Why do demyelinating peripheral lesions a decrease in CMAP/SNAP amplitudes across the area of focal demyelination ("conduction block")? I know very well that this is the common finding/teaching, but it occurred to me after a discussion with a medical physicist at my institution that this is hard to make sense of this in light of the teaching that amplitude reflects axonal integrity alone.
Conduction block
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From what I've been taught, a pure demyelinating lesion will cause increased temporal dispersion of the SNAP/CMAP. The area under the curve of the signal will be the same, bit the peak will be lower as the shape is flatter.
Still not grasping why would the SNAP/CMAP be reduced across that area resulting in "conduction block" ?
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Still in training, but from my understanding:
in the more acute/subacute stage, when you get a complete block, there is less transmission across the site so this reduces amplitude. More chronically, there is secondary axonal loss which contributes to reduced CMAPS as well.
in the more acute/subacute stage, when you get a complete block, there is less transmission across the site so this reduces amplitude. More chronically, there is secondary axonal loss which contributes to reduced CMAPS as well.
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Because amplitude is measured from the peak, not as the area under the curve. Imagine that the same number of axonal impulses are reaching the other side, but dispersed over a wider span. The apparent peak would be lower even if axon-wise conduction is unaffected.
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Because of phase cancellation as above. The amplitude AND the area are reduced, as well as increased temporal dispersion. Imagine if all the negative deflections occur simultaneously and all the positive occur simultaneously--you'd have the maximal negative and positive amplitudes and area; if some of these start to shift because of going through a demyelinated area, then the amplitude and area will be reduced. The above illustration hits the nail on the head.
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