Standardization of Electromyography (EMG) instrumentation is of particular importance to ensure high quality recordings. Standardisation of instrumentation and recording will enable comparison of Electromyography (EMG) and nerve conduction studies (NCS) results within and between laboratories. Such standards and guidelines will allow a uniform practice and improve the selection of patients for research studies.
Contemporary EMG machines have a dedicated hardware unit with amplifiers, stimulators, and control panel. and a separate computer. In the EMG hardware, the amplifiers and stimulators are the most important parts. An important aspect of any machine is its ergonomics and control panel functions, including the foot switch and hand controls. When setting up a machine, it should be tuned optimally, preferably by a team including representatives from the users (physicians, technicians, local engineers) and manufacturers.
When considering which machine to purchase, these details must be considered, since they influence medical quality, operability, and in the long term, cost efficiency.
Computer hardware:
Number of screens: Multiple screens allow EMG signals and data to be shown simultaneously with other data, such as the referral or radiology.
CPU speed and RAM memory size: The minimum limits are specified by the EMG equipment manufacturer. The requirements depend on the complexity of the software.
Hard drive size: The minimum limits are specified by the EMG equipment manufacturer. If data is to be stored locally, the minimum size needs to be sufficient for the estimated number of stored patients. Greater memory is usually needed at some time, so anticipation of this is recommended.
Loudspeaker for replay of EMG signals without EMG hardware unit. A good sound quality is essential.
Printer: Local and/or network connected.
Analysis software:
Available tests, workflow, and various other features should optimize workflow and be compatible with local practice and reference limits.
Help functions such as strategies and signal quality control.
Reference limits:
Prepare for ongoing collection of reference material
Result presentation; Tables, signals, text, color
Compatibility with used algorithms
Voice control may be used for some functions
Remote viewer: View ongoing (live) recordings remotely from another computer.
Database support: Ensure the database engine used by the EMG equipment is supported and compatible with the server to be used.
Operating system: The type and version options are specified by the EMG equipment manufacturer.
Reporting:
Multiple designable templates that comply with requirements from all referring sources.
Digital delivery of text, signals, and tables.
The primary function of the electrodiagnostic system is to faithfully record and analyze various biological signals. It is important to have an optimal ‘signal to noise ratio’, i.e. amplify the neurophysiological signal voltage while attenuating background noise. This is done using analog hardware and digital signal processing techniques. The signal and noise are recorded by surface or needle electrodes. It is carried to the amplifier input via electrode leads or cable. These components behave like an antenna and may add more noise. A differential amplifier magnifies the signal while attenuating the unwanted noise, aided by analog filters. The amplified signal is measured using an analog-to-digital converter (ADC) and the voltage values are stored as an array of numbers. This digitized signal allows further computerized analysis. Some algorithms reduce the noise, e.g., digital filters, averaging, smoothing, etc., and others make measurements such as latency, amplitude, and area in NCS. More sophisticated algorithms can detect MUPs in needle EMG. Signal characteristics are also assessed from the signal sound, generated either with analog hardware or using digital technology.
An EMG machine also offers stimulation devices to excite nerves and muscles. These may generate electrical, visual, or auditory stimuli. External devices providing other forms of stimulation, e.g. magnetic field, contact heat, reflex hammer, etc. can be interfaced to provide timing signals through so-called ‘triggers’. To achieve this, some instruments pass the digitized signals through a ‘digital-to-analog converter (DAC)’, to convert digital signals (with much less noise) into analog form. This can be used for research where the investigator wants to re-sample the signals and develop algorithms for their own analysis.
The EMG machine displays signals, measurements, and the settings of the amplifier and stimulator. The latter can be changed using a dedicated control panel or using software commands via a mouse or computer keyboard. Data collection can be initiated using the foot switch. The software is responsible for signal processing and for generating reports. Databases can be created, and remote reviews used for second opinions or to help with interpretation. Constant changes in operating systems and in regulations governing patient information protection can make this a challenging task. The electrodiagnostic systems can also record non-neurophysiological events. Temperature, for instance, should be measured and recorded during NCS. A ‘patient response’ unit may be used to record the number of times the test subject acknowledges different types of stimuli in cognitive function assessments. Video cameras may be integrated into the system to observe the patient’s behavior during a study, e.g., focussing on the checkerboard pattern in a visual evoked potential investigation. Recently we have seen the addition of ultrasound imaging probes to the device. The handling of these inputs is very different from that of the neurophysiological potentials, and is outside the scope of the current discussion.
EMG equipment uses digital computers for data sampling, storage and signal processing. After the analog signal has been amplified, the analog-to-digital (AD) converter discretizes the signal in both time and amplitude, and assigns a digital value to the amplitude at defined time points. This assignment of the amplitude to a digital value is performed by using a finite number of digital amplitude values. In this conversion process, two important criteria must be satisfied. First, the sampling frequency should be sufficiently high to reliably represent the original analog signal. Second, the digitization of the amplitude should be sufficiently fine to accurately represent the amplitude of the original signal in the digital domain. The digitizing an analogue signal, consisting of a sine wave with frequency 1 Hz and amplitude 1 mV using different sampling rates and AD converters. The phenomenon is known as ‘‘aliasing”: if the sampling frequency, fs, is lower than the highest frequency present in the signal of interest, fmax, the frequency obtained has an erroneous value, in this example 0.1 Hz. Using appropriate algorithms, it is possible to reconstruct the waveform in detail if the sampling rate is more than twice the highest-frequency component of the waveform (Nyquist theorem). In practice, the sampling frequency used is typically 2–5 times the highest frequency component in the signal of interest. In order to guarantee that the maximum frequency in the signal is known, an analog ‘‘anti-aliasing” filter is used before the signal is digitized.
