What is MG?

Myasthenia Gravis (MG) comes from the Greek and Latin words meaning “grave muscular weakness.” The most common form of MG is a chronic autoimmune neuromuscular disorder that is characterized by fluctuating weakness of the voluntary muscle groups. The prevalence of MG in the United States is estimated to be about 20/100,000 population. However, MG is probably under diagnosed and the prevalence may be higher.

Clinical Features and Symptoms

MG occurs in all races, both genders and at any age.

MG is not directly inherited nor is it contagious. It does occasionally occur in more than one member of the same family. MG may affect any muscle that is under voluntary control. Certain muscles are more frequently involved and these include the ones that control eye movements, eyelids, chewing, swallowing, coughing and facial expression. Muscles that control breathing and movements of the arms and legs may also be affected. Weakness of the muscles needed for breathing may cause shortness of breath, difficulty taking a deep breath and coughing.

The muscle weakness of MG increases with continued activity and improves after periods of rest. The muscles involved may vary greatly from one patient to the next. Weakness may be limited to the muscles controlling eye movements and the eyelids. This form of myasthenia is referred to as ocular MG. In its severest form, MG involves many of the voluntary muscles of the body, including those needed for breathing. The degree and distribution of muscle weakness for many patients falls in between these two extremes. When the weakness is severe and involves breathing, hospitalization is usually necessary.

Diagnosis

There are many disorders that cause weakness. In addition to a complete medical and neurological evaluation, a number of tests may be used to establish a diagnosis of
MG. A blood test for the abnormal antibodies can be performed to see if they are present. Electromyography (EMG) studies can provide support for the diagnosis
of MG when characteristic patterns are present. The edrophonium chloride (Tensilon®) test is performed by injecting this chemical into a vein. Improvement of strength immediately after the injection provides strong support for the diagnosis of MG. Sometimes all of these tests are negative or equivocal in someone whose story and examination still seem to point to a diagnosis of MG. The positive clinical findings should probably take precedence over negative confirmatory tests.