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Thursday, May 5, 2016
EMG test for ALS vs. MG
Would a patient with untreated MG experience muscle atrophy? If so could an EMG test distinguish between muscle atrophy caused by MG or ALS?
Patients with severe myasthenia gravis that is not well controlled can have muscle atrophy. Occasionally, be atrophy can be fairly prominent. The EMG can be very helpful in distinguishing the two. While both diseases can have fibrillation potentials, the motor unit potentials are usually quite large in ALS while they are small in myasthenia gravis. Also, the firing pattern of the motor unit potentials are different in ALS and myasthenia gravis. The physical examination findings are also helpful in distinguishing one disease from another. Myasthenia gravis does not have exaggerated reflexes or other signs of spasticity that one looks for in ALS. In summary, it is exceptional that the two diseases would be confused. It does require, however, that an experienced neurologist weighs all the evidence in order to come up with the most accurate answer. I have faced cases where I considered both possibilities initially but with careful follow-up and additional testing, the picture became clear. I hope this helps. Best of luck.
John G Quinlan, MD
Professor of Neurology
College of Medicine
University of Cincinnati