Normal NCS

Motor NCS

What are we measuring in motor NCS?

Compound muscle action potential (CMAP), which is the electrical activity across all muscles that are activated.

CMAP is NOT:

• a measure of strength of muscle contraction or mechanical twitch

Make sure you understand early on that a CMAP is the sum of MUAPs (motor unit action potential). 

• A MUAP is the electrical activity of a single motor unit. 

• Thus by definition, a CMAP is the sum of electrical activity of all the motor units that are activated when a muscle is stimulated. 

CMAPs are recorded in motor NCS whereas MUAPs are recorded on needle EMG.

Figure 3.1: Animation demonstrating that the CMAP is a summation of electrical activity across all the muscles that are activated, i.e. all the muscles that depolarize. The stronger the stimulus, the more axons depolarize, and the more muscle fibers depolarize.

Process of conducting motor NCS

You must stimulate and record the CMAP at TWO sites along one nerve. Place the recording, reference, and ground electrodes as per the figure below.

Figure 3.2: Setup for median nerve conduction study at the wrist. G1 = active recording electrode (placed on abductor pollicis brevis), G2 = reference electrode (placed on a bony prominence). Anode and cathode of stimulator are labelled.

‍The numbered steps listed below correspond to Figure 3.3 below.

1. Place the stimulator at the distal stimulation site (e.g. wrist for median nerve) and zap (press the button to deliver electrical impulse).

2. Try to get the optimal waveform via supramaximal stimulation. Record the latency based on this waveform.

3. Measure the distance between stimulation site (e.g. wrist for median nerve) and recording electrode site (muscle, e.g. APB) with a tape measure and enter this data into the machine. It's helpful to remember to measure "black to black" i.e. the black ends of the stimulator and recording electrode should face each other.

4. Place the stimulator at the proximal stimulation site (e.g. elbow for median nerve) and zap.

5. Try to get the optimal waveform via supramaximal stimulation. Record the latency based on this waveform.

6. Measure the distance between stimulation site (e.g. elbow for median nerve) and recording electrode site (muscle, e.g. APB) with a tape measure and enter this data into the machine.

NCS machine should now automatically calculate latency, amplitude, and conduction velocity for the nerve you just tested! 

Practical tip: One thing to note is that in practice the NCS machine automatically marks the key points on the waveform, e.g. height of the wave which affects the calculation of amplitude, and the start of the waveform which affects the calculation of latency. It is very important to check that these markers are correctly placed, especially if you notice that the conduction velocity or amplitude is unexpectedly low/high. Adjusting the markers to the correct key points on the waveform is very important in ensuring accurate NCS studies and diagnosis.

Figure 3.3: Animation of major steps of motor NCS for the median nerve. To test one motor nerve, you must stimulate at a distal and proximal site to obtain a good CMAP waveform. The distance between stimulation site and muscle is measured in order to calculate conduction velocity (see below).

‍CMAP waveform components

Why do these matter?

Abnormal values of latency, amplitude, and duration help differentiate broadly between an axonal vs demyelinating processes.

‍Distal Latency

Time from initial stimulus to initial muscle fiber depolarization (i.e. negative deflection from baseline). It represents the time it takes for an impulse to travel from the stimulation site to the muscle, i.e. the time taken to travel from stimulator device to the recording electrode. Importantly, this is the sum of time taken to travel down the axon + cross the neuromuscular junction + for the post-synaptic muscle fiber to generate an action potential.

Amplitude

Height of the wave (from baseline to peak of negative deflection). It represents the number of muscle fibers activated in the CMAP. 

Duration

Time between start and end of wave (i.e. time from initial negative deflection to initial return to baseline). The longer this is, the greater variability in velocities of the nerves. More on this in NCS pathologic findings.

Area under the curve

Area under the waveform (i.e. initial negative deflection to initial return to baseline). This represents the number of motor units depolarized in that CMAP. When this is abnormally low it can indicate the loss of motor units or decrease in the size of motor units. It is especially useful in diagnosis of conduction block.

Motor conduction velocity

The conduction velocity is not directly measured during NCS studies. It is derived from the data obtained during the study using the well-known formula: velocity = distance / time. In motor NCS (but not sensory), we HAVE to record TWO CMAPs for the same nerve at different locations in order to calculate that nerve's conduction velocity.

Let's use the example of the median nerve motor NCS once again (see figure 3.3 above). Why can't we just use distance (between stimulation site and recording electrode) and time (latency) from a single CMAP, i.e. why not just calculate d1 / t1 ?

Because the latency (t1) is not just the AXON conduction time (which is the part we care about). Latency is the conduction time across the axon + neuromuscular junction + post-synaptic muscle fiber. So our study would not actually represent purely nerve conduction.

So, to resolve this issue, we subtract one CMAP's data from the other so that the time taken to cross the NMJ cancels out, and we're just left with the axon conduction. We calculate (d2 - d1) / (t2 - t1) = (distance between elbow and wrist) - (time between elbow and wrist). This way, for example in median nerve studies, our analysis becomes focused on just the motor nerve segment between wrist and elbow.