The amplifier is perhaps the most critical component for the quality of the electrodiagnostic system. Selective amplification of a neurophysiological potential while attenuating background noise can be accomplished using a ‘differential’ amplifier (DA). DA requires inputs or connections from three electrodes. In past the electrodes were called ‘G1 , ‘G2 and ‘ground’. The terms G1 and G2 refer to the grids of vacuum tubes used in old amplifiers which are no longer available. Later these inputs were called ‘active’, ‘reference’ and ‘ground’. The term ‘ground’ is confusing. The ‘ground’ in electrodiagnostic recording refers to a point on the amplifier circuit that is used as a point of reference for voltage measurement. Outside electrodiagnostics, it is also used to describe one of the connections in the power supply and wall outlets. The ‘reference’ electrode is presumed to be electrically silent but does record large volume-conducted potentials, such as the electrocardiogram (ECG). Given the confusion of terms and their origins, the terms ‘E1 , ‘E2 and ‘E0 are recommended for the three connections to the amplifier. On most systems these inputs are colour-coded as black (E1), red (E2) and green (E0).
The amplifier does not amplify the voltage at E1 or E2 inputs. It magnifies their difference, and hence it is called a differential amplifier. E1 electrode (a monopolar needle) recording a 50 uV fibrillation potential. The E2 is a surface electrode placed on the skin surface and for simplicity it is assumed that it records no electrical activity, i.e. 0 uV. Their difference is amplified and the fibrillation potential is seen as a 50,000 uV signal. This amplification by 1,000 is the ‘differential’ gain of the amplifier. The ambient noise is also recorded by both electrodes and their cables. Here the ‘common’ noise is 1,000 uV, but the difference between signals at E1 and E2 is zero, and the noise will not be seen at the amplifier output. Similarly, the very large ECG potential can be eliminated by differential amplification. This enables the selective amplification of the small neurophysiologic signal in the presence of high-amplitude noise. The example reflects an ‘ideal’ DA. In practice the ‘common signal’ at E1 and E2 inputs is also amplified, but much less. The noise voltage at the output is 500 uV. The ratio of output to input noise voltage, produces a ‘common mode gain’ of 0.5 for the amplifier. In this example, the signal-to-noise ratio at the amplifier input is 0.05 and would make it difficult to recognize the fibrillation potential. However, at the output of the amplifier, the signal-to-noise ratio is 100, and this would allow it to be recognized quite easily. A DA should have a high differential gain and low common mode gain. These properties are defined in a single characteristic called the common mode rejection ratio (CMRR). It is reported in units of decibels and calculated as CMRR (dB) = 20 x Log (Differential Gain/Common mode gain.
In our schematic amplifier, the CMRR is 66 dB. Modern electrodiagnostic systems have amplifiers with CMRR exceeding 100 dB. It is important to note that the CMRR decreases at higher frequencies. Most vendors specify the value at 50 or 60 Hz (i.e., power line frequency).
Another characteristic of the amplifier is the input voltage range. As example, if the range is 50 to + 50 uV, then signals with amplitudes between these ranges can be handled without distortion. If the signal amplitude is outside the range, it will saturate the amplifier and the true signal amplitude cannot be measured. This is recognized from the ‘clipped’ peaks of the signals on the display. So, the amplifier range should be set higher than the amplitude of signals recorded in the test. In sensory NCS, the stimulus artifact can be much bigger than the nerve response. If the amplifier saturates, it can generate a long-duration artifact that interferes with the sensory potential to be recorded. The range is controlled in the software to avoid such distortion. The electronic components of the amplifier also produce some noise This is addressed by the E1, E2, and E0 electrodes being connected together and the amplifier output measured. The peak-peak amplitude or root mean square (RMS) value of the signal is reported. RMS is usually less than 1 uV, but also depends on the amplifier range and the filter settings. A high amplifier range (i.e. low gain setting) and short band width will give lower noise. The amplifier is also characterized by its ‘input impedance’. As the amplifier needs a tiny amount of current to measure a voltage, the input impedance needs to be several orders of magnitude larger than the input impedance of the generator of the voltage, i.e. muscle, nerve, and body fluids. Essentially, the voltage measured (Vm) is given by: Vm = Vsource*(Ropamp)/(Rsource + Ropamp). With Rsource the ‘internal impedance’ is meant. If Ropamp = Rsource, then Vm = 0.5*Vsource. Without going into details of circuit analysis, the amplifier impedance should be high. Low impedance makes the system more sensitive to environmental noise. It may also underestimate signal amplitude. Fortunately, modern systems report impedances in excess of 100 to 1000 mega-Ohms. Just like CMRR, the impedance decreases at higher frequencies.
Modern systems offer ‘switching amplifiers’. The unit provides a ‘head box’ with many input connections. The user can select the inputs in software to select any pair of inputs to make a recording. This facility is used mainly for evoked potential studies where a small set of electrodes is used to create multiple channels of recordings. These channels usually have a much lower CMRR. Such channels may not be suitable for recording signals with high frequencies (e.g., needle EMG) or when the electrodes differ in their impedances (surface versus needle). The best strategy for high quality recordings is to reduce the ambient noise and to ensure that the noise is not different on E1 and E2 electrodes.
