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Myasthenia Gravis: A chronic autoimmune neuromuscular disease

Eyes, General | November 28, 2014 | Author: The Super Pharmacist

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Myasthenia Gravis: A chronic autoimmune neuromuscular disease

Myasthenia gravis is a chronic autoimmune neuromuscular disease which leads to various degrees of weakness within the skeletal, or voluntary muscles. The literal meaning of the term myasthenia gravis is grave muscle weakness. However, with modern treatments, most cases of this disorder are not grave as the name would imply. As a matter of fact, many people affected by this condition actually have an average life expectancy.

The trademark of myasthenia gravis is muscle weakness that has a tendency to increase while active and decrease during times at rest. Some muscles that are often related to the disease include those that are responsible for the following:

  • movement of the eyes and eyelids
  • chewing
  • facial expressions
  • talking
  • swallowing

In addition, the muscles that are responsible for moving the limbs and neck as well as those that control breathing can also be affected by myasthenia gravis.

Causes of Myasthenia Gravis

A defective transmission of the nerve impulses to the muscles causes myasthenia gravis. The disease occurs when the normal communication between the muscles and nerves is interrupted at the neuromuscular junction, which is the area where the nerve cells connect to the muscular cells that they control.

Causes of Myasthenia GravisUsually, when the impulses move down the nerve, the nerve endings will release acetylcholine. Acetylcholine is a neurotransmitter substance that moves from the neuromuscular junction, binding to the acetylcholine receptors. The receptors are activated, which causes the muscle to contract. However, for people who are affected by myasthenia gravis, the receptors for the acetylcholine are altered, blocked or destroyed by antibodies at the neuromuscular junction. This makes it difficult for the muscle to contract.

These antibodies are produced by the bodies own immune system, which functions normally to protect the body from foreign entities. With an autoimmune disease, like myasthenia gravis, it is this same immune system which attacks systems within the body without rational cause.

Symptoms of Myasthenia Gravis

While this disease can affect any of the voluntary muscles in the body, the ones that are most commonly affected are those that control facial expressions, movement of the eyes and eyelids and swallowing. The onset of myasthenia gravis can be quite sudden and the symptoms are frequently not recognised immediately.

In most cases of myasthenia gravis, the first symptom that is noticed is the weakness of the muscles in the eyes. For others, the initial signs of the disease come in the form of slurred speech and swallowing difficulties. The degree of muscle weakness varies from one person to the next.

Symptoms of Myasthenia GravisOcular myasthenia is a localised form of the disease that is limited to the muscles of the eyes. More severe or generalised cases affect several muscles, including those that are responsible for breathing. Symptoms may include:

  • ptosis, or the drooping of one or both eyelids
  • diplopia, or double or blurry vision
  • waddling or unstable gait
  • change in facial expression
  • shortness of breath
  • difficulty swallowing
  • dysarthria, or speech impairment
  • weakness of the neck, arms, fingers, hands and legs

Diagnosing Myasthenia Gravis

Since weakness is typical in a number of other conditions, diagnosing myasthenia gravis can be missed or delayed for years. This is especially the case for those who only experience mild weakness or those with weakness that is limited to just a few muscles.

The initial step in the diagnosis involves a review of the patients medical history, as well as neurological and physical examinations. The evaluation is to look for any impaired movements of the eye as well as weakness of muscles that does not change the patient’s sense of touch. If myasthenia gravis is suspected, a number of available tests are conducted in order to confirm diagnosis.

The presence of acetylcholine receptor antibodies or immune molecules is detected through a specialised blood test. In most cases of myasthenia, the levels of such antibodies will be elevated.

Diagnosing Myasthenia GravisAnother antibody, which is known as the anti-MuSK antibody has been present in 30% to 40% of myasthenia gravis patients, even though no acetylcholine receptor antibodies were present. A blood test can also be used to test for this antibody. In some cases, neither the anit-MuSK or acetylcholine receptor antibodies are present in the patient. However, most often with patients who experience ocular myasthenia gravis, the antibodies are present.

An edrophonium test can be performed by administering edrophonium chloride intravenously to temporarily relieve weakness. The drug will block the breakdown of acetylcholine and increase acetylcholine temporarily at the neuromuscular junction.

Additional diagnosis methods include a type of study on nerve condition. This test uses repetitive stimulation of the nerves to determine specific muscle fatigue. The weakness in muscle response to small electrical pulses is recorded and may indicate gradual muscle action reduction.

Impaired nerve to muscle transmission can be detected with single fibre electromyography, or EMG. This test measures the electrical potential of the muscle cells as the individual fibres are electrically stimulated. The muscle fibres in patients with neuromuscular disorders do not respond well to this type of stimulation.

Magnetic resonance imagining (MRI) or computed tomography (CT scan) can be used in order to indicate the occurrence of thymoma, a tumor originating from the epithelial cells of the thymus.

In addition, pulmonary function tests measure breath strength in order to predict respiratory failure, a myasthenic crisis.

Treating Myasthenia Gravis

In general, myasthenia gravis may be controlled by reducing and improving muscle weakness.

Anticholinesterase medications, such as pyridostigmine and neostigmine may be used to increase muscle strength and improve neuromuscular transmission.

Immunosuppressive medications, such as azathioprine, tacrolimus, prednisone, mycophenolate mofetil and cyclosporine can be used as well to suppress abnormal antibody production and increase muscle strength. However, these drugs may have serious side effects as they unselectively suppress the entire immune system, leaving the body susceptible to other issues.

The thymus gland, which is usually abnormal in cases of myasthenia gravis, can be removed through a procedure known as a thymectomy. This may reduce the presence of symptoms and may even be a cure for some by restoring balance to the immune system.

Other therapies that may be used include:

  • Plasmapheresis - A serum that contains abnormal antibodies is taken out of the blood and the cells are replaced
  • High-dose intravenous immune globulin - The immune system is temporarily modified by the infusion of antibodies from donor blood

Such therapies can be beneficial during periods when the weakness become especially difficult. The treatment options chosen will depend on the muscles that are affected, the age of the patient, the severity of weakness as well as any additional medical issues associated with the disorder.

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References

The Australian Myasthenic Association in NSW

Brain Foundation, Medical Info, Myasthenia Gravis

Virtual Medical Centre, Diseases, Myaesthenia Gravis

Better Health Channel, Conditions & Treatments, Immune System – Autoimmune Disorders, Myasthenia Gravis

Reddel SW; Australian Prescriber; Volume 30: Number 6; Treatment of Myasthenia Gravis; 2007

myDr, Seniors’ Health, Myasthenia Gravis

Myasthenia Gravis Friends & Support Group

Muscular Dystrophy Australia Neuromuscular Junction: Myasthenia Gravis

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