Volume Conduction And Its Role In Specific Tests

4.1. Sensory Nerve Conduction Studies

Let us begin our discussion by looking at a standard antidromic median sensory study recording from digit 3, a form of "bipolar study," in that both E1 and E2 are in proximity to the potential generators (the digital nerves of the middle finger). Figure 3A shows the standard recorded response. Note that there is an initial negativity and a final positivity. In Fig. 3B, the reference electrode is moved to digit 5, essentially changing the biopolar arrangement to a referential recording arrangement, where E2 is picking up no electrical activity. An initial positivity now appears on the recorded waveform, because of the approaching wave front, as described in Fig. 1. The reason that it was not visible with both electrodes on digit 3 is that both were sensing it simultaneously and, hence, it was subtracted out by the differential recording. With the reference electrode on digit 5, only the active electrode on digit 3 senses it and, thus, it appears. Note also that the amplitude of the negative peak is slightly higher. In Fig. 3C, the contribution of E2 is clarified by keeping it as the reference electrode and placing E1 on digit 5. What is seen is essentially an upside-down version of that seen in Fig. 3B, with an initial negativity and slightly lower amplitude at the now "upside down" negative peak. The amplitude is slightly reduced because the digit 3 electrode is very distal and the underlying electrical potential is smaller. The initial negativity seen here normally counterbalances the positivity of E1 (seen in Fig. 3B) in the bipolar recording (A), producing the flat baseline leading up to the major negativity. In Fig. 3D, the waveforms seen in Fig. 3B and 3C are digitally added, to produce approximately what is observed in Fig. 3A.

Thus, antidromic median and ulnar bipolar sensory studies typically do not demonstrate an initial positivity because they are truly bipolar and the approaching wave front is sensed simultaneously by both electrodes and is cancelled out. However, in standard antidromic radial sensory studies (recording from the snuff box) and antidromic sural sensory studies (recording from the ankle) a small positivity is typically identified. This likely reflects the fact that E1 and E2 are seeing slightly different environments and that the potentials are not truly bipolar with incomplete cancellation of the approaching wave front.

4.2. Motor Studies

In motor studies, there is generally no initial positivity on the compound motor action potential, similar to that observed on bipolar digital sensory studies. However, the reason why this occurs is different. Rather than the wave front being sensed by both electrodes and canceling, in this situation, the depolarization of the muscle fibers is actually originating directly beneath the active electrode, E1, and, hence, there is no approaching wave front. This can be demonstrated by moving E1 slightly off the motor point, which will then create a small positivity in front of the major negative phase.

Fig. 3. Median digit 3 sensory studies. Comparisons are made between the standard bipolar arrangement (A) and other arrangements to better understand how the standard potential is generated. In A, the active electrode "A" and the reference electrode "R" are both placed on digit 3 to obtain the standard bipolar recording. In B, the reference electrode is moved to digit 5 to produce a true "referential recording." Note how an initial positivity is observed (because of the approaching wavefront) and the peak-to-peak amplitude is higher than the baseline-to-peak observed in A. In C, the reference electrode is maintained on digit 3, but the active electrode "A" is moved away. This reveals what is being recorded by the reference electrode alone: a small initial negativity and larger second positivity. In D, the responses from B and C are digitally added, giving approximately what is observed in A.

Fig. 3. Median digit 3 sensory studies. Comparisons are made between the standard bipolar arrangement (A) and other arrangements to better understand how the standard potential is generated. In A, the active electrode "A" and the reference electrode "R" are both placed on digit 3 to obtain the standard bipolar recording. In B, the reference electrode is moved to digit 5 to produce a true "referential recording." Note how an initial positivity is observed (because of the approaching wavefront) and the peak-to-peak amplitude is higher than the baseline-to-peak observed in A. In C, the reference electrode is maintained on digit 3, but the active electrode "A" is moved away. This reveals what is being recorded by the reference electrode alone: a small initial negativity and larger second positivity. In D, the responses from B and C are digitally added, giving approximately what is observed in A.

Regardless, the negative peak amplitude that is being measured is still very much a product of what the two electrodes are sensing. It is appealing to think that the active electrode is sensing the depolarization of the muscle and the inactive is seeing a flat line. This is actually pretty much the case for standard median and peroneal motor conduction studies, in which only a small subset of muscles on the hand or foot are depolarizing. However, this is definitely not true in ulnar and tibial motor studies, in which the majority of the muscles in the hand and foot are all depolarizing, essentially "electrifying" the entire structure. This was demonstrated by Kincaid et al., in a study in which the authors evaluated what the reference electrode was actually seeing in different parts of the hand (Figs. 4 and 5). In these studies, the active electrode was placed on different parts of the hand and the reference electrode was moved to the opposite side of the body so that E2 could be viewed as a true reference. As can be seen, nearly the entire hand is "electrified" when the ulnar nerve is stimulated (Fig. 4) and volume-conducted waveforms can be picked up all over the hand (even on the tips of the fingers); hence, there really is no inactive site for this muscle. When stimulating the median nerve, however, virtually no activity is picked up in any location, except in the region of the thenar eminence (Fig. 5). Similar results are also found by evaluating the tibial and peroneal nerves in the feet; the tibial nerve analogous to the ulnar and the peroneal nerve is analogous to the median.

