myasthenia gravis vs lambert eaton

3 min read 18-03-2025

Myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) are both autoimmune neuromuscular disorders. This means the body's immune system mistakenly attacks parts of the neuromuscular junction, the point where nerves communicate with muscles. However, they differ significantly in their causes, symptoms, and treatment. Understanding these differences is crucial for accurate diagnosis and effective management.

Understanding the Neuromuscular Junction

Before diving into the specifics of MG and LEMS, let's briefly review the neuromuscular junction. This is the specialized synapse where motor neurons release acetylcholine (ACh), a neurotransmitter, to stimulate muscle contraction. Both MG and LEMS involve disruptions to this crucial communication process.

Myasthenia Gravis (MG)

What is it? MG is characterized by fluctuating muscle weakness and fatigue that worsens with activity and improves with rest. The immune system produces antibodies that block or destroy acetylcholine receptors (AChRs) at the neuromuscular junction. This reduces the ability of muscles to contract effectively.

Key Symptoms:

Muscle weakness: Often starts in the muscles controlling eye movement (ptosis, diplopia), facial expression, chewing, swallowing, and speech. It can also affect limb muscles.
Fluctuating weakness: Symptoms vary throughout the day, often worsening with activity and improving with rest.
Fatigue: Significant tiredness and exhaustion are common.
Bulbar symptoms: Weakness in the muscles of the mouth, throat, and face, leading to difficulty swallowing and speaking.

Diagnosis:

Diagnosis often involves a physical examination, electromyography (EMG), and blood tests to detect AChR antibodies. The Tensilon test, which involves administering edrophonium, a short-acting cholinesterase inhibitor, can also be helpful.

Treatment:

Treatment focuses on improving neuromuscular transmission. Options include:

Cholinesterase inhibitors: Medications that prolong the action of ACh at the neuromuscular junction.
Immunosuppressants: Drugs that suppress the immune system to reduce antibody production.
Thymectomy: Surgical removal of the thymus gland, which plays a role in the immune response in some cases of MG.

Lambert-Eaton Myasthenic Syndrome (LEMS)

What is it? LEMS is a rarer autoimmune disorder affecting the presynaptic nerve terminals at the neuromuscular junction. In LEMS, antibodies attack voltage-gated calcium channels on the nerve terminals, reducing the release of ACh.

Key Symptoms:

Proximal muscle weakness: Weakness primarily affects the muscles closest to the body's core (proximal muscles), such as those in the legs and shoulders.
Autonomic symptoms: Dry mouth, constipation, blurred vision, and impotence can occur due to the involvement of autonomic nerves.
Improved muscle strength with repeated contractions: Unlike MG, muscle strength in LEMS often improves with repeated muscle use. This phenomenon is known as "post-tetanic facilitation."
Reflexes: Deep tendon reflexes are often diminished or absent, unlike MG.

Diagnosis:

Diagnosis relies on clinical presentation, EMG studies showing characteristic changes, and the detection of voltage-gated calcium channel antibodies in blood tests.

Treatment:

Treatment aims to increase ACh release and improve neuromuscular transmission. Options include:

Calcium channel blockers: Paradoxically, some calcium channel blockers can improve symptoms.
Immunosuppressants: Similar to MG, immunosuppressants can help suppress the immune response.
Guillain-Barré Syndrome Immunoglobulins: High-dose intravenous immunoglobulin (IVIg) therapy has shown efficacy in managing some LEMS cases.