What suggests segmental demyelination on NCV-
Correct Answer: Decreased CAMP amplitudes
Description: Ans. is 'a' i.e., Decreased CAMP amplitudes * The specific features of segmental demyelination of nerve on nerve conduction studies are:# Conduction Block# Decreased Compound Muscle Action Potentials (CAMP) amplitudes due to abnormal temporal dispersion of CAMP.* Following are the findings of demyelinating lesions on Nerve Conduction Studies:Demyelinating injuriesA) Uniform Demyelinating : In uniform demyelinating lesions, nerve conduction studies (NCS) will be slowed throughout the nerve because the entire nerve is affected. Distal latencies will be prolonged, again because of the uniform slowing (distal latencies representing the most distal conduction velocity). Amplitdes should not be significantly affected, because the axons are intact and the slowing is consistent through all fiber of the nerve.B) Segmental Demyelination: Because there is variable slowing or different nerve fibers withing the nerve, conduction velocity will be slowed and distal latencies will be prolonged in segmentally demyelinatiing injuries. CMAP amplitudes may be decreased because of temporal dispersion, not because of axonal damage. Therefore, the CMAP may be longer in duration. However, the area under the CMAP will be normal.C) Focal Demyelination : In focal demyelination, only one segment of the nerve is affected. Therefore velocity will be normal unless stimulation is across the area of focal demyeclination In this segment, you will see slowing of conduction velocity. Slowing is usually evident if there is a more than 10 m/sec drop in conduction velocity across the segment, compared to the distal or proximal segments. Distal latencies and all amplitudes should be norma.D) Conduction Block : As stated earlier, conduction block is a focal demyelination that is so severe that the action potential cannot propagate past that point. Therefore, distal latencies and conduction velocities remain normal. Distal amplitudes will be normal. However, when the nerve is stimulated across the area of conduction block, a drop in amplitude is noted. To be considered significant, this drop in proximal amplitude should be more than 20% of the distal amplitude. For example if the distal amplitude is 10 m V and proximal amplitude is 7 m V, conduction block should be considered.
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Orthopaedics
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