myasthenia gravis vs lambert eaton

3 min read 19-03-2025

Myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) are both autoimmune neuromuscular disorders causing muscle weakness. However, they differ significantly in their symptoms, causes, and responses to treatment. This article will delve into the key distinctions between these two conditions.

Understanding Neuromuscular Junction Dysfunction

Both MG and LEMS involve problems at the neuromuscular junction (NMJ), the point where nerve signals are transmitted to muscles. The NMJ relies on a neurotransmitter called acetylcholine (ACh) to trigger muscle contraction. In both conditions, this crucial communication process is disrupted, leading to weakness. However, how this disruption occurs is the key difference.

Myasthenia Gravis (MG): A Deficiency of Acetylcholine Receptors

Myasthenia gravis is characterized by the body's immune system attacking the acetylcholine receptors (AChRs) at the NMJ. This leads to a reduced number of functional receptors, impairing the transmission of nerve impulses.

Key Features of Myasthenia Gravis:

Fluctuating muscle weakness: Weakness worsens with exertion and improves with rest.
Symptoms often begin in the eyes: Ptosis (drooping eyelids) and diplopia (double vision) are common initial signs.
Weakness progresses to other muscles: This can include facial muscles, swallowing muscles, limb muscles, and respiratory muscles.
Symptoms are worse later in the day: Muscle fatigue accumulates throughout the day.
Diagnosis often involves an edrophonium test: This test involves administering a medication that temporarily improves muscle strength.
Treatment focuses on improving ACh transmission: Medications like pyridostigmine and immunosuppressants are commonly used. Thymectomy (surgical removal of the thymus gland) may also be beneficial.

Common Symptoms of Myasthenia Gravis:

Eye muscle weakness: Drooping eyelids (ptosis), double vision (diplopia)
Facial muscle weakness: Difficulty smiling, chewing, speaking
Limb weakness: Difficulty raising arms, walking, climbing stairs
Respiratory muscle weakness: Shortness of breath, difficulty breathing
Swallowing difficulties: Dysphagia

Lambert-Eaton Myasthenic Syndrome (LEMS): A Deficiency of Calcium Channels

Lambert-Eaton myasthenic syndrome (LEMS) is different. In LEMS, the body's immune system attacks the voltage-gated calcium channels (VGCCs) at the NMJ. These channels are crucial for the release of acetylcholine from nerve endings. Damage to these channels reduces ACh release, leading to muscle weakness.

Key Features of Lambert-Eaton Myasthenic Syndrome:

Proximal muscle weakness: Weakness starts in the muscles closest to the body (proximal muscles) – legs and arms.
Muscle weakness improves with exercise: This is the opposite of MG, where weakness worsens with exertion.
Autonomic nervous system symptoms: These can include dry mouth, constipation, and impotence.
Often associated with small cell lung cancer (SCLC): LEMS is frequently linked to SCLC, especially in men.
Diagnosis often involves electromyography (EMG) and antibodies tests: EMG can reveal characteristic changes related to reduced ACh release.
Treatment often includes medications to enhance ACh release: This can include 3,4-diaminopyridine (3,4-DAP) or aminopyridines. Immunosuppressants may also be helpful.

Common Symptoms of Lambert-Eaton Myasthenic Syndrome:

Proximal muscle weakness: Difficulty standing up from a chair, climbing stairs, lifting objects.
Dry mouth
Constipation
Impotence
Decreased reflexes

Distinguishing Between MG and LEMS: A Summary Table

Feature	Myasthenia Gravis (MG)	Lambert-Eaton Myasthenic Syndrome (LEMS)
Onset of weakness	Fluctuating, worsens with activity, improves with rest	Improves with activity, worsens with rest
Muscle involvement	Often begins in eye muscles, then spreads	Typically proximal muscles (legs, arms)
Autonomic symptoms	Usually absent	Often present (dry mouth, constipation, etc.)
Associated cancer	Less common	Frequently associated with SCLC
Response to edrophonium test	Usually positive	Usually negative
EMG findings	Decreased amplitude of CMAP	Decreased amplitude of CMAP, with incremental response to repetitive stimulation

Conclusion: Seeking Professional Diagnosis

Both myasthenia gravis and Lambert-Eaton myasthenic syndrome are serious conditions requiring medical attention. While this article highlights key differences, accurate diagnosis requires a thorough neurological examination, including electromyography (EMG), nerve conduction studies, and blood tests to detect specific antibodies. If you experience muscle weakness or other symptoms described here, it is crucial to consult a neurologist for proper evaluation and treatment. Remember, early diagnosis and management significantly improve prognosis and quality of life for individuals affected by MG and LEMS.