Lambert-Eaton myasthenic syndrome (LEMS): A neuromuscular disorder frequently mistaken for myasthenia gravis (MG)

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The following content is provided by Catalyst Pharmaceuticals and does not reflect the views of Myasthenia Gravis News or BioNews, Inc.

This article provides information about LEMS. It is intended for people who’ve been diagnosed with MG or seronegative MG and may not be responding to treatment as well as they’d hoped or expected.


Like MG, LEMS is a rare neuromuscular disorder caused by antibodies that disrupt the signals between the body’s nerves and muscles. This breakdown in nerve-muscle communication eventually leads to muscle weakness and fatigue. It can also cause eye problems, such as drooping eyelids and double vision.

All these symptoms are common to both LEMS and MG, and it’s one reason that many LEMS patients are first diagnosed with MG. But these are different disorders caused by different antibodies that require different treatments, so getting the right diagnosis is critical.


Both LEMS and MG are disorders of the immune system. Immune system disorders occur when antibodies—the body’s natural defenses against foreign or invading organisms—begin to disrupt the operation of healthy tissues for unknown reasons.

  • In LEMS, specific antibodies disrupt the ability of nerves to send movement signals to the muscles
  • In MG, a different set of antibodies disrupt the ability of the muscles to receive movement signals from the nerves

Fortunately, the presence of these distinct antibodies in the blood makes it possible to diagnose LEMS or MG with a simple antibody blood test in most cases.


An antibody blood test will yield one of two results, either seropositive or seronegative.

  • If your doctor tested you for MG and the result was seropositive, it means there are detectable levels of antibodies associated with MG in your blood. Combined with your medical history and a physical exam, this is usually enough to confirm a diagnosis of MG
  • If your test result was seronegative, it means that the antibodies that cause MG are not detectable in your blood. This doesn’t rule out MG as the cause of your symptoms; in fact, about 10% to 15% of MG patients are seronegative. However, if your symptoms have not improved or have gotten worse after starting treatment for MG, you may benefit from having a LEMS antibody test to see if that may be the true cause of your muscle weakness.


If you’ve been diagnosed with seronegative MG but are still experiencing symptoms even after starting treatment, ask your doctor about LEMS and whether you’ve had or might benefit from a LEMS antibody test. Even though the disorders are similar, the treatment for each is different. Ensuring you have the right diagnosis is the key to getting the best outcome from your treatment.

To learn more about LEMS, visit or visit @beLEMSaware on Facebook, Instagram, or Twitter.

LEMS Aware serves as a resource for the LEMS community, providing LEMS patients with the opportunity to further their knowledge of LEMS through educational content and other resources.

© 2023 Catalyst Pharmaceuticals, Inc. All Rights Reserved.

LEM-0005 August 2023


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National Institutes of Health: National Institute of Neurological Disorders and Stroke. Myasthenia Gravis Fact Sheet. Accessed May 6, 2022.

National Organization for Rare Disorders (NORD) website. Rare disease database: Lambert-Eaton myasthenic syndrome. Accessed August 18, 2022.

National Organization for Rare Diseases (NORD). Rare Disease Database: Myasthenia gravis. Accessed May 2, 2022. 

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