Myasthenia gravis is the commonest disorder of neuromuscular transmission.
It is usually an autoimmune disease, i.e. the immune system of the body produces antibodies against body tissues. Autoimmune myasthenia gravis needs to be distinguished from congenital myasthenic syndromes. Congenital myasthenia gravis is caused by gene mutations. It can occur at any age; affect any race. Both genders are affected. Women are affected more frequently than men. Babies born to mothers with myasthenia gravis can acquire neonatal myasthenia.
Causes of Myasthenia Gravis
Voluntary muscles of our body are controlled by nerve impulses fired from the brain. There is a space between the nerve fibres supplying muscles and the muscle fibres, the neuromuscular junction. A chemical called Acetylcholine gets released into this space from the nerve ending at the arrival of a nerve impulse. This acetylcholine traverses the space, attaches to the receptors; activates them and then the muscle contracts.
In Myasthenia gravis, the number of receptor sites reduces (up to 80% reduction may occur). Hence due to the lack of the number of activated receptors muscle contraction doesn’t happen. This reduction is due to the attack of receptors (or their blockage) by antibodies. Antibodies are proteins that our immune system produces to fight foreign bodies. But, in some individuals (with autoimmune diseases) the immune system loses its capacity to recognise ‘self from non-self’. As a result antibodies attack the body’s own tissues.
The exact mechanism why such an autoimmune mechanism initiates is not yet known. Some evidences suggest an association with thymus, a ductless gland at the base of the neck that normally takes part in immunity but atrophies with age.
Association with other autoimmune diseases, esp. Thyroid diseases is also suggested. The antibodies may get transferred from an affected mother to the newborn resulting in transient features (neonatal myasthenia).