Myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) are two rare diseases that affect the neuromuscular junction — the space where nerves communicate with muscles. Because they impact muscle function, they share many symptoms, including muscle weakness and fatigue (tiredness that doesn’t improve with rest).

However, myasthenia gravis and LEMS also differ in a few key ways, including how they develop and who they most affect. Understanding the nuances can help you and your healthcare provider pinpoint the correct diagnosis and find the most effective treatment.

How Are These Conditions Related?

Myasthenia gravis and LEMS have quite a bit in common — so much so that doctors sometimes have trouble telling these conditions apart. Both can get worse over time and have a major impact on quality of life.

Fortunately, myasthenia and LEMS also share several treatment options that can help manage symptoms, sometimes leading to long-term remission (a period of fewer or no symptoms). When MG and LEMS are well controlled, people with these conditions can have a life expectancy similar to that of the general population.

How Are Myasthenia Gravis and LEMS Similar?

Here are some key ways myasthenia gravis and Lambert-Eaton myasthenic syndrome overlap.

1. Both Are Autoimmune Diseases

LEMS and nearly all MG cases are autoimmune disorders, meaning the immune system mistakenly attacks the body’s own tissues. Researchers don’t yet know exactly why some people develop autoimmune diseases, but they believe it’s likely due to a combination of genes and environmental factors.

Normally, the immune system protects the body by targeting harmful invaders, such as viruses. But in autoimmune diseases, the immune system launches a misguided attack on the body’s healthy tissues. Treatment generally involves tamping down the immune response to help prevent long-term damage.

Most MG cases are considered autoimmune, but a few rare subtypes are not. Neonatal myasthenia occurs when a baby is exposed to certain antibodies in the womb. Symptoms usually disappear around 3 months of age. Congenital myasthenia, another rare subtype, is caused by inherited genetic mutations (DNA changes passed down by parents) rather than an immune system attack.

2. Both Affect the Way Nerves Communicate With Muscles

A major reason it can be difficult to distinguish myasthenia gravis from LEMS is that both conditions cause problems with the neuromuscular junction. At the neuromuscular junction, nerve cells send chemical signals (including acetylcholine) to muscle cells, telling them when to contract and relax. This process controls both conscious, voluntary movements (like kicking a soccer ball or raising your hand) and involuntary ones (like moving the diaphragm to breathe).

In autoimmune MG, the immune system produces antibodies that attack acetylcholine receptors on muscle cells. In LEMS, antibodies block calcium channels at the nerve endings, reducing acetylcholine release. In both cases, fewer messages reach the muscles, leading to weakness and fatigue.

3. Muscle Weakness and Fatigue Are Primary Symptoms for Both

The hallmark symptoms of both MG and LEMS are muscle weakness and fatigue. People with either condition may notice that they’re not as strong as they once were, and their muscles tire quickly, making daily tasks harder. Common challenges include difficulty walking, reduced stamina, and trouble forming facial expressions.

How Do Myasthenia Gravis and LEMS Differ?

Myasthenia gravis and Lambert-Eaton myasthenic syndrome are classified as different disorders for several reasons. Here are some key differences between these neuromuscular diseases.

1. LEMS Is Often Linked With Cancer

In about 60 percent of LEMS cases, the person also has cancer — most often small cell lung cancer. However, symptoms of LEMS can appear months before cancer is diagnosed. This type of LEMS is usually found around age 60, mostly in men and in people with a history of tobacco use, according to Cleveland Clinic. Fortunately, LEMS triggered by cancer may go away completely if the cancer is successfully treated.

It’s also possible to have LEMS without cancer. In these cases, the condition is most likely to be diagnosed at two age peaks — around 35 and 60.

Myasthenia gravis, on the other hand, isn’t linked to lung cancer. However, around 10 percent of people with MG develop tumors in their thymus gland (a small organ in the chest that helps regulate the immune system). These tumors, called thymomas, may be benign (noncancerous) growths.

2. Myasthenia Gravis Brings a Higher Risk of Respiratory Failure

People with MG have a higher chance of experiencing myasthenic crisis — a serious complication that can cause life-threatening breathing problems. This type of respiratory failure is much less common in LEMS. Myasthenic crisis can occur at any stage of myasthenia gravis, though the risk may rise with disease progression or inadequate treatment.

Respiratory failure is treatable but requires immediate medical attention. First responders must know if someone with breathing problems has MG so they can provide the right treatment. Keeping medical records up to date and carrying or wearing a medical alert card or bracelet can help ensure this information is available in an emergency. If you have myasthenia gravis and experience breathing concerns, be sure to tell your doctor right away.

3. Some Symptoms Progress Differently

Both myasthenia gravis and LEMS can cause muscle weakness and fatigue, but they typically affect different muscle groups first. MG often starts with eye and facial muscles, and early symptoms include ptosis (eyelid drooping) and trouble swallowing. LEMS usually begins with weakness in the upper legs and arms. The eye muscles can also be affected, but eye symptoms are less prominent in LEMS than in MG.

In addition, people with LEMS are more likely than those with MG to experience symptoms related to the autonomic nervous system — the part of the nervous system that controls automatic functions like digestion, heart rate, and sweating. These symptoms include:

• Constipation
• Dry mouth and eyes
• Erectile dysfunction
• Reduced ability to sweat

In some cases, these symptoms help neurology teams (specialists who diagnose and treat nerve and brain disorders) tell the difference between MG and LEMS.

Work With Your Healthcare Team

Because MG and LEMS share many symptoms, getting an accurate diagnosis is essential for finding the right treatment. If you’ve been diagnosed with MG or LEMS, work closely with your healthcare team to monitor symptoms, adjust treatment as needed, and address any complications early. Staying informed and proactive can help you manage your condition and maintain a good quality of life.

Talk With Others Who Understand

On MGteam, the social network for people and their loved ones living with myasthenia gravis, more than 2,000 members come together to ask questions, give advice, and share their stories with others who understand life with myasthenia gravis.

Did you have trouble getting diagnosed with MG or LEMS? Share your experience in the comments below, or start a conversation by posting on your Activities page.