Review Note
Last Update: 04/01/2024 02:31 AM
Current Deck: Neurology::Prior Rite Questions
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Text
A 60-year-old man presents with a 4-month history of progressive proximal arm and leg weakness and dry mouth. Nerve conduction studies reveal normal sensory responses and reduced compound muscle action potential amplitudes. With 2-Hz repetitive stimulation, there is a 15% amplitude decrement, and after 1 minute of exercise, there is a 100% amplitude increment. Which of the following medications has been demonstrated to improve strength in patients with this condition?
- 3,4-diaminopyridine
- Edaravone
- Eteplirsen
- Pyridostigmine
- Riluzole
Answer: {{c1::3,4-diaminopyridine.}}
Notes
Explanation of the Correct Answer:
- The clinical presentation and nerve conduction study findings are suggestive of Lambert-Eaton myasthenic syndrome (LEMS).
- In LEMS, there is a characteristic decrement in compound muscle action potential amplitudes with low-frequency repetitive nerve stimulation and a significant increment after exercise.
- 3,4-diaminopyridine is used in the treatment of LEMS. It works by blocking potassium channels on the presynaptic nerve terminal, which prolongs the action potential and increases calcium influx, thereby enhancing acetylcholine release into the synaptic cleft and improving neuromuscular transmission.
Analysis of Incorrect Answers:
- Edaravone:
- Used in the treatment of amyotrophic lateral sclerosis (ALS), not effective for LEMS.
- Eteplirsen:
- Used to treat Duchenne muscular dystrophy by skipping exon 51 during mRNA processing; not relevant for LEMS.
- Pyridostigmine:
- Although used in myasthenia gravis (which has a clinical presentation somewhat similar to LEMS), it is less effective in LEMS as the primary issue is with presynaptic calcium channels rather than acetylcholinesterase activity at the neuromuscular junction.
- Riluzole:
- Used in ALS to slow down disease progression; not effective in LEMS.
Pathophysiology Outline:
- Lambert-Eaton Myasthenic Syndrome (LEMS):
- Etiology: Often associated with small cell lung cancer as a paraneoplastic syndrome, though it can also be autoimmune in nature.
- Mechanism: Autoantibodies target voltage-gated calcium channels on presynaptic nerve terminals, leading to reduced acetylcholine release.
- Progression: This leads to muscle weakness, typically starting in the proximal muscles and often accompanied by autonomic symptoms like dry mouth.
Use of Analogies:
- Think of the neuromuscular junction as a conversation between two people (the nerve and the muscle). In LEMS, the nerve (presynaptic terminal) speaks too softly (reduced acetylcholine release), making it hard for the muscle (postsynaptic receptor) to hear and respond. 3,4-diaminopyridine works like a microphone, amplifying the nerve's message, thus improving the muscle's ability to hear and respond (increased muscle strength).
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