What is Myasthenia Gravis?
Myasthenia Gravis (MG) is a neuromuscular disease characterized by weakness of the voluntary muscles. The key characteristic feature of MG is fatiguable or fluctuating muscle weakness. Patients tend to find that their muscle power gradually declines with increase use or exercise and improves following a period of rest.
The Cause of Myasthenia Gravis
The site of disease in MG is the neuromuscular junction. There is a “block” in the transmission of signals from the nerve to the muscles at the neuromuscular junction. (see figure) In the normal situation, when impulses travel down the nerves, the nerve endings release neurotransmitters called acetylcholine. Acetylcholine travels through the neuromuscular junction and binds to the acetylcholine receptors (AChR) which in turn allows the muscle to contract.
In MG patients, there are antibodies which block these acetylcholine receptors at the neuromuscular junction and prevent muscle contraction from occurring. These antibodies are produced by the patient’s own immune system. This is why MG is considered an autoimmune disease as the patient’s own immune system which would normally protect them from foreign organisms recognizes itself as foreign and mistakenly attacks itself.
The role of the thymus gland in Myasthenia Gravis
The thymus gland, which lies in the upper chest area beneath the breastbone, plays an important role in the development of the immune system in early life. In adults with MG, the thymus gland can be abnormal. In the majority of patients, there is hyperplasia suggesting an active immune response. In some MG patients (~20%), there is the presence of a thymic tumour, referred to as a thymoma.
The relationship between the thymus gland and MG has yet to be fully understood. However, following the surgical removal of the thymus gland and improvement of symptoms of MG in some patients, we know that the thymus gland is likely to influence the process involved in the development of AchR antibodies.
Most thymoma in MG are benign, well differentiated and encapsulated and easily removed surgically. However, there are patients who have a more invasive form of thymoma that not only requires surgical removal but may also require either some form of either radiotherapy or chemotherapy.