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Myasthenia Gravis

Q & A: Myasthenia gravis is a chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles, which are responsible for breathing and moving parts of the body, including the arms and legs.

Myasthenia gravis is a chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles, which are responsible for breathing and moving parts of the body, including the arms and legs.

Burning and Weakness: Peripheral Neuropathy vs. Myasthenia

Question: If a patient presented with leg burning, bilateral, and leg weakness, would not an EMG/nerve conduction study be one of the first tests a doctor might do?

Answer: Your symptoms are a definite indication for EMGs. The burning feet are due to what we call a small fiber neuropathy, usually seen in patients with Diabetes or nutritional disorders. Symptoms of Myasthenia include fatiguability, double vision, droopy eyelids etc. It is important to find out how sure is the Doctor who did the study on you of the Myasthenia diagnosis, or if he/she needs to refer you to a specialist in this disease, because if you have it, you need to be treated.

EMG Assessment for Myasthenia Gravis

Question: I was diagnosed with myasthenia gravis with a positive aChR antibody test and positive tensilon test. If my EMG should be negative, will I be diagnosed as NOT having MG?

Answer 1: A positive antibody test is a very specific indication that it is Myasthenia. Repetitive stimulation needs to be performed in an involved muscle in order for it to be positive (it is important that you do not take the Mestinon prior to the test). A more sensitive test for Myasthenia is called Single Fiber EMG which has a higher diagnostic yield than repetitive stimulation.

Answer 2: You are already diagnosed as MG. Therefore, it seems that the reason behind repetitive nerve stimulation test is to check the degree of weakness or probably trying to reduce your medication. He advised you to stop mestinon before the test because it affects the result. A normal repetitive nerve stimulation test (decrement test) does not mean that you do not have MG. We know the test can be normal in MG.

Understanding MCD and MSD Results

Question: Can you explain what MCD and MSD mean in Single Fiber EMG results?

Answer: The neuromuscular junction is concerned with impulse transmission from the nerve to muscle. Normally, it takes time, and this time is calculated very precisely for each muscle in normal persons. Normally there is little variation in this transmission. It is termed jitter—there is normally a jitter up to a limit, once exceeded it is considered as abnormal for that muscle.

This is calculated mathematically using:

  • MCD (Mean value of Consecutive Differences)
  • MSD (Mean Sorted-data Difference)

Standard Deviation (SD) was not used because it would give erroneous results. All these are done nowadays by computer analysis in the EMG machine. Practically the MCD value is used. The normal value for MCD (jitter) for EDC muscle is 40 μs for the axonal stimulation method.

Attacks of Fasciculations and Nystagmus in MG Patient

Question: I have muscle fasciculations every day. What is happening when this activity becomes amplified with fasciculations occurring all over the body, and severe nystagmus?

Answer: Mestinon overdosage is known to cause fasciculation and urinary retention. But severe nystagmus is an unusual feature. I would recommend seeing a neurologist with MRI of the brain. Also, he would check if the dose of Mestinon is appropriate or not.

Tensilon Swallowing Test in MG

Question: I am going for a tensilon swallowing test. What kind of test is that?

Answer: Tensilon test in general is used in diagnosis of MG. To be specific, if a patient complains of swallowing difficulty caused by MG, then a test injection of Tensilon should improve your swallowing. This is done by asking the patient to drink water before and after the injection. This is the meaning of Tensilon swallowing test.

Arm Tested Weak but EMG Tested Negative

Question: My arm on testing began drooping after a few seconds, but my EMG was negative. Is it black and white—negative EMG means negative myasthenia?

Answer: The answer is yes and no. It is difficult to have black and white in medicine, although we wish to.

We have two kinds of EMG tests used for Myasthenia Gravis (MG):

  1. Repetitive Nerve Stimulation (RNS) - Positive in about 55-77%, but it correlates very well with the severity of MG
  2. Single Fiber Electromyography (SFEMG) - More sensitive at 77-100%

If the test is done by an experienced electromyographer and the arm is weak, then a negative SFEMG test in that weak arm means no MG.

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