EMG/NCS Apparatus

NCS: Electrodes and what they do

Some of the concepts discussed here are a little in the weeds; focus on understanding the big picture of what each electrode's basic function is and get a basic idea of electrode placement. You can refer back to the details as you continue going through other sections of the website.

Stimulator

What is it?

• Delivers electrical stimulation in order to depolarize nerves being tested.

• Consists of an anode (+) and a cathode (-), which are red and black respectively by convention.

Note: the cathode (black end) of the stimulator electrode should always face the black end of the recording electrode. An easy way to remember this is the electrode placement is "black to black" i.e the black ends are facing each other. The only exception to this is F wave and H reflex studies, but more on that later.

‍*the anode has the potential to hyperpolarize the nerve and block the depolarization as it propagates towards the recording electrode; this could cause a falsely reduced or absent potential

‍How does it work?

• ‍‍Stimulation opens voltage-gated sodium channels in nerve fiber membranes. 

• Sodium flows into the neurons and causes depolarization. If depolarization reaches threshold, then a self-sustaining electrical impulse is generated. This is known as the action potential.

• The action potential propagates in both directions from the stimulation point. 

Supramaximal stimulation

To ensure that you are truly recording the best possible electrical response to stimulation, and therefore ensure reliability of your study, the stimulus MUST be "supramaximal". This means that all axons should be activated. To achieve supramaximal stimulation, you gradually increase the stimulus (usually the current) until the response (CMAP) no longer increases, and then give a stimulus 30% greater than that (just to be sure!).‍

This concept is particularly important for motor NCS; supramaximal stimulation can be difficult to achieve in sensory NCS because these potentials are smaller, so larger stimulus can lead to motor nerve stimulation i.e. "motor artifact".

Ground electrode

What is it for?

It's primary purpose is actually not safety. It decreases electrical interference from the equipment, surroundings, and the patient.

Where is it placed?

In between the recording and reference electrodes.

Figure 2.3: Photo of ground electrode. The placement on the dorsum of the hand is typical for upper extremity studies.

‍Recording and reference electrodes

The recorded potential during nerve conduction studies (i.e. the CMAP and SNAP) represents the potential difference between the recording and reference electrodes.

What is the recording electrode?

It records the electrical activity where it is placed, and creates the waveforms you see on the screen during NCS. It is important to remember that the black end of the recording electrode should face the black end of the stimulator.

What is the reference electrode?

It is important to know that the reference electrode is NOT inactive or silent. Its placement shapes the response that is recorded by the recording electrode.

How do you decide where to put them during NCS?

It depends on whether you are doing a motor or sensory study.

Motor NCS recording and reference electrode placement

• Recording electrode is placed at the muscle belly (i.e. the motor endplate) to record electrical activity. 

• Reference electrode is placed at a tendon or bony prominence DISTAL to the recording electrode. Why? This is to minimize the reference electrode's contribution to the CMAP.

Figure 2.4: From left to right: disc electrodes, bar electrode (top view), bar electrode (bottom view), ring electrodes.

Figure 2.5: Example diagram of placement of electrodes in a median nerve motor nerve conduction study.

‍Sensory NCS recording and reference electrode placement

• Recording electrode is placed over the skin that is innervated by the nerve being tested. It is important to remember that the black end of the recording electrode should face the black end of the stimulator.

• Reference electrode is placed 2-4cm distal to the recording electrode, still over the skin that is innervated by the nerve being tested.

Why? Placing the reference electrode on the skin and only a couple centimeters away from the recorded impulse gets you a higher amplitude recording, increasing the sensitivity of the study. So, in sensory NCS you actually want to maximize the reference electrode's contribution to the SNAP since the voltages in sensory conduction are so much tinier and harder to pick up than motor nerves (microvolts instead of millivolts). 

EMG: the needle

This is simple: there is a small needle connected to the EMG machine that records the electrical activity of the muscle into which it is inserted.

It is important to keep in mind the following points when performing and interpreting an EMG needle study. 

• The EMG needle inserted into a very small volume of muscle, but we are interpreting it to represent a larger volume of muscle.  Only a portion of the whole muscle is investigated, so occasional false negative results can be expected in conditions with patchy injury.

• Slight movements in the needle position can change which muscle fibers are being recorded from the same motor unit. This can change the shape of the MUAP (motor unit action potential) seen on the EMG recording screen. 

• Large needle movements or increased muscle contraction can lead to the recording of completely different motor units.

Figure 2.6: EMG needle inserted into the tibialis anterior during EMG. Muscle electrical activity is tested at rest (patient not contracting any muscles), and then with voluntary muscle activation (in this case ankle dorsiflexion).