Ideally, our measurement system should reproduce the signal of interest as exactly as possible while rejecting undesired signals. In clinical practice, however, we typically obtain a mixture of signals and ‘noise’, where the latter refers to any signal that does not contain relevant information for our diagnostic procedure. The undesired signal components can result from ambient power line noise (50 Hz or 60 Hz) or movement artifacts, but can also include EMG signals in a frequency range that is outside the region of interest for a particular procedure. For example, in recording single-fiber action potentials, the signal of interest is in the frequency range 0.5–5 kHz, where lower-frequency components (such as distant EMG potentials) can be safely suppressed. While filtering refers to any process where irrelevant signals are suppressed, in most clinical situations filtering is limited to attenuation of particular frequency components in the signal. The name of a filter is then defined by the frequency values that the filter attenuates (low or high frequency filters) or passes (high-pass filter, bandpass filter). The amount of attenuation depends on the ‘‘steepness” of the filter (expressed as the attenuation in dB/octave) and the cut-off frequency. The cut-off frequency is defined as the frequency where the original signal’s amplitude is attenuated by 3 dB. In today’s equipment, all filters (except for the anti-aliasing filter) are digital, which also allow filtering of frequency components with minimal phase distortion of the signals.
Low-frequency filters (LFF; high pass filters): Low-frequency (or, synonymously, high-pass) filters attenuate low-frequency components in the signal. An increase in the low-frequency cut-off causes initial amplitude loss of slowly changing signals, waveform distortion, but more importantly it also decreases the latency to the peak of the waveform and can introduce artifacts (i.e. a tail of the motor unit action potential). When recording motor unit potentials (MUP), the duration as well as the amplitude decreases when the cut-off is increased up to 500 Hz. Using a 500 Hz cut-off the contribution from distant muscle fibres is attenuated because of the soft tissue itself acts as a high-frequency filter. Ideally, the low limiting frequency should be one decade (factor 10) lower than the lowest frequency of the signal, to make sure that a phase shift, if present at all, does not affect latencies. Movement artifacts contain slow frequencies. In some cases, the only way to remove this artifact is to increase the lower limiting frequency. This is commonly done for surface EMG recording of movements (e.g., tremor recordings; gait) and may be necessary for motor evoked potential recordings to transcranial magnetic stimulation (TMS). It is usually required if EMG traces are rectified before averaging, e.g., for averaged F waves.
High-frequency filters (HFF; low pass filters). High-frequency filters attenuate high frequencies. A decrease in the high-frequency cut-off reduces the amplitude and rise time. If using a high-frequency cut-off that is too low, the system will not be able to record adequately the rise of the potential (containing the highest frequencies of the signal), and this may lower the amplitude, reduce the number of phases, and prolong the duration of the main peak component of the signal. An inappropriate HFF can also affect the measurement of onset latency of a potential. It will be prolonged because the abrupt decline from the baseline is missed and more time elapses before the beginning of the potential can be appreciated.
Band-pass filters: Most filters used are band-pass filters, a combination of a high and low-frequency filter.
Notch filter: A notch filter is a special type of band-stop filter. In electrophysiology it is normally designed to reduce power line interference (50 Hz or 60 Hz). Ideally, it should not be used because most neurophysiological signals contain significant components at this frequency, and their use may hide interesting components. In addition, the phase changes abruptly back and forth around the notch frequency, and this may distort the waveform.
The widespread adoption of signal averaging in the 1970s has greatly enhanced the precision of NCS, particularly those on sensory nerves, where the signal-to-noise ratio is lower than during motor conduction studies. In clinical practice, signal averaging is indicated whenever there is a low signal-to-noise ratio, i.e., when the background ‘‘noise” obscures the potential to be recorded or the latencies of that potential. In these recordings, the signal may be so small that the noise inherent in the recording obscures the potential. Sensory nerve action potentials (SNAPs) may be recorded with surface electrodes (or near-nerve needle electrodes) but with both techniques the SNAP is often difficult to define in single sweeps; somatosensory evoked potentials (SSEPs) are always so. Signal averaging is recommended routinely, even when the potential can be visualised readily, because it is critical that onset latency be defined accurately.
Electrical safety and leakage currents. The maximum current which leaks to ground is dependent on the type of the medical devices but usually it should not exceed 10 mA at 50 Hz according to IEC 60601–1 (International Electrotechnical Commission 2005) recommendations (IEC, 2005).
Equipment must be checked annually and a certification plaque be fixed to the EMG machine.
All ground sockets in the laboratory should be connected to a single installation ground lead point. Ideally, all other electrical devises including heating lamps must also be connected to this point. No other ground terminals should be used (e.g. water pipes).
Stray currents in power cords, which correlates to the length of the cords, for example with extension cords (AAEM, 1999). In normal conditions, the 3rd prong on an electrical plug serves as a ground. This enables dissolving the leakage currents on a power cord safely, but in case of a malfunctioning ground, the leakage current may induce arrhythmia while passing across the patient´s body. This may happen if the ground electrode is placed on the contralateral side to the stimulating electrode although this is more a theoretical risk rather than an established risk (London, 2017). To avoid this;
The ground electrode should never be placed contralaterally to the stimulation site and
Extension cords should not be used.
Special attention should be paid in intensive care units where the patients are connected to several electrical medical devices. Implanted pacemakers, cardiac defibrillators and stimulators.
