Myasthenia Gravis Diagnosis and Evaluation

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Myasthenia Gravis: Diagnosis and Tests

Diagnosing Myasthenia Gravis can be hard because weakness is a common symptom in many illnesses. This often means there is a delay before getting a clear diagnosis. Doctors use a mix of physical exams, blood tests, nerve tests, and scans to confirm MG and rule out other conditions like stroke or thyroid problems.

Clinical Bedside Assessments

The Physical Exam

The neurologist will perform a detailed examination to elicit “fatigability,” the signature of MG.

Sustained Up-Gaze: The patient is asked to look up at the ceiling for prolonged periods to see if ptosis (eyelid drooping) develops or worsens.
Repetitive Testing: The doctor may ask the patient to hold their arms out or count out loud to 100 to check for developing weakness or slurred speech.
Reflexes: Importantly, in MG, deep tendon reflexes and sensation remain normal, which helps distinguish it from other nerve disorders.

The Ice Pack Test

This is a simple, non-invasive test used when a patient has a drooping eyelid.

Procedure: An ice pack is placed over the drooping eyelid for two minutes.
Principle: Cooling the muscle improves the efficiency of acetylcholine transmission.
Result: If the eyelid opens significantly wider after the ice is removed, the test is considered positive for MG.

Utility: It is susceptible for ocular myasthenia but cannot diagnose generalized weakness.

Serological Testing (Blood Tests)

Blood tests are the first-line confirmatory tests. They look for the specific autoantibodies that cause the disease.

Anti-AChR Antibodies: Antibodies to the acetylcholine receptor are found in about 85% of patients with generalized MG and 50% with ocular MG. A high level is virtually diagnostic.
Anti-MuSK Antibodies: Found in about 40% of patients who test negative for AChR antibodies. These patients often have more severe bulbar (face/throat) symptoms.
Anti-LRP4: A newer antibody test for patients who are negative for both AChR and MuSK (double-seronegative).
Anti-Striational Antibodies: Often associated with thymoma and severe disease, particularly in older patients.

Repetitive Nerve Stimulation (RNS)

RNS is the most frequently used electrodiagnostic test for MG.

Procedure: Small electrodes are placed on the skin over a specific muscle. Minor electrical shocks are delivered to the nerve supplying that muscle.
Measurement: The test measures the muscle’s electrical response to repetitive stimulation (usually 2-3 shocks per second).
The Decrement: In healthy muscles, the response stays strong. In MG, the response progressively declines (decremental response) as the nerve runs out of available acetylcholine receptors.

Sensitivity: It is more sensitive in generalized MG than in purely ocular cases.

Single Fiber Electromyography (SFEMG)

When other tests are inconclusive, SFEMG is considered the most sensitive diagnostic tool available.

Precision: It records the electrical activity of a single muscle fiber compared to another fiber in the same motor unit.
Jitter: The test measures “jitter,” which is the variability in the time it takes for the nerve signal to trigger the muscle. Increased jitter is a sign of defective neuromuscular transmission.
Specialization: This test is technically complex and requires a highly skilled neurologist to perform and interpret.

Indication: Usually reserved for complex cases or seronegative patients.

Imaging Studies

Once the diagnosis is suspected or confirmed, imaging is crucial to evaluate the thymus gland.

CT Scan of the Chest: The standard method to check for a thymoma (tumor) or thymic hyperplasia (enlargement).
MRI: May be used if a CT scan is contraindicated or provides insufficient detail.
Importance: Identifying a thymoma is urgent because it requires surgical removal, which can also improve the MG symptoms.
Thyroid Imaging: Given the high prevalence of thyroid disease, thyroid ultrasound may also be indicated.

Pharmacological Testing

Though less common today due to cardiac risks, pharmacological challenge tests were historically the gold standard.

Edrophonium (Tensilon) Test: Edrophonium is a drug that temporarily prevents the breakdown of acetylcholine.
Response: If weakness dramatically improves within seconds of injection, it suggests MG.
Current Status: This test has largely been replaced by antibody testing and the ice pack test, but may still be used in equivocal cases with careful monitoring.
Oral Trial: Sometimes, a trial of oral pyridostigmine (Mestinon) is given to see if symptoms improve clinically over days.

Pulmonary Function Tests

Because respiratory weakness is the most dangerous aspect of MG, evaluating lung function is part of the diagnostic workup.

FVC (Forced Vital Capacity): Measures the total amount of air a patient can exhale.
NIF (Negative Inspiratory Force): Measures the strength of the diaphragm and breathing muscles.
Baseline: Establishing a baseline helps doctors monitor for an impending crisis or deterioration.

Sleep Studies: May be used to check for sleep apnea, which is dangerous in MG patients with bulbar weakness.