Interpreting CMAP distal latency

In motor NCS we always comment on distal onset latency. A common scenario where distal latency might be prolonged is with carpal tunnel syndrome involving entrapment of the median nerve at the wrist. In this scenario the conduction velocity of the median nerve is often normal because the axon between elbow and wrist is conducting normally, but distal latency will be prolonged because it looks at just the segment of the nerve that is entrapped.

The important thing to note here is that you need to use a standard distal distance when comparing your distal latency to standardized values. Why?

Recall that distal motor latency is the time it takes for an impulse to travel from the stimulation site to the muscle, i.e. the time taken to travel from stimulator device to the recording electrode. That time depends on how fast the nerve segment is conducting (conduction velocity) AND the distance between the stimulator and recording electrode (conduction time = distance / velocity). So if you change the distance, you change the latency (time)—even if the nerve is conducting at a normal velocity.

Example using median nerve study with stimulation at the wrist: 

• Reference value for median distal motor latency is <4.2ms and assumes distance of 7cm from stimulation site to recording electrode at APB

• If you stimulate at 10 cm instead, you might get falsely prolonged distal motor latency of 4.8 ms ina patient who is actually normal. Similarly, stimulating at 4cm may give you a falsely decreased distal motor latency value and cause you to miss pathology.

Due to the short distances we are measuring in distal latency, the results are especially prone to artifact and thus it is particularly important to use standardized distances when performing NCS. This ensures accuracy and comparability over time for individual patients, as well as comparability across different patients.

Sensory NCS

What are we measuring in sensory NCS?

We are directly measuring the actual sensory nerve action potential (SNAP). 

Process of conducting a sensory NCS

Place the recording, reference, and ground electrodes as shown in the figure below.

Figure 3.5: Setup for median sensory nerve conduction study at the wrist. G1 = active recording electrode, G2 = reference electrode.

The numbered steps listed below correspond to Figure 3.6 below.

1. Place the stimulator at the distal stimulation site (e.g. wrist for median nerve) and zap (press the button to deliver electrical impulse)

2. Try to get the optimal waveform via supramaximal stimulation. Record the onset latency based on this waveform.

3. Measure the distance between stimulation site (e.g. wrist for median nerve) and recording electrode site (usually the ring electrode at the proximal interphalangeal (PIP) joint) with a tape measure and enter this data into the machine. It's helpful to remember to measure "black to black" i.e. the black ends of the stimulator and recording electrode should face each other.

NCS machine should ideally now automatically calculate latency, amplitude, and conduction velocity for the nerve you just tested! 

Practical tip: One thing to note is that in practice, the NCS machine automatically marks the key points on the waveform, e.g. height of the wave which affects the calculation of amplitude, and the start of the waveform which affects the calculation of latency. It is very important to check that these markers are correctly placed, especially if you notice that the conduction velocity or amplitude is unexpectedly low/high. Adjusting the markers to the correct key points on the waveform is very important in ensuring accurate NCS studies and diagnosis.

Figure 3.6: Animation of major steps of sensory NCS for the median nerve.

‍SNAP waveform components

Why do these matter? 

Abnormal values of latency, amplitude, and duration help differentiate broadly between an axonal vs demyelinating processes.

Onset latency

Time from stimulation to initial deflection of the SNAP. It represents the fastest and largest nerve fibers. 

Peak latency

Time from stimulation to the midpoint of the first negative peak of the SNAP.

Do we care about onset or peak latency for the SNAP? Why? 

Both! When looking at the raw value of latency, we look at peak latency. It is more reliable and less prone to artifact than the onset latency. However, when calculating conduction velocity (see below), we use the onset latency.

‍Amplitude

Height of the wave (from baseline to peak of negative deflection). Represents the sum of sensory fibers that depolarize.

Duration

Time between start and end of wave (i.e. time from initial negative deflection to initial return to baseline). The longer this is, the greater variability in velocities of the nerves. More on this in NCS pathologic findings.

Sensory conduction velocity

The conduction velocity is not directly measured during NCS studies. It is derived from the data obtained during the study using the well-known formula: velocity = distance / time. In sensory studies (but not motor), we can just use data from ONE SNAP to calculate conduction velocity. 

Let's use the example of the median nerve motor NCS once again (see Figure 3.6 above). Unlike in motor NCS, the neuromuscular junction and muscle are not involved, so the latency only reflects the time it takes for the sensory nerve to depolarize. So, we can just say conduction velocity = d1 / t1 = distance between wrist and APB muscle / onset latency. 

Note on antidromic sensory NCS

Sensory NCS can be performed antidromically (the electrical stimulus is measured coming from the spinal cord towards distal nerves, which is the non-physiologic direction for sensory nerves), or orthodromically (the electrical stimulus is measured on its way from distal nerves toward the spinal cord, which is the physiologic direction for sensory nerves). Each technique has advantages and disadvantages, the discussion of which is beyond the scope of this website. The technique described above is for antidromic sensory NCS.

Normal NCS: Self-test