Pacemakers are used to treat bradycardic episodes while intracardiac defibrillators treat tachycardic events such as ventricular fibrillation. Both function as sensing and stimulating devices, and most electromyographers are concerned that these devices may be charged improperly during NCS. Existing studies showed that pacemakers or intracardiac defibrillators cannot sense NCS stimulations including supraclavicular stimulations even by using stimulus intensities and durations that probably exceed those routinely. The only risk may arise in patients with old pacemakers with monopolar sensing configurations in case high intensity proximal repetitive stimulation is used. This may alter the pacing for 2–3 seconds which will in fact not cause more than lightheadedness in these patients (London, 2017). Some pacemaker or automatic implantable cardioverter-defibrillator companies require the placement of a ‘‘magnet” on the devices to monitor heart rhythm during NCS. However, an earlier study showed that magnet placed patients reported more symptoms so this is not recommended during NCS. While modern pacemakers with bipolar configuration and automatic implantable cardioverter-defibrillators implanted below the skin are shown to be safe,
NCS are still contraindicated in patients with temporary transvenous cardiac pacemakers because they may have a direct electrical conduit from the surface of the skin to the heart (London, 2017).
Peripheral intravenous lines in extremities are safe but for the safety of central venous catheters, further research is necessary. Some authors have suggested in these patients that proximal NCS or NCS in the ipsilateral extremity should not be performed because the catheters in the internal jugular or subclavian veins extend to the heart (Al-Shekhlee et al., 2003) while others did not show any influence of NCS on electrocardiographic monitoring (London, 2017).
No interaction was found between NCS and deep brain stimulator devices (London, 2017). It is suggested that caution should be excised in this area.
Recommendations on electrodiagnostic studies and anticoagulation: In a recent review, no precautions were recommended during needle EMG examinations in patients on warfarin therapy if INR < 3.0 or on antiplatelet medication (London, 2017). If the INR > 3, the studies may be performed at the discretion of the examiner. In both conditions, close surveillance during and immediately after the examination is essential. There is no literature on the risks of heparin or other oral anticoagulant therapy, but precautions are likely to be unnecessary at therapeutic doses. However, a recent survey showed that some electromyographers are as cautious as with warfarin about the novel (direct) oral anticoagulants, particularly for EMG of the paraspinal and facial muscles and single fiber EMG (Lee and Kushlaf, 2018). Prospective risk–benefit studies are necessary to establish safety guidelines.
Surface electrodes are used as stimulating or recording electrodes for NCS, for recording surface EMG data, for serving as reference electrodes for monopolar needle EMG, and as a common reference or ‘‘ground” electrode. In the past, surface electrodes generally consisted of small round or square reusable metal disks or metallic wire loops (the last for use in measuring sensory potentials from the digits), all employed in conjunction with a conductive electrode gel. However, to reduce the risk of infection and for reasons of convenience, this approach has been increasingly replaced with the use of self-adhesive, disposable electrodes. These are generally silver-silver chloride electrodes, with an adhesive conductive gel overlying the electrode surface; they can often be used several times on a single patient before they need to be replaced, generally because the adhesive loses its efficacy. Reusable, saline-saturated Velcro fabric band electrodes are also used to record sensory potentials from the digits. For surface recordings of electrical impedance myography, both reusable metal and disposable carbon-based electrodes have been used. Electrical impedance myography has been discussed in detail in a separate IFCN guidelines ‘‘Standards for Quantification of EMG and Neurography” (Stålberg et al., 2019).
A variety of surface stimulating electrodes are used for nerve conduction studies. Most commonly these include metal ‘‘prong” electrodes used with a small amount of adhesive gel or saline soaked felt electrode pads or pledgets. In either case, the electrodes are embedded within a handheld stimulator that can be easily repositioned to help identify the best region for stimulation. Small metal disk or adhesive electrodes, described above for recording, can also be used for stimulation, especially if repeated recordings from a single nerve are desired over an extended period of time. This is the case, for example, when performing studies of nerve excitability (including measurements of strength-duration time constant, threshold electrotonus and recovery cycle). The details of excitability techniques are beyond the scope of the present recommendations and have been discussed in detail in a separate IFCN guidelines, ‘‘Measurement of axonal excitability”.
Although surface electrodes are widely used for NCS, conventional surface EMG methods consisting one bipolar signal from two electrodes cannot provide detailed information about motor unit morphology, including their distribution across the muscle endplate. Therefore, special multi-channel high-density surface EMG (HD-sEMG) techniques have been developed. HD-sEMG requires sophisticated instrumentation and signal analysis of measures from multiple closely spaced electrodes, overlying a restricted area of the muscle. This enables measurement of both temporal and spatial EMG activity, thus providing different aspects of motor unit characteristics, such as muscle fiber conduction velocity measurements, motor unit number estimation and the evaluation of single motor unit characteristics. HD-sEMG has been shown to be beneficial for the assessment of different clinical conditions including motor neuron disorders (Wood et al., 2001; Drost et al., 2004) and disorders (Huppertz et al., 1997) and pathological changes have been shown at the motor unit level in neurogenic and myopathic muscles. However, HD-sEMG has not yet been incorporated into clinical practice as a diagnostic tool. Recent research has also mostly focused on physiological studies (Sleutjes et al., 2018; Lapatki et al., 2019), sports medicine (Martinez-Valdes et al., 2017) and rehabilitation (Gallina et al., 2016) rather than the primary diagnosis of neuromuscular disorders. Detection of the HD-sEMG signals requires high quality amplifiers with suitable specifications. Several kinds of amplification chains have been described for different strategies. As HDsEMG remains a research method, further details of these techniques are beyond the scope of the present report.
