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A 35-year-old female asked:

Is an abnormal rns with a >12% decrement (not done on an area that is really weak) specific for myasthenia gravis? please assume patient has signs and symptoms of generalized mg, as this is the case.

2 doctor answers2 doctors weighed in
Dr. Marisa Johnson
Specializes in Internal Medicine - Nephrology & Dialysis
Maybe: An rns study is considered positive if the decrement is > 10% but a decremental response is not specific for myasthenia gravis. Decrements may be seen in other disorders of neuromuscular transmissions (lamber-eaton myasthenic syndrome, botulism) and motor neuron disease.
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Dr. Bennett Machanic
Neurology 54 years experience
Not necessarily: Really require more than one area of decrement, and even so, nonspecific as can be seen with metabolic problems, muscle disease, neuropathies, and even can be artifactual. Maybe a tensilon test would be helpful, but of course, better approach via anti-neuromuscular junctional antibody or anti-musk results. In the end, if responding to mestinon, (pyridostigmine) may be a good confirmatory sign, also.
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Last updated Apr 14, 2014


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