This is a neuromuscular disease characterized by the weakness in the skeletal muscles that are under voluntary control. It can affect the muscles that control the eyelid movement, the eye, walking, swallowing and facial expression. The muscles involved in the controlling of breathing and limb movements could also be affected. The muscle weakness can become worse after activity and improves with rest. This develops because of the breakdown of the communication between the nerves and the muscles. This disease appears to be more common among women under 40 and men older than 60. The life expectancy of the affected cases appears to be normal.
Causes
Myasthenia gravis develops as the result of interruption at the neuromuscular junction, the place where the nerve cells and the muscles connect. Neurotransmitters are chemicals that the body uses to communicate. Normally, the electrical signals travel down a motor nerve and the nerve endings release a neurotransmitter known as acetylcholine which binds to receptors activating the muscle that causes muscle contraction. This condition occurs when the antibodies produced by the immune system attack or destroy the receptors for acetylcholine preventing the muscle from contracting. Hence, myasthenia gravis is an autoimmune disease where the body’s immune system accidentally attacks itself.
Symptoms
In most of the cases, the symptoms can progress with time becoming worse after a few years from the onset of the symptoms. In more than half of the affected individuals, the initial sign is the eye problem presenting drooping of one or both the eyelids and double vision. When the face and throat muscle become affected, it can cause altered speaking, difficulty swallowing, a problem with chewing and limited facial expression. It can also continue to affect the muscles of the neck, arms and the legs. The severity of the muscle weakness varies greatly among the affected cases ranging from limited eye muscles to severe generalized form affecting several muscles including the muscles that control breathing.
Diagnosis
The diagnosis of myasthenia gravis is based on a physical, neurological examination and with various tests. During a neurological examination, the physician may evaluate the muscle strength, sense of touch, impairment of the eye movements and coordination. An edrophonium test injects the edrophonium chloride to relieve muscle weakness by increasing the level of acetylcholine at the neuromuscular junction and is generally used to test ocular muscle weakness. The blood test can identify the abnormally high level of acetylcholine receptor antibodies although some individuals may show negative antibody and are known as seronegative myasthenia. The electrodiagnostic involves repetitive nerve stimulation to see how the muscle fibers respond to electrical stimulation.
Treatment
Myasthenia gravis can be controlled with various therapies that are available to reduce and improve the muscle weakness. Anticholinesterase medications can help improve the neuromuscular transmission and increase muscle strength. Medications such as immunosuppressive drugs can also help improve muscle strength by reducing the production of antibodies. The plasmapheresis and intravenous immunoglobulin are used for the severe cases by destroying the antibodies although they are only effective for few weeks to months.
References
http://www.nhs.uk/Conditions/Myasthenia-gravis/Pages/Introduction.aspx
http://www.myasthenia.org/WhatisMG.aspx
http://www.mayoclinic.org/diseases-conditions/myasthenia-gravis/symptoms-causes/dxc-20200262