For nerve conduction studies, repetitive nerve stimulation test, stimulated single fibre EMG, and nerve excitability testing, an electrical stimulus is delivered to the nerve. Standard stimulation techniques use surface electrodes, whereas needle electrodes are used for nerve root stimulation or deep nerve stimulation (e.g., the sciatic nerve). Usually two surface electrodes are placed over the nerve with an inter-electrode distance of 2–4 cm with the cathode (where the nerve is stimulated and membrane depolarisation occurs) placed closest to the recording electrode. Inversion of the polarity of the stimulation electrode will affect the point of stimulation, and thereby the onset latency and nerve conduction velocity. With deep nerves, a short interelectrode distance should be avoided because a greater stimulus is required the deeper the nerve and, it is then more painful. This may be an issue when stimulating the radial nerve in the spiral groove or the femoral nerve. The duration of the stimulus is usually between 0.1 ms and 1.0 ms. The activation time of axons varies with the duration of the stimulus, even when it is supramaximal, and the measured latency includes this time. In routine nerve conduction studies, short-duration stimulus pulses are therefore preferred, and this also (1) minimizes patient discomfort, (2) restricts the site of stimulation which may spread with longer duration pulses, as well as high-intensity pulses, and (3) reduces the stimulus artifact. In Hreflex studies, a longer stimulus duration (1.0 ms) is used to favor the activation of the large sensory fibres (Ia afferents) responsible for the reflex. The stimulus can be applied as single pulses, paired pulses, or as trains with repetition frequencies between 1–2/s and 50/s for repetitive nerve stimulation test. High-frequency stimulation (20–30 Hz for 1–2 s) has been used for detection of incremental responses in Lambert-Eaton myasthenic syndrome. More recently a single shock before and after a brief maximum voluntary contraction is preferred to demonstrate increment because this reduces discomfort and is equally effective. Two types of stimulators are used. The constant voltage stimulator delivers a constant voltage (0 V to 400 V). The disadvantage of this type is that the resultant current is not constant because of the fluctuations of the electrode-tissue impedance. Constant current stimulators deliver a constant and stable current (0 mA–100 mA). Stimulus current is independent of electrode/skin impedance as long as the stimulator is not overloaded. Constant current stimulation is therefore preferred by most physicians. Supramaximal stimulation is required in standard nerve conduction studies and repetitive nerve stimulation test to activate all axons at the same time for measurement of motor and sensory response amplitudes, and nerve conduction velocity. Particularly when stimulating at proximal sites (e.g., Erb’s point) or in nerves with very high threshold due to pathology (demyelination, nerve hypertrophy), it may be difficult to reach a supramaximal level. The most proximal parts of peripheral nerves can be activated by TMS or high-voltage electrical stimulation over the spine, providing additional information to that from peripheral NCS (Matsumoto et al., 2010). Conduction across proximal segments of peripheral nerves, plexuses and nerve roots is commonly tested using H-reflexes and F-waves. An alternative technique using magnetic stimulation or high-voltage electric stimulation involves direct stimulation of the nerve roots, and this may be preferable. Conversely too-high stimulus currents will result in both distal displacement of the stimulation site, leading to a shorter onset latency, and stimulation of nearby nerves. Avoiding a stimulus that is too high is important to ensure the most information with the least discomfort or pain. However, this must be tempered by the fact that erroneous reports of conduction block occur when the stimulus was not really supramaximal, particularly when stimulating at proximal sites. If a supramaximal stimulus causes discomfort that is regrettable but preferable to an erroneous conclusion.
The concentric needle consists of two electrodes, the first electrode is a wire electrode, typically platinum that is insulated and housed within a steel cannula acting as the second electrode. The surface area of the wire electrode depends on the wire diameter and the bevel angle of the needle and is usually between 0.01 and 0.09 mm2, typically 0.07 mm2. The differential recording is then achieved by measuring the voltage between the wire electrode (active electrode (E1)) and the entire cannula shaft (reference electrode (E2)). The main spike component of the MUP is generated by approximately 2–12 fibres within a radius of about 0.5–1 mm around the tip of the needle. More distant fibers contribute to the initial and late parts of the potential. Due to the short distance between the electrodes, a great common mode voltage recorded by the active and reference electrodes is present leading to the elimination of much distant activity and providing a relatively sharp and self-contained MUP.
Monopolar needles for EMG recordings are usually constructed from a stainless-steel core that is coated with Teflon except for an exposed cone tip of 1–5 mm that acts as the active electrode. The recording area is approximately 0.03–0.34 mm2. The potential difference is measured between the exposed tip of the needle and a second reference electrode. This reference electrode may be a needle placed subcutaneously or a surface electrode at some distance from the active electrode. The reference electrode should be placed over an electrically silent area, such as a tendon or a bone. Impedance mismatch between the active monopolar needle and a surface electrode can lead to a reduced common-mode signal and greater artifacts, including power line interference (50 or 60 Hz). Monopolar needles record larger amplitudes and greater duration than concentric needles, but the number of phases is comparable. Monopolar needles can also be used as recording electrodes for sensory nerve action potentials in the near-nerve technique and as stimulation electrodes (e.g., stimulated single fiber EMG) (Kouyoumdjian and Stålberg, 2008). In the former situation, the reference electrode is also usually a monopolar needle placed subcutaneously at a distance from the nerve. Hollow core monopolar needles are also used for botulinum toxin injection, assisting with correct muscle localization prior to injection.
