- A progressive neuromuscular disease of the lower motor neurons characterized by muscle weakness andfatigue.

Causes and Incidence The cause of myasthenia gravis (MG) is unknown. More than 80% of individuals with MG also have thymic abnormalities, but the link is unclear. The incidence is 3 to 6 per 100,000 individuals in the United States. The age of onset is either 20 to 30 years (primarily in women) or 50 to 60 years (primarily in men).

Disease Process An antigen attack on the acetylcholine receptor of the postsynaptic neuromuscular junction results in dysfunction of the receptor, which fails to act on the acetylcholine. Because of this, nerve impulses do not pass on to the skeletal muscle at the myoneural junction.

Symptoms The most common manifestations are ptosis, diplopia, and muscle fatigue after exercise. Dysarthria, dysphagia, ocular palsy, head bobbing, and facial and proximal limb weakness are also reported. Symptoms are milder on awakening and become worse as the day progresses. Rest temporarily improves symptoms. Respiratory involvement leads to breathlessness and reduced tidal volume and vital capacity. Manifestations can be remitting, static, or progressive. Factors such as stress, menses, heat, and illness can exacerbate symptoms.

Potential Complications Myasthenic crisis is an acute exacerbation of symptoms; it usually involves respiratory distress and can lead to respiratory failure or aspiration and cardiopulmonary arrest.

Diagnostic Tests A characteristic pattern of fatigue and weakness on exertion that improves with rest, and a positive Tensilon test, are indicators. A computed tomography scan may indicate the presence of thymoma; electromyography may show muscle fiber contraction with progressive decremental response.
Treatments

Surgery
Thymectomy for treatment of thymoma and remission of adultonset MG.

Drugs
Anticholinesterases and corticosteroids to counteract muscle weakness and fatigue; immunosuppressants with autoimmune pathogenesis; flu shots to prevent respiratory infection.

General
Plasmapheresis to treat weakness and fatigue; ventilatory support in respiratory crisis; physical therapy to prevent disuse problems, occupational therapy to aid in activities of daily living; balance of exercise and rest; instruction about cholinergic crisis caused by excessive anticholinesterase medication; counseling for long-term adaptation to disease; information about the importance of preventing respiratory infection or recognizing and treating symptoms early.

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