Myasthenia Gravis Diagnostics

Laboratory Tests and Ancillaries

Most patients with generalized MG are positive for AChR antibodies on radioimmunoassay. The presence of these antibodies provides a definitive diagnosis. While only 50-60% of those with ocular MG are positive for ant-AChR antibodies. It can be noted that a negative anti-AChR radioimmunoassay does not exclude MG, and that as much as 66% of seronegative myasthenic patients only have low-affinity antibodies to AChR that cannot be detected by common assays.

In 50% of MG patients who are negative for AChR antibodies, they are positive for antibodies to muscle-specific tyrosine kinase (Anti-MuSK antibodies). Patients with MuSK-MG are predominantly female and may exhibit prominent bulbar, neck, shoulder, and respiratory weakness.

In approximately 10% of patients who are seronegative for both AChR and MusK antibodies, there is demonstrable antibodies to LRP4.

Diagnosis can also be made via Edrophonium Cl administration at 2-mg and 8-mg IV doses. A positive response is noted improvement of weakness within 30 seconds of administration. If there is no response at 2 mg, administer 8 mg IV, for a total of 10 mg. This test has a high sensitivity for ocular MG and generalized MG.

Another test that can be done to help with the diagnosis is the repetitive nerve stimulation test. A positive result is when the amplitude of the fourth compound muscle action potential (CMAP) is reduced >10% from the baseline value. Single-fiber electromyography (SFEMG) can also be done. This test is sensitive in detecting neuromuscular defects. An abnormal single-fiber electromyography findings may be seen in motor neuropathic and in myopathic disorders. However, normal single-fiber electromyography findings in a clinically weak muscle exclude a diagnosis of MG.

Myasthenia Gravis_Diagnostics

Imaging

Computed tomography (CT) or magnetic resonance imaging (MRI) of the chest may be done to diagnose associated thymic tumors. All patients with myasthenia gravis must be evaluated for the presence of a thymoma. MRI of the brain in cases where a brainstem lesion may also be done.