QRS: Alzheimer's disease; Myasthenia gravis

MYASTHENIA GRAVIS

GENERAL FEATURES

• Myasthenia gravis (MG) is an autoimmune disease characterized by skeletal muscle weakness and fatigue.

• The two types of MG are ocular, which involves the eyelids and extraocular muscles, and generalized, which may involve ocular, bulbar, respiratory, and extremity muscles.

• Neonatal MG is a transient condition related to the neonatal transmission of maternal antibodies that affect the neuromuscular junction.

• Although MG may occur at any age, the disease most commonly affects young women and older men, in a bimodal distribution.

• 10% to 15% of persons with MG have underlying thymoma.

CLINICAL ASSESSMENT

• Fluctuating skeletal muscle weakness, often associated with true muscle fatigue, should be distinguished from a sense of tiredness.

• Ocular symptoms include diplopia and ptosis.

• Bulbar symptoms include difficulty with chewing and swallowing.

• Less common symptoms include neck, distal limb, and isolated respiratory muscle weakness.

• Sensation and deep tendon reflexes are usually normal.

• Monitoring respiratory status, particularly negative inspiratory force, vital capacity, and tidal volume, is critical in the early stages of MG because acute respiratory failure may be caused by respiratory muscle weakness.

DIAGNOSIS

• Edrophonium (Tensilon) test results in sudden, temporary improvement in muscle strength following IV administration of the drug. However, edrophonium is no longer manufactured in the United States, so testing is limited to the available supply.

• Immunologic assays, which determine the levels of circulating acetylcholine receptor antibodies (AChR-Ab) as well as antibodies to muscle-specific tyrosine kinase (MuSK), are the most commonly used confirmatory tests. Antibodies to MuSK are present in 38% to 50% of persons with AChRAb negative, generalized MG.

• Electrophysiologic testing may include repetitive nerve stimulation and single-fiber electromyography. Results will indicate a reduction in muscle response.

• Once the diagnosis is made, a chest CT should be ordered to look for thymoma.

TREATMENT

• Symptomatic therapy with cholinesterase inhibitors is usually the primary treatment.

• Chronic immunotherapies, such as glucocorticoids and immunosuppressive drugs and rapid immunotherapies, including plasma exchange and IV immunoglobulin, may be used in patients with refractory disease.

• Thymectomy may lead to symptomatic improvement. JAAPA

1. You suspect a patient has myasthenia gravis. To confirm the diagnosis, you order

a. Thyroid function tests

b. A chest CT

c. An immunoassay for elevated levels of circulating acetylcholine receptor antibodies

d. A cortisol stimulation test

Answer: c

Explanation: The immunoassay for circulating acetylcholine receptor antibodies has good specificity (80%-90%) and is the most commonly used test to confirm a diagnosis of MG.

Dawn Colomb-Lippa is a professor of PA studies at Quinnipiac University, Hamden, Connecticut, and is a member of the JAAPA editorial board. Margaret King-Schumacher is an assistant professor in the Physician Assistant Department, Rosalind Franklin University, North Chicago, Illinois. Amy Klingler practices in primary care at the Salmon River Clinic, Stanley, Idaho, and is a member of the JAAPA editorial board. They have indicated no relationships to disclose relating to the content of this article.