Myasthenia Gravis

Dr. Belal M. Hijji, RN. PhD

February 29, 2012

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By the end of this lecture, students will be able to:

• Describe the pathophysiology, clinical manifestations, and medical and nursing management of myasthenia gravis.

Learning Outcomes

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Introduction

Myasthenia gravis, an autoimmune disorder affecting the

myoneural junction, is characterized by varying degrees of

weakness of the voluntary muscles. Women tend to develop the

disease at an earlier age (20 to 40 years) compared to men (60 to

70).

Pathophysiology• Normally, a chemical impulse precipitates the release of

acetylcholine from vesicles on the nerve terminal at the myoneural junction. The acetylcholine continuously bind to the receptor sites on the motor end plate, for muscle contraction to sustain.

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• In myasthenia gravis, autoantibodies, produced in the thymus gland, directed at the acetylcholine receptor sites impair transmission of impulses across the myoneural junction. Therefore, fewer receptors are available for stimulation, resulting in voluntary muscle weakness that escalates with continued activity. These antibodies are found in 80% to 90% of the cases. In patients who are antibody negative, it is believed that the offending antibody is directed at a portion of the receptor site rather than the whole complex.

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The thymus gland, a part of your immune system located in the upper chest beneath the breastbone, may trigger or maintain the production of antibodies that result in the muscle weakness common in myasthenia gravis.

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Chemicals messengers, called neurotransmitters, fit precisely into receptor sites on your muscle cells. In myasthenia gravis, certain receptor sites are blocked or destroyed, causing muscle weakness.

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Clinical Manifestations

• Involvement of the ocular muscles results in:– Diplopia (double vision) and ptosis (drooping of the

eyelids [next slide]). • Laryngeal involvement produces dysphonia (voice

impairment) and increases the patient’s risk for choking and aspiration.

• Patients experience weakness of the muscles of the face and throat.

• Generalized weakness affects all the extremities and the intercostal muscles, resulting in decreasing vital capacity and respiratory failure.

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Assessment and Diagnostic Findings• Anticholinesterase test. Anticholinesterase agents stop the

breakdown of acetylcholine, thereby increasing its availability.– Edrophonium chloride, 2-10 mg, is injected IV. Thirty

seconds later, facial muscle weakness and ptosis should resolve for about 5 minutes. This immediate improvement usually confirms the diagnosis.

– Keep atropine 0.4 mg ready to control the side effects of edrophonium (convulsions and restlessness).

• ↑ Acetylcholine receptor antibody titers. • MRI demonstrates thymus gland enlargement in 90% of cases.

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Medical Management

• The aim of medical management is to improve function and reduce and remove circulating antibodies. This could be achieved through:– Anticholinesterase agents such as pyridostigmine bromide

and neostigmine bromide provide symptomatic relief by increasing the concentration of available acetylcholine at the neuromuscular junction.

– Immunosuppressive therapy (corticosteroids, cyclophosphamide, azathioprine ) aims to reduce the production of the antibody.

– Prednisone appears to be successful in suppressing the disease.

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– Plasma exchange (plasmapheresis) is a technique used to treat exacerbations by a temporary reduction in the titer of circulating antibodies.

– IV immune globulin (IVIG) is nearly as effective as plasmapheresis. However, neither therapy is a cure as it does not stop the production of the antibodies.

– Thymectomy (surgical removal of the thymus gland) can produce antigen-specific immunosuppression, resulting in clinical improvement and decreasing or eliminating the need for medication.

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Nursing Management

• Nursing care focuses on patient and family teaching. Educational topics include:

Medication management where emphasis is placed on understanding their action, taking them on time, consequences of delaying administration, and the signs and symptoms of myasthenic and cholinergic crisis.

Energy conservation through identifying the best times for rest periods throughout the day. The nurse is in position to teach the patient various ways of conserving energy. How? And to schedule activities to coincide with peak energy and strength levels.

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Strategies to help with ocular manifestations. To prevent corneal damage, the nurse should instruct the patient to tape the eyes closed for short intervals, regularly instil artificial tears, and to patch one eye can help with double vision.

Prevention and management of complications. To minimize the risk of aspiration, the nurse instructs the patient to plan the mealtimes with the peak effects of medication, and to rest before meals. Further advice is related to sitting upright during meals with the neck slightly flexed to facilitate swallowing, and using soft foods; if choking occurs frequently, the nurse can suggest pureéing ( مركز .food to a pudding consistency (حساءSuction should be available at home and usable. Gastrostomy feedings may be necessary to ensure adequate nutrition.

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Finally the nurse should instruct the patient about factors that will exacerbate symptoms and potentially cause crisis such as emotional stress, respiratory infections, vigorous physical activity, and high environmental temperature.