What is myasthenia gravis?
Myasthenia gravis (MG) is a neuromuscular disease, meaning that it affects the muscles and the nerves that control them. It is caused by a disorder in the immune system that causes the body to attack the area of the muscles where the nerves connect to them.
The immune system is the body’s natural defense against disease. Normally, when bacteria or other foreign substances enter the body, the immune system produces molecules called antibodies that attack the bacteria.
In people with myasthenia gravis, the immune system produces abnormal antibodies that prevent the muscles from receiving signals from the nerves that tell them when to relax or contract. This causes muscle weakness with symptoms that can include in 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 by mistake, it is called an autoimmune disorder, with “auto” meaning “self." So myasthenia gravis is 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.
What causes myasthenia gravis?
Scientists do not completely understand what triggers the autoimmune reaction in myasthenia gravis, but they do know that the thymus gland plays a role in the disease.
The thymus is a small gland that lies in the front part of the 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 job is thought to be completed by puberty, and after that it decreases in size. Over time, fat replaces portions of the gland. In older people the thymus weighs only .1 to .5 oz.
Tumors of the thymus gland are called thymomas. Around 10% to 15% of people with myasthenia gravis 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 myasthenia gravis was made back in the early 1900’s when surgeons observed that removal of a thymoma resulted in the improvement in the patient’s myasthenia gravis. Ultimately surgeons began removing of the thymus gland in myasthenic patients without thymic tumors and a similar response was noted.
Research into the causes and treatments of myasthenia gravis will help scientists learn more about the role of the thymus in the disease.
How is myasthenia gravis treated?
The key to treatment of myasthenia gravis begins with an accurate diagnosis. The evaluation is usually directed by a Neurologist and can involve blood tests, nerve testing and tests involving administration of medicines in order to differentiate myasthenia gravis from other disease of muscles and nerves.
Once the diagnosis has been confirmed, a treatment plan is developed with the goal of reducing the number of antibodies causing the disease and/or improving the communication between the nerves and muscles. The ultimate results is improving muscle strength. Medical treatment options include:
- Medicines that suppress antibody production or improve nerve signal transmission
- Plasmapheresis, a procedure that removes antibodies from the blood
- High-dose intravenous immune globulin, the infusion of normal antibodies from donated blood to temporarily modify the immune system
Surgical treatment is thymectomy, removal of the thymus gland. This is the treatment for patients with thymomas, but is also considered for patients with MG who do not have thymomas.
What are the results of thymectomy?
The goal of a thymectomy is to remove the source of abnormal antibody production causing the disease thus leading to resolution of symptoms. The benefits of thymectomy are not realized immediately after surgery, thus patients will continue with their medical regimen after the procedure with the goal of weaning those medications over time. Individual response to thymectomy varies depending on the patient’s age, response to prior medical therapy, the severity of the disease and how long the patient has had myasthenia gravis. In general, 70% of patients have complete remission or significant reduction in medication needs within a year of the procedure. The other 30% of patients who have thymectomy experience no change in their symptoms. According to the American Association of Neurologists, patients who have thymectomy are two times as likely to experience remission as those who have medical treatment alone.
How does a doctor determine which patients with myasthenia gravis should undergo thymectomy?
Thymectomy is recommended for all patients with thymomas and for patients under 60 who have mild to moderate muscle weakness due to myasthenia gravis. Thymectomy generally is not used for treating patients with myasthenia gravis that affects only their eyes. Thymectomy appears to be most effective when it is performed six to 12 months after the onset of symptoms. It is important to talk to your doctor early in your diagnosis about thymectomy as an option for treatment.
How is thymectomy performed?
Thymectomy can be performed by several different surgical techniques:
Transsternal thymectomy: In this procedure, the incision is made in the skin over the breastbone (sternum), and the breastbone is divided (sternotomy) to expose the thymus. This approach is commonly used for heart surgery. The surgeon removes the thymus through this incision as well as any residual fat in the center of the chest which may harbor extra thymic cells. This approach is commonly used when the patient has a thymoma.
Transcervical thymectomy: In this procedure the incision is made across the lower part of the neck, just above the breastbone (sternum). The surgeon removes the thymus through this incision without dividing the sternum. This is mostly used in patients without thymoma with certain body-types.
Robotic thymectomy and video-assisted thorascopic thymectomy (VATS): These minimally invasive techniques use several tiny incisions in the chest. A camera is inserted through one of the incisions and the surgery is performed with video guidance. The surgeon removes the thymus by using special surgical tools inserted into the other incisions. In a robotic-assisted procedure, the surgeon uses robotic arms to perform the surgery. The goal is to provide the same result as the more invasive transsternal approach with less post-operative discomfort and a quicker recovery.
What type of thymectomy is the best for me?
The transsternal thymectomy is the most commonly performed procedure, however there are no proven differences in outcomes with less invasive approaches. Your neurologist and surgeon will guide you in making a decision about the type of thymectomy you should have. Your surgeon will make a recommendation based on whether a thymoma is present and other factors related to your history and anatomy. There currently is no scientific evidence that proves one type of thymectomy is better than the other in terms of outcomes. To make the best decision for yourself, you should be informed about the different types of thymectomy and consult with your neurologist and surgeon. You also may want to seek a second opinion.
Risks / Benefits
What are the risks of thymectomy?
Complications are rare, but the risks include
- Injury lung.
- Nerve injury.
Your doctor will evaluate your personal risk based on your age and other medical conditions.
How can I find a doctor who can evaluate me for thymectomy or provide a second opinion?
Thymectomy is performed by a thoracic surgeon, a surgeon who operates on the chest. This is a relatively rare procedure and should be performed by a surgeon with experience specifically in this procedure. In addition, the best outcomes are achieved by a multidisciplinary team of neurologists and thoracic surgeons with a cohesive treatment plan.
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- National Library of Medicine. Myasthenia Gravis, MedlinePlus http://www.nlm.nih.gov/medlineplus/ency/article/000712.htm.* 5/28/2013. Accessed 4/7/2015.
- Myasthenia Gravis Foundation of America.*
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