Hello everyone, my name is Dr. Monica Alcantara, and today I will talk about some problems and pitfalls of nerve conduction studies. In this episode, I will present some routine upper limb sensory studies. 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! Let me start by giving you some examples. These are antedromic median and radial nerve conduction studies, stimulating at the wrist and recording at the first finger, with a distance of 10 cm, in a case of median neuropathy at the wrist. The first two tracings represent radial snaps, where the stimulus intensity was gradually increased. Here we can see an example of insufficient stimulation, where the radial snap has prolonged latency and reduced velocity, as compared to the second tracing, where a higher intensity produced normal responses. The bottom tracings here represent median snaps, where the intensities were also gradually increased. With supermaximal stimulation, we can see that a motor response follows the sensory response. As we have good amplitudes, there is a clear separation between the sensory response and the motor artifact, but the recognition of a sensory response could be a problem if the snaps were small or had prolonged latencies. Finally, we have to be careful with the measurements, as the median nerve has a natural, slight curvature that requires a precise measurement. Here we have additional recordings from the same patient, who appears to be very slender, given the relatively low electrical stimulation intensities. The first tracing here represents a median snap, with stimulation at the wrist and recording at the index finger, and the second one here, when stimulated at the antecubital fossa. The distal snap has normal amplitude and reduced conduction velocity, whereas in the proximal segment, the velocity is normal. But we have to make sure we are not under-stimulating the median nerve at the wrist. We can see that with submaximal stimulation, the snap has reduced amplitudes, almost half of the size of the first one, and conduction velocities that are a bit slower. And everything improves when we gradually increase the stimuli, as you can see here. In the last two tracings, we can see the comparison of two sites over the same median nerve, now recording from the third digit and stimulating at the wrist, then at the palm. This antedromic median sensory palmar study can be helpful not only to show focal changes in conduction velocities, as we can see here in these tracings, but also to indicate a possible conduction block at the wrist, when the palm-to-wrist amplitude ratio is above 1.6. Which is not the case here. Here we have another example of antedromic snaps with motor artifacts that can happen even with low-intensity stimulations. This recording is from the ring finger, while stimulating both the ulnar nerve at the wrist and elbow and the median nerve at the wrist in the same patient. In this antedromic comparison, the median snap recorded at the ring finger is not only smaller but also has prolonged latencies when compared to the ulnar nerve, as you can see here in the picture. Motor artifacts are common when recording from the fifth finger, but can also happen with the ring finger. Obviously this is not a matter of stillness or relaxation, but it is in fact the effect of a volume-conducted motor response. To minimize this kind of motor artifact, especially when it starts to obscure the sensory potentials, you can try to use the smallest stimuli possible. To move the recording electrodes away from the muscle that is generating the motor response, let's say, by placing the recording electrodes at the proximal interphalangeal joint or by asking the patient to spread the fingers or even by protecting the electrodes with gauze. 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 study

electrode placement

stimulus intensity

measurement accuracies

motor artifacts

ncs

ncv