How to reduce noise interference in nerve conduction studies and EMG signals for more reproducible test results

In electrodiagnostic studies, the signal voltage can be very low, e.g. a few microvolts. To reliably detect these potentials, clinicians should reduce the ambient noise and interference generated by other electrical and magnetic devices.

The stimulation artifact generated during the nerve conduction studies can also affect the recorded response. When the noise and artifact amplitude are reduced, the recorded signals will be more reproducible. This gives the operator more confidence in the measurements and their interpretation of the pathophysiology.

In this article, we share tips on how you can optimize your EMG and neurography (also called nerve conduction studies) laboratory environment and instrumentation to reduce noise interference and artifacts.

EMG signals: From patient to clinician

A clinician and/or technologist works with the patient and the instrument controls to record the best quality signals. When the noise is unacceptable, the clinician makes adjustments and reassesses the noise. The procedure is repeated until a satisfactory signal can be recorded.

Signals: From patient to clinician

The procedure begins with placing the electrodes to record neurophysiologic potentials. Cables or leads connect the electrodes to the amplifier. Much of noise reduction is achieved by using a so-called differential amplifier and use of filters. The amplified signals are converted into a digital record by the analog to digital convertor (ADC).

When signals are sampled and displayed using a high sampling rate, the signals are without any technical distortion called aliasing. The digitized signals are analyzed by the software algorithms, also called digital signal processing (DSP). The measurements of waveforms are displayed on the computer screen. The operator can make necessary adjustments, e.g. adjust stimulator settings, move recording cables away from stimulator, etc. to reduce noise and artifacts. The process then repeats.

If there is a weak link at any of these stages, the recordings will not be optimal, making it difficult to compare the test results against reference values to reach a correct diagnosis. To ensure the best signal quality, you can optimize your workflow at each stage by following the tips below.

Patient preparation before EMG testing

All measurements (latency, amplitude and velocity) in nerve conduction study are affected by the limb temperature. Low conduction velocity due to cold limbs may be misinterpreted as a nerve pathology. Clinicians usually perform the conduction studies when the limb temperature is > 30-31 C.

When the tested limb is cold, you can use different methods to warm the patient. Regardless of the device or method, it is important to monitor the patient to avoid any heat burns. One popular method is to place hot packs’ wrapped in a towel. Such packs are often found in the rehabilitation services. Some laboratories use a heat lamp. These lamps may generate noise and interference in recordings.

Another method is to have patients warm their hands and feet in a sink or a tub with hot water. Some laboratories use a hair dryer. This is readily available and convenient to use. However, it may require the hospital biomedical safety department’s approval. It is the intent to warm the body tissue reaching the nerve. Hence, you should maintain the heating device for a few minutes even if the skin temperature appears satisfactory.

Many laboratories use the thermistor built in the EMG device to check the temperature. It can record the temperature automatically. Therefore, you should place it on the limb during each procedure. A probe connected to the system but not placed on the patient will record the ambient temperature, e.g. 22 C. This may cause confusion when a third party reads the nerve conduction report.

With greater awareness of germ transmission, many laboratories are using a contactless infrared thermometer. The measured temperature can be entered in the test results.

Reduce ambient noise in EMG signals

The electrodes, the cables and the amplifier are electronic devices that will record ambient noise. In order to minimize the ambient noise for better outcome, you can take the following into consideration when setting up your lab (Nandedkar, 2019):

Use incandescent lights. If you use fluorescent lights or tube lights, you will get spikes at the power line frequency.
Do not use dimmer switches, which also creates spikes.
Use short power cords and not extension cords as the power cord works as an antenna for electromagnetic radiation.
The system should be properly ‘grounded’. A biomedical engineer or electrician can check this connection in your wall power outlet. This is essential for electrical safety, but it will also reduce the 50/60 Hz power line interference. (The term ‘ground’ in this context refers to the third pin of the power plug, not the ground electrode used for recordings.)
Use short and shielded cables so they do not pick up any noise, and if possible, with braided leads.
Turn off all other devices and unplug them from the wall outlet. If you keep them plugged into the outlet, they are going to emit power line interference.

Use disposable electrodes for needle EMG and nerve conduction studies

Needle EMG examination should be done using a disposable concentric or monopolar needle electrode. These needles are for single patient use and pre-sterilized. This will significantly reduce the possibility of cross infection among patients. It also eliminates the need to sterilize needles between patients and sharpen them as reusable needle become blunt after repeated use.

Disposable surface electrodes are also a standard for nerve conduction studies in many laboratories. As indicated earlier, this eliminates the cost and time associated with cleaning the reusable electrodes. The disposable electrodes are also pre-gelled and have standardized recording surfaces. Circular surfaces for motor responses are suggested as the orientation of the electrode in relation to muscle fiber direction will not affect the potential.