Fig. 4. Volume-conducted potentials recorded from various parts of the hand with stimulation of the ulnar nerve. Note how potentials can be recorded from nearly the entire hand, even the tips of the fingers. From Kincaid et al., 1993 with permission.
Fig. 5. Volume conducted potentials recorded from the hand with stimulation of the median nerve. Note how potentials can only be recorded in the vicinity of the thenar eminence, where the median-innervated muscles reside. From Kincaid et al., 1993 with permission.

Hence, when performing a standard ulnar motor study, the ultimate configuration of the motor response is a composite of what E1 and E2 are sensing. The large positivity that is sensed over the fifth metacarpal-phalangeal joint is subtracted from the response detected directly over the hypothenar muscles. Subtracting this positivity will cause it to become a negativity, leading to the characteristic "double-humped" ulnar motor potential (Fig. 6). The same exercise for the median nerve, on the other hand, produces essentially no change in the amplitude/configuration of the motor response because the E2 activity is negligible.

4.3.1. Fibrillation Potentials and Positive Sharp Waves

Fibrillation potentials and positive sharp waves are both produced by the electrical depolarization of single muscle fibers. However, they have dramatically different morphologies, as illustrated in Figs. 3 and 4, Chapter 14. They appear so dissimilar because of volume conduction effects. Fibrillation potentials are thought to represent the depolarization of a single muscle fiber observed from a short distance by the needle electrode. Hence, the reason for its triphasic nature is very similar to a sensory nerve conduction study, with an approaching

Ulnar

E1 recording

Ulnar

E2 recording

Resulting response

Fig. 6. Approximate digital addition of the ulnar motor response recorded from the hypothenar eminence and that recorded from the fifth MCP joint, leading to the typical "double-humped" ulnar motor response.

wave front causing the initial positivity, the major negative spike being induced by the depolarization traveling underneath the electrode, and the trailing positivity caused by the departing wavefront. This is in contrast with a positive sharp wave, in which it is proposed that needle is in actual contact with the fiber being recorded. The mechanical deformation of the myocyte membrane by the needle is thought to partially depolarize the cell. Hence, the approaching wave front produces an initial positivity and then the negativity begins to develop, but essentially aborts as it comes into contact with the depolarized area of muscle membrane. Although these are only models and the real phenomenon is likely more complex, they are helpful in understanding how two different waveforms can be produced by a single generator.

4.3.2. End-Plate Spikes

These potentials are similar to fibrillation potentials in that they represent the depolarization of a single muscle fiber. However, they are different in that needle movement at the end plate induces the firing of the muscle fiber and, hence, the action potential is generated immediately beneath the needle. Accordingly, unlike fibrillation potentials, they are usually biphasic— with an initial negativity followed by a positivity. The fact that these potentials are generated immediately beneath the needle is similar to a motor conduction study, in which the depolarization of the muscle fibers begins under the surface electrode.

4.3.3. Motor Unit Potentials

When in close proximity to the muscle fibers of a single motor unit, the motor unit potential will have higher amplitude and, when more distant, the amplitude will decrease and the general sharpness of this waveform will decrease. The duration tends to be more immune to these volume conduction and tissue filtering effects, with it being relatively consistent regardless of the distance the needle electrode is placed from the motor unit potential being recorded. Hence, the single most useful parameter when evaluating a motor unit is its duration.

4.4. Somatosensory Evoked Potentials

Somatosensory potentials are usually recorded in a bipolar fashion with both recording electrodes placed near the generator source (e.g., spinal cord or scalp). This approach measures near-field potentials. However, by widely spacing the recording electrodes, early waveforms in the form of far-field potentials can also be observed. Although initially thought to be due to a specific neuronal generator of some form, such as ganglia or individual neurons, it was recognized by Jun Kimura and others that these waveforms were actually being caused by a waveform traveling from one body segment into another, the so-called standing waves or virtual dipoles noted above in Section 3. These waveforms have also been called "junctional" or "boundary" potentials. The most prominent and most carefully studied of these is the P9 potential obtained with stimulation of the median nerve. It is thought that this is caused by the intersection of the arm with the trunk. Similar "virtual dipoles" can be demonstrated in standard digital sensory nerve conduction studies, if performed in a referential fashion; however, these are usually irrelevant when performed in a standard bipolar manner.

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