Myasthenia gravis and Lambert-Eaton syndrome both cause unusual muscle weakness but in different ways. In addition to slight differences in symptoms, they produce different diagnostic test results and require different treatments.

Myasthenia gravis (MG) and Lambert-Eaton syndrome, also known as Lambert-Eaton myasthenic syndrome (LEMS), are two autoimmune disorders that attack your muscles. Both conditions affect muscle strength and function but have different causes and treatments.

In this article, we discuss the similarities and differences between MG and LEMS, including their causes and symptoms, how doctors diagnose and treat them, and the respective outlooks for people with these conditions.

LEMS is a rare autoimmune disorder that affects communication between your nerves and muscles.

In LEMS, your immune system mistakenly attacks the calcium channels in nerve endings, disrupting the release of specialized messenger molecules called neurotransmitters that trigger muscle contraction. As a result, your muscles can’t tighten (contract) properly.

Risk factors for LEMS include smoking and having certain cancers, such as small cell lung cancer (SCLC).

MG is also an autoimmune disorder, but it differs from LEMS in its target.

In MG, your immune system attacks the receptors on muscle cells that receive neurotransmitter signals from nerve endings. This impairs the communication between nerves and muscles.

MG is more common in females under age 40 years and males over age 50 years. Risk factors include a family history of MG and a personal history of autoimmune disease or thymoma.

Both conditions share symptoms like muscle weakness and fatigue, which can come and go. But there are some key differences.

If you have LEMS, your muscle weakness can temporarily improve with repeated activity or when you repeatedly use a specific muscle. But in people with MG, muscle weakness usually worsens with exercise or when you repeatedly use a specific muscle.

In addition, LEMS usually begins with weakness in the legs, which may cause difficulty walking. On the other hand, initial MG symptoms typically affect the eye muscles, which can cause eyelid drooping and vision problems, such as double vision.

To diagnose LEMS and MG, a doctor takes your medical history, performs a physical exam, and orders specialized tests. These tests may include:

  • blood tests to check for specific antibodies characteristic of each condition
  • repetitive nerve stimulation and electromyography to measure muscle and nerve function
  • a CT scan or other imaging tests to check for underlying or related conditions, such as SCLC

There’s no cure for either LEMS or MG, but treatment can help manage symptoms and improve muscle strength.

To treat LEMS, a doctor may recommend the following:

For MG, a doctor may recommend the following:

  • medications, including:
    • acetylcholinesterase inhibitors, such as pyridostigmine (Mestinon)
    • immunosuppressants
    • monoclonal antibodies, such as eculizumab (Soliris)
  • thymectomy to remove the thymus gland
  • IVIG infusions or plasmapheresis
  • exercise appropriate for this condition

LEMS typically responds well to treatment, but your outlook may depend on whether you have an underlying cancer, such as SCLC. People with LEMS who don’t have cancer usually have an average life expectancy.

Treatment is also very effective for MG. Most people with this condition also have an average life expectancy.

Let’s review a few questions people with LEMS and MG frequently ask their doctors.

Why does Lambert-Eaton syndrome get better with exercise?

Exercise can temporarily improve muscle strength in LEMS by increasing the release of neurotransmitters that stimulate muscle contraction. However, this effect is short-lived and doesn’t provide long-term relief from symptoms.

How does repetitive stimulation differentiate between myasthenia gravis and Lambert-Eaton syndrome?

Repetitive nerve stimulation tests assess how muscles respond to repeated electrical impulses.

In MG, these tests typically show decreased muscle function with repeated stimulation. In LEMS, muscle response measured with electrical testing may improve with exercise.

How rare is Lambert-Eaton syndrome?

LEMS is rare. Experts estimate LEMS affects 46 times fewer people than MG. However, it may be underdiagnosed due to its similarity to MG and other neuromuscular conditions.

LEMS and MG may share some similarities in symptoms, but their causes and treatment differ.

If you experience muscle weakness or fatigue, speak with a medical professional as soon as possible. With early treatment and ongoing care, you can live a full life despite the challenges that come with these conditions.