a single agent. The majority of patients experience significant relief of symptoms following initiation of this drug, and in most patients, it is the only drug needed. Corticosteroids, most commonly prednisone, are the most commonly used immunosuppressive agents for treating the symptoms of MG and may be necessary in the occasional patient who responds less than optimally to pyridostigmine. Other immunosuppressants, such as azathioprine, may also be used. Prior to operation, depending on the judgment of the referring neurologist, plasmapheresis may be performed, which usually consists of three plasma exchanges carried out during the week preceding the operation. This procedure significantly reduces the level of circulating anti-AChrantibodies. Depending on venous access, this procedure may be done on an outpatient basis. Following plasmapheresis, patients usually report feeling better than they have done in many months. Plasmapheresis should be performed for patients with reduced vital capacity, though this procedure becomes less important when the thymectomy is performed via the transcervical approach.

ANESTHESIA

General anesthesia can be performed safely in patients with MG following optimal preparation and adequate monitoring of neuromuscular transmission during and following the surgical procedure. Patients are requested to take their medication on the morning of the operative procedure just as they would normally. Due to the decreased number of AChrs or their functional blockade by antibodies directed against them, the use of succinylcholine or other nondepolarizing muscle relaxants is avoided. Other types of muscle relaxants can be used in smaller amounts as part of a balanced technique of anesthesia. It is important that an anesthesiologist experienced in the care of patients with MG be part of the team in order to avoid postoperative problems. Neuromuscular transmission should be monitored during the operation by peripheral nerve stimulation to aid in titrating the dose of muscle relaxants and to ensure complete reversal of neuromuscular block at the conclusion of the procedure. A detailed discussion of anesthetic technique is beyond the scope of this chapter but may be found in the paper by Baraka. ⁷

OPERATION

The standard operation for thymectomy in patients with MG consists of a median sternotomy with total removal of all thymic tissue. This operation is essentially the same operation as described for excision of an anterior mediastinal mass in Chapter9, “Anterior mediastinal lesions,” so a detailed description will not be provided in this chapter. The critical factor for thymectomy in the patient with MG is the complete removal of the thymus gland. Some have argued that aberrant rests of thymic tissue are so common that more radical operations are justified.

1. Masaoka first described the extended transsternal thymectomy where not only the gross thymus but also the surrounding adipose tissue in the anterior mediastinum is removed (see Figure 11.1).

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