Which nerve stimulation studies are indicated in the workup of Lambert-Eaton myasthenic syndrome (LEMS)?

Updated: May 23, 2019
  • Author: David E Stickler, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Repetitive nerve stimulation (RNS) studies confirm the diagnosis of LEMS by demonstrating characteristic findings (see the image below). Compound muscle action potentials (CMAPs) recorded with surface electrodes are usually small, often less than 10% of normal, and fall during 1- to 5-Hz RNS.

Characteristic responses to repetitive nerve stimu Characteristic responses to repetitive nerve stimulation in patient with Lambert-Eaton myasthenic syndrome. (A) Responses elicited from hand muscle by stimulation of nerve at 3 Hz. Amplitude of initial response is less than normal, and response is decremental. (B) Responses as in A, immediately after voluntary activation of muscle for 10 seconds. Amplitude has increased. (C) Responses in hand muscle elicited by 20-Hz stimulation of nerve for 10 seconds. Response amplitude is less than normal initially, falls further during first few stimuli, then increases and ultimately becomes more than twice initial value.

During stimulation at 20-50 Hz, the CMAP increases in size (ie, facilitation) and characteristically becomes at least twice the size of the initial response. A similar increase in CMAP size is seen immediately after the patient voluntarily contracts the muscle maximally for several seconds (see the image below).

Compound muscle action potentials elicited from ha Compound muscle action potentials elicited from hand muscle before and immediately after maximal voluntary activation of muscle for 10 seconds. Amplitude is small initially, increasing almost 10 times after activation.

In virtually all patients with LEMS, a decremental response to low-frequency nerve stimulation is observed in the hand muscles. This finding is not specific to LEMS and can be seen in MG and other neuromuscular diseases.

In LEMS, the CMAP amplitude is low in most muscles tested. This finding is also nonspecific and is commonly observed in other neuromuscular diseases.

Facilitation greater than 100% is seen in some but not all muscles (or in all patients) with LEMS. Facilitation greater than 50% in any muscle suggests LEMS. However, these findings might also be observed in MG. If facilitation is greater than 100% in most muscles tested or greater than 400% in any muscle, the patient almost certainly has LEMS. If facilitation is less than 50% in all muscles tested, the patient still may have LEMS, especially if weakness has been present for only a short time or the patient has been partially treated.

When LEMS is mild, the electromyography (EMG) findings may resemble those of MG, including normal CMAP amplitudes, decremental response to RNS at low rates, and little facilitation. One helpful feature is that in LEMS, the EMG findings are usually more severe than the clinical findings would suggest. The opposite is frequently true in MG.


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