How is tetanic contraction of the muscle used in repetitive nerve stimulation (RNS) for myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas Jowkar, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Tetanic contraction of the muscle can be achieved by applying electrical stimulation to the nerve at a rate of 50 per second for 20-30 seconds. However, this is painful. Voluntary contraction of the muscle for 10 seconds at the maximum force can achieve the same goal without discomfort and is preferred. This principle is utilized in RNS studies. If on slow RNS (3 Hz) there is no significant decrement (<10%) on RNS at baseline, the patient should peform maximual voluntary contraction of the muscle being tested for 1 minute. This is followed by RNS immediately and at 1-minute intervals for the next 4 minutes, looking for a >10% CMAP decrement that results from post-exercise exhaustion. If at any time, either at baseline or following exercise, a significant decrement (>10%) develops, the patient should perform a brief 10 seconds of maximum voluntary contraction of the muscle being tested. Immediately following this maneuver, slow RNS is performed looking for an increment in the CMAP that suggests post-exercise facilitation or “repair” of decrement. This finding should be demonstrated in at least two nerves for the definite diagnosis of a neuromuscular transmission defect.

It is useful to get a baseline RNS on patients who are clinically and have serological positive status for MG. Typically, these patients who are initially weak will on slow RNS (3Hz) demonstrate significant decrement (>10%). When such patients who are on treatment return later at some point with worsening of symptoms, a normal RNS (showing no decrement) may be helpful in ruling out worsening of weakness due to myasthenia.


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