Single fiber EMG electrodes have an active region consisting of a platinum wire approximately 25 mm in diameter exposed on a side port of a steel cannula, with the cannula itself serving as the reference lead (similar to a concentric needle). Within the pick-up range (a semicircle with a radius of 300 mm, pick-up area 0.0005 mm2) in healthy muscles there are usually no more than 2–3 fibers, allowing for pairs of fibers to be easily obtained. Concentric needles can also be used to collect ‘‘single-fiber-like” data; this is achieved by increasing the cutoff frequency of the high pass filter so as to help distinguish individual spikes within the MUP.
Physiologic factors:
Temperature
Age
Height
Proximal vs distal nerve segments
Anomalous innervations
Non-physiologic factors:
Electrode impedance mismatch and 60 Hz interference
Filters
Electronic averaging
Stimulus artifact
Cathode position: reversing stimulator polarity
Supramaximal stimulation
Co-stimulation of adjacent nerves
Electrode placement for motor studies
Antidromic vs orthodromic recording
Distance between recording electrodes and nerve
Distance between active and reference recording electrodes
Limb position and distance measurements
Limb position and waveform morphology
Sweep speed and sensitvity
NCS and temperature
Most important of all the physiologic factors and affect every parameter measure in NCS and also affects MUP morphology during the needle EMG exam.
CV slows by 1.5 to 2.5 m/sec per 1°C cooling
DML prolongs by 0.2 msec per 1° C cooling
Increased amplitude and duration of potentials on NCS (SNAP > CMAP)
Increased amplitude, and phases of MUP.
Maintain temperature between 32°C to 34°C
Thermometer to check skin temperature: Exergen DermaTemp DT-1001 Infrared Dermal Thermometers.
Age:
Full term infants have CV approximately half of adult normal values.
At 1 year of age CV becomes 75% of adult normal values.
By 3 - 5 years of age it is equal to adult normal values.
After age 50 CV slows by about 1 m/sec per decade
CV decrease slightly with age in adults, likely due to normal loss of motor and sensory neurons.
In 60 years or older, CV decreased approximately by 0.5 - 4 m/s/decade (sensory > motor).
SNAP amplitudes fall by 50% average by age 70 years.
Lower ext. SNAPs (peroneal and sural) may be absent in normal individuals >70 yrs.
MUP duration increases with age (from birth to childhood, it is due to physiologic increase in muscle fiber size and motor unit size)
MUP size and duration is generally larger in older individuals as a result of dropout of motor units from the normal effects of aging, leading to some compensatory "normal" reinnervation resulting in large amplitude and longer duration.
The loss of motor units has been estimated to be approximately 1% per year beginning in the 3rd decade of life, which then increases rapidly after age 60 years.
Height:
Tall persons have slower CV, than do shorter persons.
Longer limbs have longer nerves.
Nerves taper as they reach distally.
CV is directly proportional to nerve diameter.
Nerves are warmer proximally and less well insulated distally and thus become cooler.
Normal sural sensory CV is 5 m/s slower than median sensory CV
Normal peroneal and tibial motor CV is 6 - 9 m/s slower than median and ulnar motor CVs.
In practice, adjustments usually is no more than 2 - 4 m/s below LLN.
F responses and H reflex must be interpreted based on height.
Filters:
Filters setting can affect onset and peak latencies by affecting the waveform.
Fourier analysis: Total shape of waveform is composed of the sum and amount of its frequency components.
Waveforms that are taller than wide have a greater proportion of high frequencies
Sensory action potentials contain higher frequency components.
SNAPs, frequency composition is 0-2000 Hz
Fibrillation potentials (high pitched)
MUP with crisp (short rise time)
Waveforms that are wider than taller have greater proportion of low frequencies.
CMAPs, frequency composition is 0-500 Hz.
MUP with dull (long rise time)
Duration is primarily a low frequency response
Filter settings:
SNAP: 20 - 3 KHz
CMAP: 20 - 10 KHz
SSEP: 20 - 3 KHz
Blink reflex: Motor NCS settings, but raise the low-frequency filter to 20 Hz to reduce the amount of slow activity
Effect of changing filters on waveform:
General rule:
If the band pass is narrowed, frequencies get removed from the total sum of the waveform and therefore the amplitude and area will decrease.
If the band pass is widened, frequencies are added and thus the amplitude and area of the waveform will increase.
Narrowing and widening of the band pass occurs regardless of whether high or low frequencies are removed or added.
High frequencies are subtracted, making the waveform appear less vertical (rise time is impacted).
Amplitude is decreased.
Amplitude is a high frequency response.
The peak latency and duration is increased.
Rise time and latency are determined by high frequency components.
Duration is increased.
High frequencies are added, making the waveform appear more vertical.
Amplitude is increased.
The peak latency and duration is decreased.
Low frequencies are added to the band pass, causing the entire waveform to appear more flat and wide.
The peak latency and duration are increased.
Duration is a low frequency response.
Increasing the LFF (or high-pass filter) filters out frequencies below a set limit. Increasing the LFF will exclude more low-frequency signals from the tracing. Accordingly, the duration and amplitude of the waveform may be reduced.
Low frequencies are removed from the band pass, causing the entire waveform to appear more vertical.
Amplitude is decreased.
The peak latency and duration are decreased.
Onset latency is not affected.
Skin impedance can be thought as resistance to current flow.
V = IR. Voltage = current in mA x impedance or resistance in kOhms
High impedance can cause noisy baseline, force high stimulation intensities to compensate for lack of response > greater shock artifact and tilting of baseline > problems with accurate measurement of amplitude and latency.
