Introduction

If you’ve been dealing with ongoing muscle weakness, tiredness, or difficulty moving, you might have heard of two uncommon conditions called Lambert-Eaton Myasthenic Syndrome (LEMS) and Myasthenia Gravis (MG). These conditions affect how your nerves and muscles work together, leading to symptoms like weakness and fatigue. Although they may seem similar at first, there are important differences between them. This article will explain how LEMS and MG are different, what they could mean for your health, and what you can do if you or someone you care about is affected.

Both LEMS and most MG cases are autoimmune disorders. This means your immune system, which normally protects you from illness, wrongly attacks healthy parts of your body. Experts still are not sure exactly why this happens, but it’s likely a mix of inherited traits and environmental triggers.

In both conditions, the problem lies at the connection point between nerves and muscles, called the neuromuscular junction. Here, nerve cells release chemicals like acetylcholine that signal muscles to move. This process is essential for everyday actions such as walking, lifting objects, and even breathing. When this connection is disrupted, the muscles don’t receive the right signals, which leads to the symptoms you may be experiencing.

Understanding the distinction between LEMS and MG is crucial for receiving the right diagnosis and treatment. Although they may appear similar at first, they have different causes, symptoms, and responses to treatment. This article will empower you to recognise these differences and discuss them confidently with your healthcare provider.

What is Lambert-Eaton Myasthenic Syndrome?

LEMS is a rare autoimmune disorder. This means your immune system, which is supposed to defend your body from harmful invaders like viruses or bacteria, mistakenly targets your own tissues. In LEMS, the immune system mistakenly targets special channels on the ends of nerve cells called voltage-gated calcium channels (VGCCs). These channels are crucial because they help release acetylcholine, the chemical that activates muscle movement.¹

When these calcium channels are damaged, less acetylcholine is released, making it harder for the muscles to contract, which causes weakness. This weakness usually begins in your upper legs but can go on to involve other muscle groups in your arms, as well as muscles involved in breathing, swallowing, and speech.

One important thing to know is that LEMS is often associated with an underlying form of cancer, most commonly small-cell lung cancer. This cancer-related version is called paraneoplastic LEMS and accounts for about 50–60% of LEMS cases.² LEMS is rare in children.

What are the symptoms of LEMS?

If you are affected, you may notice weakness or fatigue in your legs, hips, or lower body. You might find yourself struggling to stand up from a low chair or feeling unsteady on your feet. However, for some people, their strength might improve after they use their muscles.³

In addition to this, other symptoms include:

• Fatigue, especially in the lower limbs
• Dry mouth and sometimes dry eyes
• Climbing stairs or walking longer distances can be hard
• Difficulty lifting objects or raising arms
• Muscle aches or cramping
• Problems with speech, swallowing, or breathing

It’s also worth noting that LEMS rarely affects the eyes early on, which is one key difference from Myasthenia Gravis (MG).

What is myasthenia gravis?

Like LEMS, MG is an autoimmune disease. However, in MG, the immune system attacks the acetylcholine receptors found on the muscle side of the nerve-muscle connection, rather than the nerve endings. Without enough functioning receptors, the muscles can’t respond properly to nerve signals. This leads to muscle weakness that can get worse with activity and improves with rest.

MG can affect anyone, but it most commonly occurs in women under 40 and men over 60. It is linked with the thymus gland. Some people with MG have a tumour in the thymus (called a thymoma), and removing this gland can sometimes improve the condition.⁴

What are the symptoms of MG?

Early symptoms of MG usually appear suddenly. Unlike LEMS, MG causes weakness that gets worse with effort and better with rest. The strength of muscle weakness often varies from one day to another. Most people feel strongest at the start of the day and weakest at the end.⁵

MG primarily affects the muscles in your eyes, face, neck, arms, and legs, causing weakness. Other common early signs include:

• Fatigue
• Droopy eyelids (ptosis)
• Blurry or double vision
• Limited facial expressions
• Difficulty speaking, swallowing, or chewing
• Trouble walking

How are these conditions diagnosed?

