Repetitive nerve stimulation studies are performed in a patient with fatigable weakness. Low-frequency stimulation produces a decremental response, while high-frequency stimulation produces a marked incremental response exceeding 100 percent. Which diagnosis fits best?
- A Motor neuron disease
- B Myasthenia gravis
- C Lambert-Eaton myasthenic syndrome ✓
- D Myotonic dystrophy
Correct answer: C. Lambert-Eaton myasthenic syndrome
Explanation
An increment greater than 100 percent on high-frequency stimulation reflects increased calcium entry into the presynaptic terminal during rapid firing and is characteristic of Lambert-Eaton myasthenic syndrome. In myasthenia gravis the defect is postsynaptic, so low-frequency stimulation shows decrement but high-frequency stimulation does not produce facilitation. Motor neuron disease and myotonic dystrophy do not show this pattern of neuromuscular junction facilitation.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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