Shock artifact can cause recorded potential to have prolonged onset and peak latencies.
Impedance is checked by using impedance button on EMG machine
Display in kOhms
Impedance should be <20 kOhms for most NCS
Difficult to achieve this when sural nerve testing
Methods to reduce skin impedance:
Rubbing alcohol and removing oils from skin. Patients should not put oils or lotion.
Use skin abrasive
Electrode gel
Needle electrode
High impedance in both recording and reference electrode mean there is a problem with either the ground electrode or both recording and reference electrodes are faulty.
Whatever is commonly detected by the two electrodes (recording and reference) is rejected. What is not rejected is amplified by differential amplifier.
4 cm difference between electrodes along the nerve to make sure they are recording the SNAP at slightly different times. If they are too close, they will record SNAP at the same time, and it will be rejected (common mode rejection).
Speed of the NCV (50 m/sec in UE) x duration of the waveform (most SNAPs are 0.8 msec wide) = 4 cm
Do not put electrodes closer than 4 cm wide.
Remove sweat, make-up, lotion, excessive gel.
Use small amount of conducting gel
Place ground between stimulator and recording electrodes
Reduce electrode impedance mismatch between the recording electrodes
Use coaxial cables
Make sure the stimulator is positioned directly over the nerve
Lower the stimulus intensity
Rotate anode of the stimulator while keeping the cathode in position over the nerve.
Increase the distance between stimulator and recording electrodess
Ensure that the stimulator and recording electrode cables do not overlap.
Active and reference recording electrodes should be of the same type
Ensure all contacts are intact without any frayed or broken connections
Clean all dirt and oil from the skin using alcohol or acetone
Apply conducting electrode jelly between the skin and electrodes
Secure electrodes firmly to the skin with tape or Velcro straps
Place ground between stimulator and recording electrodes
Use coaxial recording cables.
Gain refers to vertical magnification of the waveform.
measured in uV for SNAPs, and mV for CMAPs
Note that a sensitivity of 5 uV/mm means that, to obtain a pen deflection of 1 mm, a 5 uV input voltage is required. To obtain a 10 mm deflection, an input of 50 uV is needed. If sensitivity is decreased to 10 uV/mm (increase in numerical value), the same 1 mm pen deflection now requires a larger input of 10 uV. Correspondingly, a 10 mm pen deflection now needs an input of 100 uV. This perhaps seemingly paradoxical relationship can sound counter-intuitive.
As gain is increased (high sensitivity), it will become apparent that where the potential really begins (i.e., the onset latency) is less than at low gain, and the duration will increase. Thus, as sensitivity is increased (e.g. 5 mv > 1 mv > 100 uV), the latency usually decreases, when the sweep speed is held constant. Increasing the gain allows ability to detect changes at the baseline, more readily. For example, one will be able to see the onset latency more easily as it first deviates from the baseline.
50 uV/division for viewing insertional and spontaneous activity, and 200 uV/div to 1 uV/div for analysis of MUP during needle EMG
Mark all the waveforms for a given nerve recording at the same gain setting.
Sweep speed refers to horizontal axis of EMG display.
measure in msec
SNAPs and CMAPs: 1 msec/div
F and H waves: 5 msec/div
EMG (needle): 10 msec/div
Adjust sweep speed if waveform is prolonged and does not appear completely on the screen in order to mark the duration.
As sweep speed decreases (e.g. 2 ms > 1 ms > 0.8 ms) the latency measurement usually decreases, when the sensitivity is held constant.
Peak latencies are not affected by changes in either sweep speed or sensitivity.
Amplitude is measure by multiplying the height of the waveform (number of vertical divisions) x gain setting.
example: 4 divisions high with gain of 200 uv/divison = 800 uv
Normal range: <2000 uV and >200 uV.
If more than 3 potentials are increased than call large amplitude; same rule applies for low amplitude.
Variability in amplitude is best seen with minimal voluntary activation muscle tested in order to isolate MUP while using slow sweep speeds (50-100 ms/div).
Multiply how many division wide a waveform is with the sweep speed.
Width of waveform is determined from the time it first deviates (takes off) from baseline to where it returns back to a smooth baseline.
Normal duration in most limbs is 5 - 18 ms.
Upper limit for increased duration is 20 ms. For sweep speed 100 ms, 2 division = 20 ms, and 1/2 division = 5 ms
Criteria for duration: Point when the waveform leaves the baseline to the point when it returns to baseline.
Recruitment: If firing rate is around 10 Hz, in a screen with sweep speed of 100 ms, one will generally have one unit firing. Limb muscles should not be firing at 15 Hz without another motor unit (upper limit). if one see more than a couple of units around 5 - 6 Hz, then this is early recruitment as can be seen in myopathy.
Recruitment frequency measured is the frequency at which the first motor unit is firing as the second motor units appears
Multiply the number of times a motor unit appears in 200 msec (screen has 20 boxes width) by 5.
example: If you see the same motor unit twice on the screen, then 2 x 5 = 10 cycle/sec or Hz
How?
200 msec is 200/1000 or 1/5 of a second (1000 msec = 1 sec)
2 motor units appear in 1/5 of a second. How many times would they appear in 1 sec at the same firing rate?