Amplifiers with high CMRR and high input impedance ensure EMG signal quality

The electromyograph uses a differential amplifier to attenuate noise. The ability for noise attenuation is defined by the so-called ‘common mode rejection ratio (CMRR)” (Nandedkar, 2019). An amplifier with high CMRR is preferred. Note that the CMRR is measured using a logarithmic scale in decibels. If the CMRR is higher by 10 dB (e.g. 124 db vs 114 db), it attenuates the noise 5 times better.

The amplifier should also have a high imputer impedance (> 1000 MOhms) to make distortion free recordings. Finally, an amplifier should support a wide range of input signals to allow recordings of very low voltage (e.g. brain stem auditory evoked potential) to high amplitude signals (e.g. compound muscle action potential).

You should always use an amplifier with low noise, which is usually measured by the Root-Mean-Square (RMS) of the voltage, typically 0.7 µV, or peak-peak at 2 µV. Modern amplifiers can offer even less noise (e.g. 0.4 µV). Keep in mind that the noise level will depend upon further settings. If you reduce the bandwidth, you can get less noise but also distort the neurophysiologic signals.

Filters for EMG and nerve conduction studies

All EMG machines use filters to reduce noise. The settings also affect neurophysiologic signals and their measurements. Therefore, you should use standard filter settings. In hostile situations such as an intensive care unit, you may need to reduce the signal bandwidth (increase low frequency and/or decrease high frequency setting). The EMG machines may provide default settings that meet the guidelines.

The notch filter is most useful in sensory nerve conduction and needle EMG studies. It should not be used in motor nerve conduction study or late response recordings (F wave, H reflex, etc). The filter reduces signal amplitude and may create a ringing artifact.

Save and analyze EMG signals using high sampling rate

The analog-to-digital converter (ADC) is used to create a digital record of the EMG signals. Its principle is similar to that of a video camera. The camera takes 30 pictures per second. Although each picture is a static image, when they are played back you get the perception of a continuous motion. If the camera took only 4 pictures per second, the video will be dis-jointed.

The ADC measures the EMG signals at regular intervals. Each measurement is called a sample. The time interval between successive samples is called the sampling rate. The number of samples gathered over 1 second period is the sampling rate or sampling frequency (Nandedkar, 2019). Using our analogy, the video camera has a sampling rate of 30 Hz.

To preserve the signal characteristics, you should sample the signals at a minimum of two times the maximum frequency it contains. EMG machines should sample signals at a much higher rate. As example, the high frequency setting (i.e. low pass filter) in needle EMG is set to 10 kHz. A sampling rate of 48 kHz will give high quality EMG signals.

In nerve conduction studies, the signals are recorded using surface electrodes. They have less high frequencies (< 2 kHz). A sampling rate of 10 kHz may appear adequate. While this is adequate to get proper amplitude measurements, it may not be satisfactory to measure conduction velocity and its differences between different segments.

Using a sampling rate, e.g. 48 kHz, improves the reproducibility of these calculations, and hence the confidence in their analysis. The sampling rate varies among different programs on the EMG device. One method to describe the sampling rate is by the number of points in a trace. In nerve conduction studies, the systems usually record a 100 ms epoch (although a shorter initial portion is displayed). A sampling rate of 48 kHz will make the trace using 4800 data points.

Recently, the International Federation of Clinical Neurophysiology (IFCN) has provided guidelines for filter and amplifier settings for optimal signal display and analysis.

Optimal setting summary

Reduce stimulation artifact by cleaning the stimulus site

Stimulation artifact is inherent in the nerve conduction studies, and it occurs because of the spread of stimulus current to the recording site. You can reduce stimulus intensity by cleaning the stimulus site. This will result in lower stimulation intensity and artifacts. Dry the skin surface between the stimulation and recording site. This will reduce the current reaching the recording site.

Sensory nerve conduction studies are challenging due to their low amplitude. Stimulus artifact interferes with the response and can significantly affect the amplitude. The artifact can be reduced using many strategies. The most commonly used method is to ‘rotate’ the anode. The cathode stays over the nerve while the position of the anode is changed to get the least artifact. Bipolar stimulation also reduces the artifact. This is very useful in lower limb nerve sensory nerve conduction studies.

Finally, you can use digital signal processing algorithms. The ‘Enhance’ function estimates the slow ‘U’ shaped artifact (middle trace) and subtracts it from the recorded signal (top trace). The resulting signal has a better baseline before and after the response. This gives more confidence in analysis of the potential.

We hope that these tips will help you optimize your EMG and nerve conduction studies to record high quality signals with minimal noise and interference. If you want to know how to streamline your workflow for collecting and reporting data, you can download our guide: Six steps to a faster patient flow with accurate EMG & NCS results.