Myasthenia Gravis (MG)

Diagnosis

Weakness and fatigue are common complaints in the general population, but the degree and pattern of these symptoms — particularly diplopia (double vision), 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 minute, hold the arms out for prolonged period, or do repeated squats.

Bedside tests

An ice pack test can be used as a part of a neurologic examination for patients with ptosis (weakness of the eyelid muscles). Cooling the eyelid muscles may lead to improvement in ptosis and serve as an indication that the patient might have MG. The specificity of an ice pack test is only about 80%, as false positive results can be frequently observed. This test should not be considered as a standalone test but rather an extension of the physical examination.

Serologic testing

Serologic tests check for the presence of antibodies in the serum (a component of blood). Seropositive means that there is presence of the antibody in the blood, and seronegative means that the antibody was not detected.

If a physical exam is consistent with MG, a neurologist usually orders a blood test designed to detect antibodies to the acetylcholine (ACh) receptor. A blood test for muscle-specific tyrosine kinase (MuSK) antibodies is also available. In approximately 90% of patients with MG, a positive test result confirms a diagnosis of MG. However,6% to 12% of patients with myasthenia may test negative for both AChR antibodies and MuSK antibodies. These patients have seronegative MG. Rates of seronegativity are much higher in ocular MG as compared to generalized MG, making it more challenging to diagnose. Additional abnormal antibodies targeting LRP4 have been identified in a small subset (<1%) of individuals with MG. Their presence is not specific to MG, however, as they have also been detected in other autoimmune and neuromuscular disorders, making them less reliable for diagnosis.

Electrodiagnostic testing

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, repeated nerve stimulation leads to a significant (>10%) decline in the muscle’s response. There are two types of tests that can be done to confirm the diagnosis of MG:

• Repetitive nerve stimulation (RNS) is commonly used, with 75% to 80% accuracy and is described above.
• Single-fiber electromyography (SFEMG) is a more sensitive test (with 95% accuracy) but is less widely available and technically demanding.

If a diagnosis of MG is confirmed, imaging studies such as a chest CT scan or magnetic resonance imaging (MRI) are performed to evaluate the thymus gland for abnormalities, including thymoma, which is found in approximately 10–15% of individuals with MG.

Additional Reading

• Kaminski HJ, Sikorski P, Coronel SI, Kusner LL. Myasthenia gravis: the future is here. J Clin Invest. 2024 Jun 17;134(12):e179742. doi: 10.1172/JCI179742. PMID: 39105625; PMCID: PMC11178544.

Last reviewed June 2025 by Neelam Goyal, MD.