Lambert-Eaton Myasthenic Syndrome (LEMS) Workup

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

Approach Considerations

In the emergency setting, very few tests are of importance in regard to Lambert-Eaton myasthenic syndrome (LEMS), because the diagnosis is not made in the emergency department (ED). It would be reasonable, however, to consider basic tests in any patient with cancer who reports weakness and dry mouth. These basic tests would include the following:

  • Complete blood count

  • Basic chemistry

  • Pulse oximetry

Other, more specific tests are ordered as indicated (see below).

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Antibody Assays

Voltage-gated calcium channel antibodies

Antibodies to voltage-gated calcium channels (VGCCs) have been reported in 75-100% of LEMS patients who have small cell lung cancer (SCLC) and in 50-90% of LEMS patients who do not have underlying cancer.

They are also found in fewer than 5% of patients with myasthenia gravis (MG), in up to 25% of patients with lung cancer without LEMS, and in some patients who do not have LEMS but have high levels of circulating immunoglobulins (eg, those with systemic lupus erythematosus or rheumatoid arthritis).

The sensitivity and specificity of the VGCC antibody assay are affected by the source of the antigen and the specific laboratory measuring the antibody.

Reports suggest that SOX1, an immunogenic tumor antigen in SCLC, may play a role in identifying LEMS patients with lung cancer. [6]

Acetylcholine receptor antibodies

ACh receptor (AChR) antibodies are most commonly associated with myasthenia gravis (MG) and are occasionally found in low titers in LEMS. The only true methods of differentiating MG from LEMS are the detection of AChR antibodies and the presence of underlying malignancy.

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Imaging Studies and Bronchoscopy

SCLC is the malignancy most frequently associated with LEMS. In all adult patients with LEMS, diagnostic imaging (eg, computed tomography [CT] or magnetic resonance imaging [MRI]) of the chest for cancer detection should be performed. Screening strategies may help to detect SCLC in patients with newly diagnosed LEMS and therefore offer a better approach to treatment.

If imaging findings are negative in a patient with a substantial risk of having lung cancer, bronchoscopy should be performed. If both imaging and bronchoscopy results are initially negative and risk factors for lung cancer are present, positron emission tomography (PET) scanning should be considered. If all imaging study results are negative in such patients, periodic reassessment thereafter is indicated.

In a large cohort study, Titulaer et al screened for tumors using various methods (CT, radiography,18 F-fluorodeoxyglucose PET (FDG-PET), bronchoscopy, or mediastinoscopy) and found that CT of the thorax detected 93% of the tumors. [7]

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Repetitive Nerve Stimulation Studies

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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Electromyography

Needle electromyography

Conventional needle EMG in LEMS demonstrates markedly unstable motor unit action potentials, which vary in shape during voluntary activation.

Single-fiber electromyography

The jitter and blocking measured by single-fiber EMG is increased markedly in LEMS, frequently out of proportion to the severity of weakness. In many endplates, jitter and blocking decrease as the firing rate increases. This pattern is not seen in all endplates or in all patients with LEMS.

Because jitter and blocking may also decrease at higher firing rates in some endplates of patients with MG, this pattern does not confirm an LEMS diagnosis unless it is dramatic and seen in most muscles.

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Edrophonium (Tensilon) Test

Testing with edrophonium (Tensilon) may be performed to help differentiate LEMS from MG. However, such testing is highly subjective, and it is of little value in the diagnosis of LEMS in the ED.

The test may produce an improvement in strength, but rarely is the response in patients with LEMS as noticeable as the typical response in patients with MG.

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