Myasthenia gravis: Review of the literature and report of a case of malignant thymoma

Abstract

Myasthenia gravis, a disorder without known etiology, has been studied extensively by the neurophysiologists; it is thought to be associated with either a defect of synthesis of acetylcholine or with a lowered sensitiveness of the muscle endplates to acetylcholine, possibly because of a curare-like substance. Hyperplasia or benign and malignant tumors of the thymus occurs not infrequently. Microscopically lymphorrhages in skeletal muscle or in various organs are frequently found. The disorder occurs at any age, in frequent association with upper respiratory infections, and occasionally with hyperthyroidism. It is characterized by gradual, at times sudden onset with weakness usually involving the eyelid, muscles of mastication, larynx, and less frequently the extremities. It is aggravated by exercise, quinine and curare. It is relieved by rest, neostigmine and often by pregnancy. There are normally remissions and exacerbations. The disease has been confused with neurasthenia, polyneuritis, muscular dystrophies, muscular atrophies, degenerative lesions of the bulb, pseudobulbar paralyses, epidemic encephalitis, syphilis involving the bulb, hyperthyroidism and Addison's disease. Careful evaluation of the history, examination, response to quinine, curare, and neostigmine, and spinal fluid examination usually suffice to make the diagnosis. The chief drug therapy is neostigmine which is sometimes combined with ephedrine. Recently tetra-ethylpyrophosphate, which has a longer and more even action than neostigmine, was introduced in the treatment of myasthenia gravis. X-ray therapy has been used but thus far without great success. Surgical removal of the thymus whether it is composed of hyperplastic tissue or tumor in a significant number of cases gives definite improvement. Further study is necessary for proper evaluation of surgery of the thymus gland in myasthenia gravis. The thymus gland may show involvement by hyperplasia and benign or malignant thymoma in a significant number of patients with this symptom-complex; it is not, however, the etiologic agent of this disease. The case presented emphasizes the early symptoms and signs with x-ray evidence of a mediastinal mass, muscular weakness, increasing symptoms at the beginning of x-ray therapy followed by temporary improvement, exacerbation of symptoms with an upper respiratory infection, temporary relief by neostigmine and finally a respiratory type of death. At autopsy the patient was found to have a malignant thymoma with local extension and metastases of the tumor limited to the thoracic cage.