Myasthenia Gravis (MG)
Weakness and fatigue are common complaints in the general population, but the degree and pattern of these symptoms — particularly diplopia, ptosis and other signs of weakness in the eye muscles — should alert a neurologist to the possibility of myasthenia gravis (MG).
A neurologist will ask many questions and conduct a physical exam to determine the extent of weakness. To look for evidence of increased weakness following exertion, a neurologist might ask the patient to look up without blinking for one or two minutes, hold the arms out for as long as possible or climb up steps.
If the physical exam is consistent with MG, the neurologist usually orders a blood test designed to detect antibodies to the ACh receptor. A blood test for MuSK antibodies is also available. Positive test results confirm a diagnosis of MG.
If the blood tests are negative, the next step is usually electrodiagnostic testing, in which electrodes are used to measure the electrical signals in muscle. Surface electrodes (similar to those used in electrocardiograms) deliver small shocks to a nerve in the arm, leg or face, while other surface electrodes record the responses in muscle. In MG, a muscle’s response to repeated nerve stimulation declines rapidly.
In addition to or in place of electrodiagnosis, the neurologist might try giving an intravenous injection ofedrophonium (Tensilon), a fast-acting cholinesterase inhibitor. (Cholinesterase inhibitors are used in the treatment of MG.) A temporary increase in strength after this "Tensilon test" is consistent with MG.
If a diagnosis of MG is confirmed, a CT scan, chest X-rays or magnetic resonance imaging (MRI) will be used to examine the thymus and look for evidence of a thymic tumor, which occurs in about 15 percent of people with MG.