Myasthenia Gravis And Thymoma

Myasthenia gravis is a condition in which the patient develops progressive muscle weakness secondary to acetylcholine receptor antibodies. Medications used to treat myasthenia gravis include anti-cholinesterase medications which block the cholinesterase-mediated removal of the acetylcholine from its receptor. The classifications of myasthenia gravis include ocular and generalized. In the ocular group, there is involvement of ocular muscles only with ptosis and diplopia. This is a very mild form of myasthenia gravis with no mortality. The generalized type of myasthenia gravis may be mild or progress to severe disease. In the more severe forms, the response to drug therapy is poor and the prognosis is poor. Ninety percent of patients with myasthenia gravis have adult onset disease. It affects women twice as often as men. Ten percent of patients with myasthenia gravis have thymomas. One-third to one-half of patients with thymomas have myasthenia gravis.

Myasthenia crisis is an episode of severe, rapid progression of the disease with respiratory compromise. Patients with ocular symptoms alone or those whose conditions are stable and well-managed with medication are not generally candi dates for thymectomy because their prognosis is generally good. On the other hand, patients in myasthenia crisis are also not candidates for emergency surgery and thymectomy can be delayed until better medical control is obtained.

Myasthenia patients are not particular anesthetic risks, although no muscle relaxant should be used during the procedure. For patients with severe myasthenia gravis, several options for treatment exist, including medications and plasmapheresis which may be a useful preoperative adjunct by reducing circulating levels of acetylcholine receptor antibodies.

The reason that thymectomy is effective in myasthenia gravis is unknown. It has been shown that patients with a normal thymus who undergo thymectomy for myasthenia gravis do better in terms of decreased requirement for medication than those patients with myasthenia gravis who have thymomas resected. The reason for this is unknown. In general, it can be expected that in those patients with myasthenia gravis who undergo thymectomy, one-third will be cured of their disease; one-third will be somewhat improved; and one-third will have no benefit. A poorly understood paradoxical phenomenon is the rare de novo appearance of myasthenia gravis after resection of a thymoma, occurring in less than 5% of thymomas.

The thymus lies in the anterior mediastinum overlying the pericardium and great vessels. The arterial supply is predominantly from the internal mammary vessels, whereas the venous drainage is central into the left innominate vein over which it is draped. It is an H-shaped organ with two limbs going up into the neck and two limbs going down into the anterior mediastinum (Fig. 18.2).

Surgical approaches to the thymus include a transcervical approach, a limited median sternotomy and a full median sternotomy. In the cervical approach, a collar incision is made and the thymus is dissected out through this approach. As can be imagined, it is extremely difficult to resect the thymus in its entirety using this approach, and we would generally not recommend it. The limited median sterno-tomy approach allows better access to the upper mediastinum. In this approach, an upper midline incision is made and a saw is used to perform a median sterno-tomy down to the level of about the 3rd intercostal space, at which point the saw is turned transversely and the sternum transsected. A small retractor is then used to spread the transsected upper sternum, and thus a limited upper median sterno-tomy is performed. A full median sternotomy allows the best visualization of the anterior mediastinum.

The thymus is completely dissected free and one may need to go all the way down on either side of the pericardium towards the phrenic nerves for complete resection.

In the case of a thymoma, lack of an enveloping capsule or local invasion of adjacent tissues including pericardium, pleura, great vessels or phrenic nerve is a good indication of malignancy. Thymomas usually do not metastasize by hematog-enous or lymphatic roots, but metastasize directly to pleural surfaces by seeding or by direct extension. The thymoma may recur locally if its capsule has been violated. If the tumor appears invasive, radical excision may be warranted. The phrenic nerve needs to be identified and the tumor may be cleared away from the phrenic nerve if it is not actively invading the nerve.

It is controversial whether or not a phrenic nerve should be taken. Generally, if the patient does not have myasthenia gravis, then sacrifice of one of the phrenic nerves is acceptable, since these patients will generally still be able to be extubated, if they have a normally functioning hemidiaphragm on the other side. If, however, they have myasthenia gravis, it is controversial whether or not resection of a phrenic nerve is acceptable. It appears logical that if it is being actively invaded, then it probably is not functional and its sacrifice may be warranted. Removal of invaded pericardium may be performed as well as removal of adherent mediastinal pleura and wedge excision of portions of lung that are invaded.

Preoperative radiation therapy may be considered if the thymoma is huge or if radiologic evaluation suggests vascular invasion. The dose is in the range of 4000 rads over 4 weeks followed by surgery in another 4 weeks.

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