Myasthenia gravis vs Lambert-Eaton syndrome
How to tell Myasthenia gravis and Lambert-Eaton syndrome apart when a board question makes them look alike.
MG worsens with use and is postsynaptic; Lambert-Eaton has post-exercise facilitation and is presynaptic.
Side by side
Fluctuating ocular or bulbar weakness that worsens with repeated use.
Proximal leg weakness with autonomic symptoms, small-cell lung cancer association, and post-exercise facilitation of strength or reflexes.
Autoantibodies target postsynaptic acetylcholine receptors or related junction proteins.
Autoantibodies target presynaptic voltage-gated calcium channels.
Decremental response on repetitive stimulation supports myasthenia gravis.
Incremental response after exercise or high-frequency stimulation supports Lambert-Eaton; evaluate for an underlying small-cell lung cancer pattern.
The trap
Both can cause proximal weakness; fatigability direction and autonomic findings separate the pair.
Watch it explained
The repair move
Use fatigability direction and reflex response: worse with use means MG; brief improvement after contraction plus autonomic symptoms suggests Lambert-Eaton.
Try a free question
CoreA patient has ptosis and diplopia that worsen at the end of the day. Reflexes are normal and weakness worsens with repeated use. Which disorder is most likely?
Show answer and explanation
Answer: A. Myasthenia gravis. Fatigable ocular weakness that worsens with use points to myasthenia gravis.
Why the other choices are tempting
- B. Lambert-Eaton usually improves briefly with use and often begins in proximal legs.
- C. Botulism causes descending paralysis with autonomic symptoms.
- D. GBS causes ascending weakness with areflexia.
- E. ALS causes UMN and LMN signs without ocular fatigability as the core clue.
Flashcards
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What is the fastest separator for Myasthenia gravis vs Lambert-Eaton syndrome?
MG worsens with use and is postsynaptic; Lambert-Eaton has post-exercise facilitation and is presynaptic.
What clue should make you choose Myasthenia gravis?
Fluctuating ocular or bulbar weakness that worsens with repeated use.
Quick answers
What is the fastest way to tell Myasthenia gravis from Lambert-Eaton syndrome?
MG worsens with use and is postsynaptic; Lambert-Eaton has post-exercise facilitation and is presynaptic.
What is the key clue for Myasthenia gravis?
Fluctuating ocular or bulbar weakness that worsens with repeated use.
What is the key clue for Lambert-Eaton syndrome?
Proximal leg weakness with autonomic symptoms, small-cell lung cancer association, and post-exercise facilitation of strength or reflexes.
What is the common trap with Myasthenia gravis vs Lambert-Eaton syndrome?
Both can cause proximal weakness; fatigability direction and autonomic findings separate the pair.