Myasthenia gravis (MG) is an autoimmune disorder caused by antibodies that destroy acetylcholine receptors (AChR) at the junction between motor nerves and muscles, resulting in progressive weakness. Only skeletal muscles are weak in MG. While any skeletal muscle can be affected, it most commonly affects the eye lids, eye movements, breathing, swallowing and the limbs.
A thymoma is a growth of the thymus gland. The thymus gland is centered in the front of the chest, behind the breastbone. It extends into the neck and down to the diaphragm, and about halfway towards the spine on either side of the middle of the chest. The center portion of the chest, between the lungs, is known as the mediastinum. The front of the mediastinum is called the anterior mediastinum, so abnormalities in the thymus gland are often called Anterior Mediastinal Masses.
Patients with MG may have thymic gland abnormalities and thymic tumors, such as thymoma and follicular hyperplasia. Approximately a third of patients with MG will have a thymoma, and up to half of patients with a thymoma will have MG.
Thymomas tend to grow slowly, and it usually takes months to years to notice much growth. These tumors usually don't spread outside the thymus, but there are situations in which thymomas are more aggressive and involve neighboring organs in the chest or the lining of the lung (the pleura). They can also grow through the capsule surrounding the tumor and may invade into the pericardium, the fibrous sac surrounding the heart. Often, the tumors are discovered incidentally during an evaluation for unrelated illnesses.
Thymic hyperplasia is a condition in which the thymus gland is inflamed. This is a benign condition and can be associated with a number of other medical conditions, such as thyroid abnormalities. Thymic hyperplasia can also be seen in association with MG.
The diagnosis of myasthenia gravis is made by learning the patient's symptoms and doing specialized blood tests. Also, several special electrical tests on muscles and nerves called electromyography (EMG) can be performed. At Penn, we have several clinical laboratories that do a super-specialized test called a single fiber electromyography (SFEMG).
Thymic abnormalities are most frequently identified on a CT scan. Sometimes a special type of MRI (magnetic resonance imaging) scan is used, called a chemical shift MRI. This type of study can sometimes be helpful in distinguishing thymoma from lymphoma.
We usually don't perform biopsies, in part because the scans and the clinical situation may be highly suggestive, and in part because thymomas reside in a location that's challenging to access. The sternum often blocks access for a needle biopsy, and with the proximity of the heart and great vessels, it can be difficult to safely obtain biopsy specimens. If the mass or tumor is large, it may be easier to perform a biopsy.
The Penn Myasthenia Gravis and Thymoma Program was developed to treat patients with myasthenia gravis (MG) and thymoma, which are often comorbid disorders. Because these diseases are related and can be complicated to treat, they require multiple specialists. We created the program to coordinate care across specialties in the management of these patients. The team is comprised of neurologists, thoracic surgeons, radiologists, ophthalmologists and many others specialists, who together will develop a comprehensive treatment plan for your condition.
To improve the muscle weakness caused by myasthenia gravis, we provide medicines to suppress the immune system. In addition to pills, there are several kinds of emergency treatments that are available for people who are short of breath or are having trouble swallowing. These include intravenous gamma globulin (IVIG) and plasma pheresis. At Penn, we also have clinical trials for patients with a severe disease who are not responding to conventional treatments.
Treatment for thymomas is centered around their removal. If the tumor has not invaded the capsule, surgery is usually the only treatment recommended. For locally advanced cases, we may consider radiation therapy or chemotherapy plus radiation therapy after surgery. In the less common circumstance of a thymoma that does not appear to be completely removable, we may do chemotherapy first, followed by surgery and then radiation therapy. Rarely, thymic tumors can metastasize (spread to other sites), in which case chemotherapy is the mainstay of treatment.
Thymic hyperplasia by itself does not require any treatment, but the associated conditions, such as hyperthyroidism, may. In patients with MG and thymic hyperplasia, we will probably recommend surgery to remove the thymus gland.
If radiation therapy is included in the treatment plans, there are various types of radiation therapy available at Penn. Proton beam therapy, housed at the Roberts Proton Therapy Center at Penn, is a special type of radiation that is extremely precise: When it enters the body, it targets just the area where the thymoma was but spares the healthy, normal surrounding tissue, such as the heart and lungs.
The Roberts Proton Therapy Center is one of the few in the world to use pencil beam scanning, technology that "paints" the tumor and is ideal for complex targets located near critical structures, like thymomas. For this reason, proton therapy can result in fewer side effects both during the time that a patient receives radiation and years afterwards.
The Penn Myasthenia Gravis Support Group meets quarterly at Penn Presbyterian Medical Center. Please call 215-294-9459 for more information.
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