Myasthenia gravis (MG) is a neuromuscular disease caused by a disorder in the immune system. It occurs in approximately 14-20 patients out of 100,000. In people with MG, the immune system produces abnormal antibodies that prevent the muscles from appropriately receiving signals from the nerves that tell them when to relax or contract. This causes muscle weakness and symptoms that can include double vision or blurred vision (eye muscle weakness), drooping eyelids (eyelid muscle weakness), difficulty with speaking and swallowing (throat muscle weakness) and weakness of the limbs.
When the immune system acts against healthy tissue in an inappropriate manner, it is called an autoimmune disorder, with “auto” meaning “self. MG is considered to be a neuromuscular autoimmune disease.
Myasthenia gravis is most common in young women and older men, but people of any age or either sex can get it.
Scientists do not completely understand what triggers the autoimmune reaction in MG, but they do know that the thymus gland plays an important role in the disease.
The thymus is a small gland that lies in the front (anterior) portion of the central chest, beneath the breastbone, and extends into the lower part of the neck. It is most important early in life during immune system development. A baby’s thymus gland weighs between .7 and 1.1 oz. The gland continues to grow and by puberty weighs 1.1 to 1.8 oz. The thymus gland’s importance in immune development is felt to dissipate by puberty and diminish in size. Over time, fat replaces the majority of the gland.
Tumors of the thymus gland are called thymomas. Around 10-15 percent of people with MG have a thymoma. Another 60%, however, will have other abnormalities of the gland including thymic hyperplasia (an enlarged gland). The original association between the thymus gland and MG was made back in the early 1900’s when surgeons observed that removal of a thymoma resulted in improvement in the patient’s myasthenic symptoms. Based on this observation, surgeons began removing the thymus gland in myasthenic patients without thymic tumors and noted a similar response.
The diagnosis of MG is usually made by a neurologist (a physician expert in diagnosing and treating neurologic disorders. Infusion of a specific medication (edrophonium chloride) under close observation should cause improvement of a patient’s symptoms if the patient has MG and therefore confirm the diagnosis. Additionally, blood tests can be performed that may demonstrate the presence of an antibody against acetylcholine receptor (AchR) and muscle-specific receptor tyrosine kinase (MuSK). This can help confirm the clinical diagnosis.
Once the diagnosis of MG has been confirmed, a treatment plan by your neurologist is developed with the goal of reducing the number of antibodies causing the disease and/or improving the communication between the nerves and muscles.
This results in improvement in muscle strength and symptoms. Medical treatment options include:
The surgical treatment of MG is thymectomy (complete removal of the thymus gland). Thymectomy should be performed by a thoracic surgeon with specific expertise in removal of the thymus gland for MG. Theoretically, this removes the source of abnormal antibody production.
The role of and indications for thymectomy are debated. However, in general, thymectomy is recommended for all patients with thymomas and for patients under 60 who have mild to moderate muscle weakness due to MG. Thymectomy is not generally recommended for patients with MG that affects only their eyes (ocular MG).
Thymectomy appears to be most effective when it is performed 6 to 12 months after the onset of symptoms and therefore it is important to talk to your doctor early in your diagnosis about this option. Your surgeon and neurologist will choose the best treatment for you.
Individual response to thymectomy varies depending on a patient’s age, response to prior medical therapy, severity of disease and duration of symptoms. In general, 70 percent of patients have complete remission or significant reduction in medication needs within a year of the procedure. The other 30 percent of patients who have thymectomy experience no improvement in their symptoms. Data suggests that patients who have thymectomy are two times as likely to experience remission compared to medical treatment alone.
Thymectomy can be performed by several different surgical techniques:
Your surgeon will choose the best approach for you.