The diagnosis of myasthenia gravis is made by a positive edrophonium test, repetitive nerve stimulation test of a weak muscle, and antiacetylcholine receptor antibody assay. MRI of the mediastinum is now indicated to
|A||Rule out tuberculosis before starting prednisone|
|B||Rule out thymoma|
|C||Look for small cell carcinoma and Lambert-Eaton syndrome|
|D||Rule out sarcoidosis|
Rule out thymoma
1). Ten percent of myasthenia patients have thymic tumors. Surgical removal of all thymomas is necessary because of local tumor spread. Even in the absence of tumor, 85% of patients clinically improve after thymectomy.
2). Thymectomy be performed in all patients with generalized MG who are between puberty and age 55). Sarcoidosis causes peripheral neuropathy and aseptic meningitis, but not a myasthenia syndrome.
3). Small cell carcinoma is associated with Lambert-Eaton syndrome, a paraneoplastic syndrome similar to myasthenia. In Lambert-Eaton syndrome, an autoimmune response results in anti–calcium channel antibodies A chest x-ray would be sufficient to screen for malignancy or infection.