Demyelination

1. What is conduction block and how do you recognize it?
2. What are 2 common causes of demyelination?
3. What are the NCS findings of demyelination?
4. What are the EMG findings of demyelination?
5. How long does recovery take and what do subsequent NCSs show?

Answers:
1. Conduction block is the failure of an AP to get past an area of demyelination even though axons are intact. It presents as a >50% block in CMAP amplitude going from distal to proximal.
2. Compression and peripheral neuropathies.
3. NCS shows prolonged latency, decreased amplitude across the site of injury, increased temporal dispersion, and decreased conduction velocity.
4. EMG shows normal insertional activity and MUAPs with decreased recruitment. There may be myokymia.
5. Once the insult is removed, recovery takes a few weeks. The new myelin is thinner with shorter internodal distances, resulting in slower than normal CV.