Understanding Near and Far-Field Potentials

CNIM Exam Preparation: Far-Field Responses

I’ve asked a couple of readers what they were having trouble with when preparing for the CNIM exam. In the last post, I went over?when to use constant voltage and constant current. This time, I’m going to go over far-field potentials and how those are formed.

Near-Field vs. Far-Field

Near-field potentials are pretty easy to understand. You just put your recording electrode as close to the generator point as possible. If you want a nice N20, you stick a needle in the contralateral head from the wrist you’re stimulating. You wouldn’t want to stick it in the shoulder because that’s not where you want to record over. Consider yourself lucky to get any kind of reliable response with that poor setup.

Needle placement is especially important when recording a near-field response. The further away from the generator, the smaller the signal. (HINT: possible CNIM exam question there).

But far-field potentials aren’t the same. There are a couple of principles of conducting electric current through the body and recording a wave that you need to understand before you’ll fully grasp it.

List of Far-Field Potentials You Need To Know For The CNIM Exam

When you go to take the CNIM or DABNM written exam, you need to have an understanding of how to generate a far-field potential to better figure out if a response is a near-field or far-field response. But you need to also have a list of those memorized, like this one here…

• Far-Field Potentials: P14 UE SSEP
• N18 UE SSEP
• P31 LE SSEP
• N34 LE SSEP
• Wave III ABR
• Wave V ABR

But you need to really understand why these are far-field responses.

And here’s a hint…

It’s not because they are a specific distance from the recording site.

There is no such distance that distinguishes a near-field from a far-field response… that’s too easy.

First off, far-field responses have a larger spread than near-field responses. You will equally record the same generator from various distances.

How is that possible?

Because we’re not just recording electrical impulses carried through a nerve or tract. Electrical current transmits through any solution that has high enough sodium levels (just like in your middle school science project).

Our body’s tissue is able to conduct these electric currents because of our tissues, blood, CSF, etc.

Here’s an example… the EKG artifact that you pick up on EEG recordings. The electrical current is created in the heart, but it’s not transmitted through some tract directly from the heart to the brain (cardiospinal tract?). The current flows through the tissue in the body and gets picked up by the head electrodes.

Creation of electric current from all over occurs. Luckily, it follows some rules to manage the chaos.

3 principles that electric current follow

1. ?If resistance is equal, the current will flow evenly in all directions.
2. The current will always take the path of least resistance.
3. The amplitude varies by the square of the distance between the recording electrode from the generator… the closer the recording electrode, the bigger the amplitude.

Now that you know what kind of current acts on the generator points, and the rules that this current follows, you’re able to understand why far-field potentials are recorded from a wide area (and near-field in a small area), why some areas are better to record from than others, why these various recording sites are not always the same in amplitude and why it is theoretically possible to record any potential from anywhere on the body.

In order to benefit from this, we need to utilize this information to give us a clue as to what’s going on and where. We need to record from areas that are strong enough to give reproducible signals, along areas of interest. These are called generator points.

Generator points

The generator points are the areas believed to give off strong enough signals to create a reliable waveform. Most generator points are approximations (pretty much a guess) as to exactly where or how this signal is being created.

And now that you understand that the generator points are not created just by activation of a nerve, you’ll appreciate the possibilities of what it is that we are actually recording. Here’s what is currently accepted as the actual source of the evoked potential.

1. activity from a cluster of sensory neurons (for example, the N13 cervical response has a larger amplitude over the C5 segment than the C2 spinal segment. That means that this is a near-field response, with the active electrode over the sensory neurons)
2. synaptic activity between neurons (for example, the P100 response in VEP is from a cortical activity from light stimulation)
3. AP along axons (for example, when stimulating over the brachial plexus or popliteal fossa, move your electrode along the nerve and still get a very similar response in amplitude size, with a different latency. This traveling response is a good example of generating the potential along the nerve axon)
4. Non-neural events, like a change in direction of a tract, or a change in the compartment that houses that neural structure (for example, as the?P14 SSEP response?comes from changes in direction after entering the foramen magnum)
5. Volume conduction, or electrical current through the conduction through the body (for example, Wave I BAER is considered a near-field response because even small changes in the distance of the Ai electrode (Cz-Ai montage) will decrease the amplitude. However, the Cz reference electrode is also active and gives off a far-field potential at far distances. It has a large spread or distribution)

And this does not happen in isolation. That’s why Wave I of BAER has both near-field and far-field potentials (but if I had to guess one or the other on the CNIM or DABNM, I would put it as near-field).

Positive vs. Negative?

A wave will either have a peak or a trough for you to put your markers on. How that is determined is the relationship between your active and reference electrodes and the polarity of the electric current. That polarity is determined by the orientation of the dipole.

Dipole

A dipole is a pair of electric charges or magnetic poles, of equal magnitude but of opposite sign or polarity, separated by a small distance. So on one side, you have a positive charge, and on the other, you have a negative charge. This dipole takes different paths, some horizontal and some vertical. With the round cortex with sulcus causing a 90-degree change in tissue orientation, you will see how recording electrodes are influenced by various electrical sources, from various distances, with various polarities. Some of these potentials are additive, others will cancel out the potentials.

The dipole helps explain why a wave is positive or negative, depending on the area in the recording electrodes are placed. For example, the N13 cervical response is a near-field, horizontal dipole. That means that the active electrode placed on the posterior neck will result in a negative peak while placement over the anterior neck will result in a positive valley.

The dipoles determine the polarity of the potential in relation to where your active and reference electrodes are placed.

Far-Field Potentials Final Verdict

Now you see why far-field potentials are a little tricky, thanks to volume conduction, various influencing dipoles and their orientations, the direction of spread, changes in the direction of tracts, changes in resistance of surrounding tissue, etc. What happens is a response is near-field (like P37), far-field (like P31) or both (like Wave I and N13/P13).