Since the symptoms of LEMS and MG can overlap, it’s essential to undergo tests to confirm the diagnosis. Your doctor will likely start with a physical examination, followed by specialised tests.

Blood Tests

• LEMS: Blood tests may detect antibodies to VGCCs. About 85% of people have these antibodies in their blood if they have this condition¹
• MG: Tests often find antibodies to the acetylcholine receptor (AChR) or to a related protein called MuSK.⁵ About 85% of people with MG have AChR antibodies, and about 6% have MuSK antibodies

Electromyography (EMG) – To measure the muscles' response to nerve stimulation.

• LEMS: EMG may show a "waxing" pattern — where muscle strength increases with repeated stimulation⁶
• MG: EMG usually shows a "waning" pattern — where muscle response decreases over time⁶

Imaging tests

• LEMS: An MRI or CT scan can be used to look for signs of lung cancer since LEMS is sometimes linked to small-cell lung cancer
• MG: An MRI or CT scan can check for a thymus gland tumour

How Are LEMS and MG Treated?

Treating LEMS

If LEMS is associated with cancer, it must be treated with chemotherapy, radiation, or surgery. Immunosuppressive medications such as corticosteroids or azathioprine may also be used to reduce the immune system’s attack.⁷

Beyond cancer treatment, most therapies for LEMS focus on symptom relief using medication. Several medicines are used to manage LEMS, most of which aim to boost acetylcholine release from the nerve endings.

The most commonly used medication is Amifampridine (Firdapse®) or Guanidine, which improves muscle strength and nerve signalling.⁸ Another option is pyridostigmine (Mestinon®), which increases the amount of acetylcholine available at the neuromuscular junction.⁸

Treating MG

Currently, there is no cure for MG, but effective treatments are available to help manage symptoms. The first line of treatment is usually pyridostigmine, which increases acetylcholine activity. Many patients also need immunosuppressive drugs to reduce the production of harmful antibodies.⁹

If a thymoma is present, surgical removal of the thymus (thymectomy) can offer long-term benefits, especially in younger patients.⁴ In more severe cases, treatments like intravenous immunoglobulin (IVIG) or plasmapheresis (a procedure that filters harmful antibodies from the blood) may be used during a crisis or flare-up.

Key differences between LEMS and MG

Although both LEMS and MG cause muscle weakness, they do so in distinct ways. Here are some key differences:

• Site of attack: LEMS affects the nerve endings; MG affects the muscle receptors
• Pattern of weakness: LEMS usually starts in the legs and may improve with use; MG often starts in the eyes and gets worse with activity
• Associated conditions: LEMS is often linked to lung cancer; MG is sometimes linked to thymus abnormalities
• Respiratory failure: People with MG have a higher chance of experiencing life-threatening breathing problems; this is much less common in LEMS
• Symptoms: Dry mouth is more common in LEMS; eye problems are more common in MG 

Summary

There are two uncommon conditions that you may be familiar with if you have been experiencing unusual fatigue, weakness, or difficulty moving: Myasthenia Gravis (MG) and Lambert-Eaton Myasthenic Syndrome (LEMS). Because they are autoimmune diseases, your immune system is mistakenly targeting certain parts of the body. The attack at the ends of nerves in LEMS prevents the muscles from receiving the signals necessary for correct movement. Often beginning in the legs, this could be connected to a particular kind of lung cancer. In MG, the immune system targets the muscles, particularly those surrounding the throat, face, and eyes. This may result in double vision, drooping eyelids, or trouble speaking and swallowing. Your doctor may perform scans, nerve tests, and blood tests to distinguish between these two conditions. These are useful for checking for antibodies, cancerous signs, and other related issues. 

Although there currently isn’t a cure, immune-based therapies, medication, and surgery can help control symptoms and increase muscle strength. In order to begin the most effective treatment plan for your condition and feel more in control of your health, it is imperative that you receive the correct and timely diagnosis.