2 x 5 = 10 cycles/sec = 10 Hz
So when the first motor unit fires away at a frequency of 10 Hz, a second motor unit appears. The recruitment frequency is therefore 10 Hz, but he recruitment ratio is 5:1. This is derived from the equation: frequency at which the fastest motor unit fires (typically the 1st motor unit) divided by the total number of motor units firing across the screen. So if the 1st motor unit is firing at a rate of 10 Hz and there are 2 motor unit firing away on the screen, the the recruitment ratio is 10/2 = 5.
Criteria: 4 or more crossing the baseline = polyphasic, <4 = simple
Upto 25% polyphasic in deltoid muscle. Other muscles, 20%
So in 20 motor units one needs to see 5 - 6 polyphasic motor units in order to call polyphasic motor units.
Stable or unstable
Use the trigger: Enable the trigger on the MUP of interest. This center the MUP. If the MUP has variable in it trailing morphology it is unstable, if not it is stable. Check a few MUP to conclude.
Variability in amplitude is best seen with minimal voluntary activation muscle tested in order to isolate MUP while using slow sweep speeds (50-100 ms/div).
Variability in morphology is best associated with fast sweep speeds (1-5 ms/div) and slow activation of muscle tested.
EDX feature of DNMT
Immature NMJ, traumatic nerve injury, ALS.
Stable = >3 months old
Unstable = <3 months old
NCS:
Motor study:
Sensitivity: 5 mV/div
LFF: 3 Hz
HFF: 10 KHz
Sweep speed: 2 ms/div
Duration of stimulus: 0.2 ms
Sensory study:
Sensitivity: 20 microvolt/div
LFF: 20 Hz
HFF: 2 KHz
Sweep speed: 1 ms/div
Duration: 0.05 to 0.1 ms
Needle EMG
Insertional/spontaneous activity: amplifier at 50 microvolt/div
Normal insertional activity <300 ms
If sweep speed is 100 ms, insertional activity greater than 3 divisions (boxes) is considered increased.
Voluntary MUAP acvitity: amplifier 200 microvolt/div to 1 mV and higher.
Sweep speed: 20 ms/div
LFF: 10-30 Hz
HFF: 20-30 Hz
The most frequent cause of artifact is the electromagnetic radiation from power sources of 50 or 60 Hz, since its frequency is within the physiological range of the EMG signal.
Technical origin:
Cable motion artifact, with possible additional triboelectric (electrostatic) effect (low frequency range, 1–10 Hz).
Transducer noise from displacements in the gel-skin interface, including changes associated with skin stretch.
High electrode skin-electrode impedance
Intrinsic noise from the EMG machine (e.g. from amplifiers, semiconductors).
Biomedical devices (e.g. pacemaker).
Biological origin
ECG
Neighbouring muscles (crosstalk)
It is important to consider a number of procedures to reduce the impact of artefact on the quality of the recording.
Isolate electrical circuit of the EMG machine from the ones for other electrical devices, unplug and disconnect unnecessary electrical devices and lights located in the room, avoid use of fluorescent lights and dimmer switches (they give high frequency noise spikes).
For conduction studies and surface EMG, skin surface should be cleaned using sand paper, abrasive gel or 70% alcohol to reduce skin impedance by removing electrically non-conducting elements forming a high-impedance transcutaneous potential generator, which is increased by stretch-deformation. Skin abrasion with or without a drop of peeling paste is generally effective and although puncturing the skin with a needle has been proposed as a well-tolerated option, this is often not necessary.
Select the appropriate electrode size and their distance according the muscle volume to reduce the chance of cross-talk. It should be considered that smaller surface electrodes have higher impedance, requiring more careful skin preparation.
Double differential recording can be used to eliminate crosstalk in demanding protocols, in the latter technique signals arriving simultaneously at both electrodes are deleted, since propagating signals are time-delayed.
Filtering the signal to remove frequencies outside the known physiological source is important (like mechanical and electrical noise). However, elimination is not complete for the frequencies above and below the setting limits. Filtering is not useful for cross-talk, since desired and cross talk signals have similar frequency ranges.
Proper patient grounding is essential to reduce electromagnetic noise. The patient ground electrode is attached to the amplifier as a reference to differential inputs to improve rejection ratio mode. A large surface ground electrode or felt band ground electrode is recommended, positioned close to the recording electrode (between stimulator and recording electrode in conduction studies), not overlying electrically active surfaces like as muscle, and with a low electrical resistance (<3–5 kOhms). Sometimes it is advisable to ground the examiner too, in order to reduce power line artifact. Make sure the power outlet used for the electrodiagnostic system (equipment grounding) has good connection to ‘earth’ (i.e. to a pole buried in the earth outside the building). This is also required for safe operation of the instrument. Cables should be short, fixed, shielded, and separated from others (in particular recording and stimulator cables).
Stimulus artifacts depend on its intensity, duration and distance between recording and stimulation sites. They can distort the waveform and interfere with the accurate measurement of latency with short nerve segments. As always, it is recommended that the lowest supramaximal intensity is used.
Ideally, EMG should be performed in a quiet, temperature controlled room, separated from any source of electrical noise.
Screening of the rooms is not necessary anymore due to better amplifiers.
It should be considered that when the amplitude and/or decay of the stimulus artifact is an issue, attention needs to be paid to the grounding, skin perspiration, the orientation of the stimulating and recording electrodes, the quality of the skin-electrode interface, and the high-pass filter, for instance, are the recording electrodes dry? If attention to these factors does not fix the problem, the amplification may need to be reduced so that the trace remains within the linear range of the amplifier and A/D converter. Removing artifacts may be even more difficult during electrophysiological examinations in intensive care units. To minimize artifacts, all unnecessary plugs should be removed.