Nerve Conduction Studies

1. What is the difference between peak and onset latency?
2. How is conduction velocity (CV) affected by age and temperature?
3. What is temporal dispersion?
4. What is phase cancellation?

Answers:
1. Onset latency is recorded at the initial deflection from baseline and represents the fastest axons. Peak latency is recorded at the peak of the waveform and represents the majority of axons.
2. CV is 50% adult in newborns, 80% in one year olds, and adult speed by 3-5 years. After 60 years, CV decreases by 1.5% per decade. CV decreases 2.4m/s per 1C drop in temp below 32C for upper limbs and 30C for lower limbs.
3. Temporal dispersion occurs when the waveform spreads out with proximal stimulation due to the difference between the CVs of the fastest and slowest fibers, and results in a smaller amplitude.
4. With proximal stimulation, the APs of different axons (based on their speeds) may be out of phase with one another, resulting in cancellation of their potentials and lower amplitudes and longer durations. This happens most frequently with SNAPs because the AP has a short duration, resulting in proximal SNAP 50% of distal. CMAP does not have as much of a drop (15%) because of its longer duration.