Myasthenia gravis (MG)

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.

What is thymoma?

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.

What is thymic hyperplasia?

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.