What is the role of repetitive nerve stimulation (RNS) in the evaluation of myasthenia gravis (MG)?

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

RNS is abnormal in more than 50% to 70% of patients with generalized MG but are often normal in patients with purely ocular form of MG. During low-frequency (1-5 Hz) RNS, the locally available acetylcholine (ACh) becomes depleted at all neuromuscular junctions (NMJs), and less is therefore available for immediate release. This results in smaller excitatory postsynaptic potentials (EPSPs).

In patients without MG, all EPSPs exceed the threshold to generate an action potential (ie, there is a safety factor). No change in the summated compound muscle action potential (CMAP) is noted. In patients with MG, the number of AChRs is reduced, lowering the safety factor. During RNS, some EPSPs may not reach threshold, which means that no action potential is generated. This results in the decrement in the amplitude of the CMAP.

In patients with myasthenia gravis, this decremental response usually has a maximum decrement at the fourth or fifth response, followed by a tendency toward repair (see the image below). A stimulation rate of 1-5 per second should result in a 10% or more decrease in amplitude by the fourth or fifth action potential; any decrement over 10% is considered abnormal. The most common employed stimulation rate is 3 Hz.

Repetitive nerve stimulation at frequency of 2 Hz Repetitive nerve stimulation at frequency of 2 Hz showing increasing decrement in amplitude of compound muscle action potential up to fourth response (42% amplitude loss), after which it stabilizes.

Patients with MG rarely have a decreased response in a clinically normal muscle. Thus, testing a proximal weak muscle gives a better yield than testing a unaffected distal muscle, even if the latter is technically easier. Testing a facial muscle (eg, the orbicularis oculi) is useful because most patients suffer from eyelid weakness or ptosis. RNS results are less likely to be positive in patients with ocular MG. Facial RNS is especially important to perform in suspected anti-MuSK MG due to much higher facial and bulbar involvement than a limb muscle.


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