Chapter 23 Nerve Action Potential Recording
Assessing Distal Motor Function
• After external neurolysis, the next logical step in evaluation of any lesion in continuity is to stimulate the nerve both above and below the lesion, to determine whether there is any motor function distal to the lesion.
• If there is muscle contraction, then enough axons have grown to the muscle in question to provide voluntary contraction, even though the patient may not be able to do it voluntarily at the time of the stimulation.
• If stimulation above gives no muscular contraction distal to the lesion but stimulation below does, there is clearly an element of neurapraxic block involved.
• If a few months have passed since the injury and there is no response, then the lesion is predominantly neurotmetic.
• Alternatively, if the lesion is axonotmetic, either it is too far proximal to meaningful distal muscles to expect adequate regeneration, or it is too early to expect excellent regeneration to have reached them.
• When using stimulation alone, care must be taken not to mistake contraction of muscles proximal to the lesion for distal contractions. One also has to be certain that the current does not spread to adjacent and often more intact muscles or nerves. The muscles seen to contract must be in the distribution of the injured nerve.
• If one records as well as stimulates, the presence or absence of adequate regeneration can be detected relatively early after injury. Nevertheless, several months must elapse for even nerve action potential (NAP) recording to be useful. Thus, after external neurolysis and before internal neurolysis or repair (split or otherwise), we attempt to record NAPs.
Technique
• If the lesion and the requisite surgical exposure permit it, we first record a NAP proximal to or above the lesion. We then move the recording electrodes into and then distal to the lesion to see whether the NAP transmits and how far it travels. Even to record a proximal NAP, the stimulating and recording electrodes need to be separated by at least 3 cm, or 1½ inches (Figure 23-1).
• Muscle action potentials (MUAPs) picked up by recording electrodes can be mistaken for NAPs. Latency is much greater for MUAPs than for NAPs, and responses are larger and much more complex. MUAPs have a relatively broad base and a rounded apex. NAPs can, of course, be recorded without any visible or palpable muscular contraction, whereas sizable MUAPs can seldom be recorded without such.
• When a plexus lesion is close to the spine, stimulation may have to be done close to the lesion, either just proximal or even distal to it, and NAP recording must be done more distally.
• Usually, if a proximal NAP is recorded, we gradually move the electrodes distally and re-record as we go, to see how far a response will be transmitted. Distance between the recording electrode pair should be increased to 5 mm or more when attempting to record from large nerves or when studies are done over a relatively long distance.
