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.

MG and LEMS share many symptoms, but they have key differences, including other health conditions they’re associated with, potential symptoms and complications, and how they’re treated.

Understanding the differences between MG and LEMS can help you and your healthcare provider arrive at the correct diagnosis and find the most effective treatment for your symptoms.

Here’s more on how MG and LEMS differ, along with common treatments used to manage these two conditions.

Key Differences Between Myasthenia Gravis and LEMS

Myasthenia gravis and Lambert-Eaton myasthenic syndrome are classified as different disorders for several reasons. For example, the two conditions affect different parts of the neuromuscular junction. Here are three other key differences between these neuromuscular diseases.

In about 60 percent of LEMS cases, the person also has cancer — most often small cell lung cancer. Symptoms of LEMS often appear months before cancer is found. When LEMS is linked to cancer, it is considered a paraneoplastic disease.

LEMS that occurs with cancer is usually diagnosed around age 60, mostly in males and in people with a history of smoking, according to StatPearls. LEMS that’s triggered by cancer may go away completely if the cancer is successfully treated.

No cancer is found in about 40 percent of people with LEMS. In these cases, the condition is most likely to be diagnosed around ages 35 and 60. LEMS rarely occurs in children.

Myasthenia gravis isn’t linked to lung cancer in the same way. However, many people with MG have problems with the thymus gland, a small organ in the chest that helps regulate the immune system.

About two-thirds of people with MG have thymic hyperplasia, or overactive thymic cells. About 10 percent develop tumors called thymomas in their thymus gland. Thymomas may grow slowly and remain confined to the thymus, but they can also spread to nearby tissues.

People with MG have a higher chance of experiencing myasthenic crisis — a serious complication in which severe muscle weakness causes life-threatening breathing problems.

Severe respiratory muscle weakness is possible in LEMS, but it’s much less common.

Myasthenic crisis can occur at any stage of myasthenia gravis. It can be triggered by factors such as infection, stress, surgery, or certain medications.

Respiratory failure requires immediate medical attention. First responders need to know if someone with breathing problems has MG so they can provide appropriate care. If you have myasthenia gravis and experience trouble breathing or swallowing, seek emergency care right away.

Keep medical records up to date, and carry or wear a medical alert card or bracelet so that first responders know you have MG. Talk with your healthcare team about emergency planning if you’re living with MG.

Both myasthenia gravis and LEMS can cause muscle weakness, but they typically affect different muscle groups first. MG often starts in the eye and facial muscles. Early symptoms often include diplopia (double vision) and ptosis (drooping eyelids). Trouble swallowing can also occur early.

LEMS usually begins with weakness in the upper legs, which can cause difficulty walking, and later affects the arms. The eye muscles may also be affected, but eye symptoms are generally less prominent in LEMS than in MG.

People with LEMS are also more likely than those with MG to experience symptoms related to the autonomic nervous system (ANS). The ANS is the part of the nervous system that controls automatic functions like digestion, heart rate, and sweating. These symptoms include:

In some cases, ANS symptoms can help neurology specialists tell the difference between MG and LEMS.

Work With Your Healthcare Team

Because myasthenia gravis 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.