Hello everyone, my name is Dr. Monica Alcantara and today I will talk about some pearls and pitfalls of nerve conduction studies. In this episode, I will present some additional upper limb sensory studies as a continuation of the last episode. Before I give you some examples, I will briefly discuss common technical problems that can affect your waveform recordings. To begin with, it's important to check the machine settings to select the right electrodes and to place them according to anatomic landmarks. The next step is to make sure you apply sufficient stimuli in order to depolarize all axons within the nerve while avoiding overstimulation. We have to pay special attention to the orientation of the cathode to avoid inaccurate measurements that may affect conduction velocities, distal latencies, and potentially result in a node block. Finally, we have to precisely measure the distance between the stimulation and recording sites and to reveal the waveforms from any misplacement of the cursors and always make sure the limb temperature is appropriate. After you took care of everything in this list, let's go practice. To begin with, I will give you some examples of antidromic sensory nerve action potentials where we occasionally struggle to make good recordings, either because of limb movement or motor artifacts caused by volume-conducted motor responses. This is an antidromic superficial radial nerve conduction study. I find that positioning the hand between pronation and supination is a good way to maintain stillness and to minimize motor artifacts instead of choosing either the entirely pronated or supinated positions. The first tracing represents a radial snap with insufficient stimulation resulting in low amplitudes around 13 microvolts. With supermaximal stimulation, the amplitude more than doubles, as you can see here in the middle tracings. However, if we go past the ideal threshold, we get this motor artifact. We should also remember that the amplitudes are usually higher with this type of electrode than with the ring electrodes that we frequently use. Here we have an antidromic dorsal onorcutaneous sensory study with the recording electrode placed over the web space between the little and the ring fingers and the reference placed over the metacarpophalangeal joint. Others may prefer to place both electrodes more distally, but the potentials are very similar with either recording technique. Also, I wanted to show you an alternative hand position. This is a position that I think can improve stillness while avoiding displacement of the electrodes. In the first tracing, we have the duck snap, again with insufficient stimulation resulting in low amplitudes at 10.4 microvolts and prolonged latencies. In the bottom tracings, we can see good amplitudes and conduction velocities. However, when we increase the stimulation a bit more, we get this big motor artifact. There will be situations where you cannot get any response. If that's the case, check everything again, but remember that the radial nerve branches may also supply the duck territory. To exclude this possibility, we have to stimulate the distal radial sensory nerve while recording over the duck. Another examples are the lateral and medial antebrachial cutaneous sensory studies. For both nerves, most of the times we'll get a good response with a distance of 12 centimeters approximately. One of the main issues with the lack is that the stimulation site at the antecubital fossa is slightly lateral to the biceps tendon. Therefore, excessive stimuli may result in direct stimulation of the muscle, even with low intensities, as we can see here in the left side picture. The first tracing represents understimulation, where we see a small amplitude snap. In the second tracing here, we see the supermaximal stimulation, and in the third tracing, where we increase the stimulus from 8 to only 11 milliamp, we got this big motor artifact. Motor artifacts are less common with the medial antebrachial cutaneous nerve, as we can see here when we stimulate at the midpoint between the biceps tendon and the medial epicondyle. This nerve is usually very superficial, and supramaximal stimulation can be achieved with low intensities, as we can see here. As we increase the stimuli, there are no significant changes in amplitudes, but a motor artifact is gradually appearing. As a last remark, to maximize the snap amplitudes in MAC or LAC studies, the recording electrodes may have to be repositioned either slightly medially or laterally to the original position. I hope this teachable moment was helpful. The intention is to stimulate you to go beyond your results table, to examine the waveforms and to repeat tests, especially to avoid misdiagnosing a patient with a condition they don't have. These are some of my references. Thank you for watching this teachable moment.

nerve conduction studies

ncs

electrode placement

stimulus intensity

antidromic sensory nerve action potentials

waveform analysis

nerve conduction studies

nerve conduction study

ncs