Myasthenia gravis vs Lambert-Eaton syndrome
How to tell Myasthenia gravis and Lambert-Eaton syndrome apart when a board question makes them look alike.
Step 2 CK pairs are in open beta while clinical review finishes. Spot an issue? Tell us.
Reflexes that are reduced at baseline with post-exercise facilitation point to Lambert-Eaton syndrome over myasthenia gravis.
Side by side
Fluctuating ptosis and diplopia that worsen with repeated use and improve with rest supports myasthenia gravis.
Proximal weakness with reduced reflexes and transient strength facilitation after repeated contraction suggests Lambert-Eaton syndrome.
Postsynaptic acetylcholine receptor autoantibodies reduce neuromuscular transmission efficiency.
Presynaptic P/Q-type calcium channel autoantibodies reduce acetylcholine release at the neuromuscular junction.
Acetylcholine receptor binding antibodies are often positive, and repetitive nerve stimulation typically shows a decremental response greater than 10% at low frequency.
P/Q-type voltage-gated calcium channel antibodies are common, and repetitive stimulation shows low baseline CMAP with more than 100% increment after high-frequency stimulation or brief exercise.
The trap
The common miss is calling all fatigable weakness myasthenia gravis. Autonomic symptoms and areflexia are the clue that should trigger Lambert-Eaton evaluation and malignancy search.
The repair move
Next best step: confirm neuromuscular junction subtype with antibody and electrodiagnostic testing, then treat the immediate weakness burden and underlying cause. For Lambert-Eaton syndrome, begin cancer evaluation and treatment plus symptomatic amifampridine 15 to 30 mg orally three to four times daily when available; for myasthenia gravis with bulbar or respiratory compromise, admit and start IVIG 0.4 g/kg/day for 5 days or plasmapheresis.
Try a free question
CoreA 58-year-old smoker has proximal leg weakness, dry mouth, and reduced deep tendon reflexes that improve briefly after repeated contractions. What is the next best step in management?
Show answer and explanation
Answer: A. Order chest imaging to evaluate for small cell lung carcinoma. Clinical findings are classic for Lambert-Eaton syndrome, where malignancy search is a central immediate management step.
Why the other choices are tempting
- B. Wrong because pyridostigmine alone does not address the high-yield cancer association.
- C. Wrong because objective neuromuscular findings are not consistent with functional neurologic disorder.
- D. Wrong because empiric immunosuppression should not replace syndrome confirmation and malignancy evaluation.
- E. Wrong because acetazolamide has no primary role in neuromuscular junction autoimmunity.
Flashcards
Tap a card to flip it.
Which bedside reflex pattern helps separate Lambert-Eaton syndrome from myasthenia gravis?
Reduced reflexes that transiently improve after exercise favor Lambert-Eaton syndrome.
What ocular pattern is most characteristic of myasthenia gravis?
Fluctuating ptosis or diplopia that worsens throughout the day is classic for myasthenia gravis.
Quick answers
What is the fastest way to tell Myasthenia gravis from Lambert-Eaton syndrome?
Reflexes that are reduced at baseline with post-exercise facilitation point to Lambert-Eaton syndrome over myasthenia gravis.
What is the key clue for Myasthenia gravis?
Fluctuating ptosis and diplopia that worsen with repeated use and improve with rest supports myasthenia gravis.
What is the key clue for Lambert-Eaton syndrome?
Proximal weakness with reduced reflexes and transient strength facilitation after repeated contraction suggests Lambert-Eaton syndrome.
What is the common trap with Myasthenia gravis vs Lambert-Eaton syndrome?
The common miss is calling all fatigable weakness myasthenia gravis. Autonomic symptoms and areflexia are the clue that should trigger Lambert-Eaton evaluation and malignancy search.