From the Guidelines
The workup for myasthenia gravis should begin with a thorough clinical evaluation, followed by specific diagnostic tests, including acetylcholine receptor (AChR) antibody testing, as recommended by the most recent study 1. The initial testing should include AChR antibody testing, which is positive in about 85% of patients with generalized myasthenia gravis.
- If AChR antibodies are negative, testing for muscle-specific kinase (MuSK) antibodies and lipoprotein receptor-related protein 4 (LRP4) antibodies should be performed.
- Electrophysiological studies are essential, particularly repetitive nerve stimulation showing decremental response and single-fiber electromyography demonstrating increased jitter, as supported by 1.
- Edrophonium (Tensilon) testing can be helpful, where a brief improvement in muscle strength after administration suggests myasthenia gravis.
- All patients diagnosed with myasthenia gravis should undergo chest imaging (CT or MRI) to evaluate for thymoma, as approximately 10-15% of patients have associated thymic abnormalities.
- Pulmonary function tests are important to assess respiratory muscle strength, especially in patients with bulbar symptoms.
- Additional laboratory tests should include complete blood count, comprehensive metabolic panel, thyroid function tests, and testing for other autoimmune disorders that may coexist with myasthenia gravis. The most recent and highest quality study 1 provides a comprehensive approach to the workup of myasthenia gravis, including:
- AChR and antistriated muscle antibodies in blood
- Pulmonary function assessment with NIF and VC
- CPK, aldolase, ESR, and CRP for possible concurrent myositis
- Consider MRI brain and/or spine depending on symptoms to rule out CNS involvement by disease or alternate diagnosis
- Troponin, ECG, and consider TTE and/or cardiac MRI to evaluate concomitant myocarditis
- Electrodiagnostic studies, under neurologic consultation, including neuromuscular junction testing with repetitive stimulation and/or jitter studies, NCS to exclude neuropathy, and needle EMG to evaluate for concomitant myositis
- Inflammatory markers (ESR and CRP)
- Consider paraneoplastic workup
- Review and stop medications with known risk of worsening myasthenia: beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolide antibiotics. Early diagnosis is crucial as prompt treatment with acetylcholinesterase inhibitors, immunosuppressants, or thymectomy can significantly improve outcomes and prevent life-threatening complications like myasthenic crisis, as highlighted in 1.
From the FDA Drug Label
ENLON Test in the Differential Diagnosis of Myasthenia Gravis ... Responses to ENLON in myasthenic and nonmyasthenic individuals are summarized in the following chart
- Myasthenic Response- occurs in untreated myasthenics and may serve to establish diagnosis; in patients under treatment, indicates that therapy is inadequate.
The workup for myasthenia gravis involves the ENLON test, which uses edrophonium (IV) to diagnose and evaluate treatment requirements in myasthenia gravis. The test can help establish a diagnosis in untreated patients and assess the adequacy of therapy in treated patients. Key features of the test include:
- Myasthenic response: increased muscle strength, no fasciculations, and no side reactions, indicating undertreatment or untreated myasthenia gravis.
- Adequate response: no change in muscle strength, presence or absence of fasciculations, and minimal side reactions, indicating adequate treatment.
- Cholinergic response: decreased muscle strength, presence or absence of fasciculations, and severe side reactions, indicating overtreatment with anticholinesterase drugs 2.
From the Research
Diagnostic Tests for Myasthenia Gravis
The diagnosis of myasthenia gravis (MG) involves a combination of clinical evaluation and diagnostic tests. The following tests are commonly used:
- Intravenous edrophonium chloride test: This test is helpful in cases of measurable ptosis and is equally efficient in detecting generalized myasthenia gravis as acetylcholine receptor antibody testing 3.
- Electromyography (EMG): EMG testing can demonstrate abnormal jitter in virtually all patients with MG, but may not be specific for the disease 4.
- Acetylcholine receptor antibody testing: This test is equally efficient in detecting generalized myasthenia gravis as edrophonium testing 3.
- Single-fiber electromyography (SFEMG): SFEMG is the most sensitive test for MG and can demonstrate abnormal jitter in virtually all patients with the disease 4, 5.
- Repetitive nerve stimulation: This test can demonstrate a decremental response in some patients with MG, but has a modest diagnostic yield 5.
Approach to Diagnosis
The diagnosis of MG is primarily clinically based, and diagnostic tests can reaffirm the clinician's impression 6, 5. A careful selection of tests is necessary to confirm the diagnosis and to assess the severity of the disease. The tests should be interpreted in light of the overall clinical picture, and a combination of tests may be necessary to evaluate some patients with MG 3, 4.
Classification and Treatment
Patients with MG can be classified based on antibody status and clinical presentation, and treatment responses may differ based on disease subtypes 7. The treatment of MG involves immunotherapy, and corticosteroids remain the first-line treatment. Other treatment options, such as oral immunosuppressants and thymectomy, may also be used 7.