SNAP and CMAP

1. Why are antidromic SNAPs preferred to orthodromic?
2. What is the consequence of the active and reference pickups being less than 4cm apart?
3. What is the normal appearance of the CMAP and what might result in an initial positive deflection?
4. How is amplitude measured in SNAP vs. CMAP?

Answers:
1. Antidromic responses are easier to record, require less stimulation, and have larger amplitudes.
2. Lower amplitude (due to similar waveforms at active and reference electrodes being dropped) and decreased peak latency.
3. CMAP should be biphasic with an initial negative deflection. An initial positive deflection could be due to placement of active electrode off the endplate region, volume conduction from other muscles or nerves, or anomalous innervations.
4. SNAP amplitude is peak to peak, CMAP is baseline to peak.