Myasthenia Gravis

What is Myasthenia Gravis ?

Myasthenia gravis produces sporadic but progressive weakness and abnormal fatigability of striated (skeletal) muscles. Muscle weakness is exacerbated by exercise and repeated movement but improved by anticholinesterase drugs. Usually, myasthenia gravis affects muscles innervated by the cranial nerves (face, lips, tongue, neck, and throat), but it can affect any muscle group. It commonly accompanies immune and thyroid disorders. In fact, 15% of myasthenic patients have thymomas. When the disease involves the respiratory system, it may be life-threatening.

Myasthenia gravis follows an unpredictable course of recurring exacerbations and periodic remissions. No cure is known, but drug treatment has improved the prognosis and allows patients to lead relatively normal lives except during exacerbations.

Myasthenia gravis occurs at any age, but incidence is highest in women ages 18 to 25 and in men ages 50 to 60. About three times as many women as men develop this disease.

Causes of Myasthenia Gravis

Myasthenia gravis is thought to be an autoimmune disorder. For an unknown reason, the patient's blood cells and thymus gland produce antibodies that block, destroy, or weaken the neuroreceptors that transmit nerve impulses, causing a failure in transmission of nerve impulses at the neuromuscular junction.

Signs & Symptoms of Myasthenia Gravis

The first noticeable symptoms of myasthenia gravis may be weakness of the eye muscles, difficulty in swallowing, or slurred speech. Symptoms vary in type and severity. Patients often have nasal-sounding speech and weak neck muscles that give the head a tendency to fall forward or backward. These symptoms occur in about 90% of MG cases, are usually intermittent (i.e., come and go), and may disappear for weeks and then recur.

Generalized weakness often develops in the trunk, arms, and legs within a year of onset. Arm muscles usually are affected most severely. Muscle weakness tends to worsen as the day progresses, especially after prolonged activity.

Diagnostic Tests

Myasthenia gravis is diagnosed using a number of tests including:

• The Tensilon test - a drug (edrophonium) given intravenously. If the person’s muscle strength temporarily improves, this indicates myasthenia gravis.
• Electromyography (EMG) - a device that measures muscular response to nerve stimulation. A person with myasthenia gravis has an atypical response to the EMG test.
• Blood test - to check for the presence of acetylcholine receptor antibodies.

Treatment

Measures to relieve symptoms may include anticholinesterase drugs, such as neostigmine and pyridostigmines. These drugs counteract fatigue and muscle weakness and allow about 80% of normal muscle function. They become less effective as the disease worsens. Corticosteroids may also help to relieve symptoms. I.V. immune globulin is also used for acute exacerbations.

Some patients may undergo plasmapheresis if medications prove ineffective. This procedure is used to remove acetylcholine-receptor antibodies and temporarily reduce the severity of symptoms. I. V. immune globulin is also used for acute exacerbations.

Patients with thymomas require thymectomy, which leads to remission in adult-onset myasthenia in about 40% of patients if done in the first 2 years after diagnosis.

Prevention Tips

Myasthenia gravis cannot be prevented, but avoiding the following triggers may help patients prevent exacerbations:

• Emotional or mental stress
• Exposure to extreme temperatures
• Fever
• Fatigue
• Illness (e.g., respiratory infection, pneumonia, tooth abscess)
• Low levels of potassium in the blood.
• Weight loss