Quiz: Diagnosis of myasthenia gravis. Have you mastered these 5 questions?

Myasthenia gravis (MG) is a rare neuromuscular junction (NMJ) autoimmune disease, mainly manifested as skeletal muscle weakness
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It is caused by autoantibodies that target acetylcholine receptors (AChRs) or NMJ structural proteins, resulting in a characteristic pattern of fatigue myasthenia
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How much do you know about MG's diagnostic tests? This article will test everyone's understanding through 5 small questions, let's try it together
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Yimaitong compiles and organizes, please do not reprint without authorization
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01 Regarding MG's laboratory inspection, which statement is the most accurate? A.
The absolute titer of serum AChR antibody is related to the severity of the disease.
B.
In the early stage of the disease, anti-AChR antibody can always be detected in the serum.
C.
The absolute titer of serum AChR antibody is of no value for the diagnosis of MG.
D.
Anti-muscle specific kinase (MuSK) ) There may be a correlation between titers and disease severity.
Compared with anti-AChR antibody titers, there is a correlation between anti-MuSK titers and disease severity in patients
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On the other hand, the serum titer of AChR antibody is not well correlated with the severity of the disease
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AChR antibodies may not always be detectable.
Importantly, about 15%-20% of MG patients are seronegative
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 02 Which inspection method is most sensitive to MG? A.
AChR antibody detection B.
Single fiber electromyography (SFEMG) C.
Rheumatoid factor and antinuclear antibody detection D.
Repetitive nerve stimulation (RNS) SFEMG provides the most sensitive measurement index for MG.
Normal SFEMG finds that the muscles are clinically stronger Weak, can effectively rule out the diagnosis of MG in most cases
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 When showing abnormal neuromuscular transmission, the examination of weak muscles with SFEMG is more sensitive than the examination with RNS; however, it is more technically demanding and requires an experienced clinician
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 Anti-AChR antibody detection is reliable for diagnosing MG, and at 98%-99% specificity, it is obviously the most specific diagnostic test for MG
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 Rheumatoid factor and antinuclear antibody testing is an important examination content to rule out comorbid autoimmune diseases that may occur in MG (ie systemic lupus erythematosus, rheumatoid arthritis)
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 03 What is the difference between the examination of ophthalmic MG and systemic MG patients? A.
The RNS test results of ophthalmic MG patients are more likely to be positive.
B.
About 70%-80% of ocular muscle MG patients have positive anti-AChR antibody test results, and about 50% of systemic MG patients have positive anti-AChR antibody test results.
C.
SFEMG is a reasonable alternative to RNS in patients with ocular muscle MG.
D.
Anti-AChR antibodies (blocking type) are more common in ocular diseases.
SFEMG provides a reasonable alternative to RNS in patients with ocular muscle MG.
The subgroup showed a sensitivity close to 100%
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RNS is more specific to postsynaptic disorders of neuromuscular transmission
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 Compared with patients with simple ocular muscle MG, patients with systemic MG are more likely to have abnormal RNS test results
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If other diagnostic test results still do not show, usually start with RNS and then complete SFEMG
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About 70%-80% of patients with systemic MG test positive for anti-AChR antibodies, but about 50% of patients with ocular muscle type MG test positive for anti-AChR antibodies
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 About half of patients with systemic MG have anti-AChR antibodies (blocking type), but only 30% of patients with eye diseases have anti-AChR antibodies
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However, it is worth noting that less than 1% of MG patients have anti-AChR antibodies (blocking type), and no binding or modulating antibodies are detected, which makes this test alone has no special clinical value
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 04 What kind of operation might be helpful for diagnosing MG? A.
Thigh thrust B.
Fluidity test C.
Deep knee flexion D.
Flexion, abduction, and external rotation fatigue.
Several stimulation techniques for specific muscle groups are useful in diagnosing MG, such as letting the patient perform deep knee flexion and back straightening , The hand rests on the palm of the examiner
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During this operation, increased palm pressure of the examiner is an early sign of weakness
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In addition, repeatedly standing up from a chair without using your arms may result in excessive forward leaning and "hip forward" movements
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In order to check for possible ocular signs of MG, continuous upward gaze (60-180 seconds) may show fatigue-prone ptosis in one or both eyes
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 05 Which statement about anti-AChR antibodies is the most accurate? A.
Anti-AChR regulatory antibodies are not associated with an increased risk of thymoma.
B.
False positive test results for anti-AChR binding antibodies are not common.
C.
About 10% of patients who test negative for anti-AChR antibodies have a positive MuSK antibody test result.
D .
Anti-striatal antibodies have nothing to do with thymoma.
Although false-positive test results of anti-AChR antibodies are rare, they are found in thymoma without clinical MG, Lambert-Eaton myasthenia syndrome, graft-versus-host disease, autoimmune liver disease, and small It can be observed in cell lung cancer, rheumatoid arthritis and motor neuron disease treated with D-penicillamine
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 Approximately 2%-4% of MG patients with negative AChR binding antibody test results will have regulatory antibodies, which is associated with an increased risk of thymoma
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Up to 73% of patients with MG and thymoma have regulatory antibodies
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 Approximately half of patients with negative anti-AChR antibody test results may have a positive MuSK antibody test result
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These patients constitute a unique MG subgroup, showing different characteristics from patients with anti-AChR antibodies
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 In patients with thymoma and MG younger than 40 years old, the incidence of anti-striatal antibodies is about 70%-80%
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In patients younger than 40 years of age, data suggesting a positive test result for anti-striatal antibodies should suggest a thymoma test
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 The answers to the above 5 questions are D, B, C, C, and B in order.
Did you get them all right? Yimaitong compiled from: Richard Nowak.
Fast Five Quiz: Myasthenia Gravis Workup-Medscape-Jul 08, 2021.