Electrodiagnostic (edx) testing provides information about the peripheral nervous system and is an extension of the physical examination. Although most commonly comprised of nerve conduction studies (NCS) and electromyography (EMG), it can also include special testing, such as late responses, repetitive stimulation, and single-fiber EMG. Each test sheds light on a different aspect of the peripheral nervous system—from the alpha motor neuron, through the neuromuscular junction, and down to individual muscle fibers. This chapter provides an introduction to EDX testing and reviews nerve electrophysiology and common instrumentation used in EDX studies. Basic descriptions of the different types of EDX studies will also be examined.
In neurons, depolarization and repolarization cycles create action potentials. Action potentials are brief changes in the electrical potential on the surface of neurons or muscle cells. Action potentials create a current I (measured in milliamperes) and potential difference or voltage V (measured in millivolts) along the membrane. The resistance to current of an alternating-current (AC) circuit (such as in human tissue) is termed impedance Z (measured in kilo-/megaohms). In electrophysiology, V = I × Z.
Every axon and motor cell has a resting membrane potential of around −70 µV. When a stimulus is strong enough to reach threshold potential (−30 µV in most cells), the membrane depolarizes. Sodium channels open, allowing sodium ions to flood into the cell. Sodium channels are slowly deactivated as potassium ions exit the cell, causing an initial hyperpolarization, before the Na+/K+ pump restores the resting membrane potential. This period of hyperpolarization is also known as the “relative refractory period” because the hyperpolarized membrane requires an even greater stimulus than normal to bring the cell membrane to threshold and cause a subsequent depolarization. The period just prior to the relative refractory period is called the “absolute refractory period” because the sodium channels have not yet reset and a second action potential cannot be elicited until they have done so. The refractory period promotes unidirectional action potential propagation (Fig. 70–1).
Figure 70–1
(A) Nerve action potential. The upstroke of the action potential results from increased Na+ conductance. Repolarization results from a declining Na+ conductance combined with an increasing K+ conductance; afterhyperpolarization is due to sustained high K+ conductance. (B) Action potential propagation. Local current flow causes the threshold potential to be exceeded in adjacent areas of the neuron membrane. Because the upstream region is refractory, an action potential is only propagated downstream. In myelinated axons, action potentials propagate faster by “jumping” from one node of Ranvier to the next node by saltatory conduction. ARP = absolute refractory period; RRP = relative refractory period. (Reproduced with permission from General Physiology. In: Kibble JD, Halsey CR, eds. Medical Physiology: The Big Picture, New York, NY: McGraw-Hill; 2014.)
The beginning of the peripheral nervous system is the alpha motor neuron. It is a large-diameter nerve (10–20 µm) that resides in the ventral horn of the spinal cord. The alpha motor neuron sends projections ipsilaterally out the ventral and dorsal rami to the corresponding muscle it innervates. The axon and all the muscle fibers it innervates are known as a “motor unit” (Fig. 70–2). The finer the movement required, the lower is the number of total muscle fibers in a single motor unit. The opposite is also true. Thus, motor units used to abduct the eye are smaller than those required to bend the elbow. Individual axons are surrounded by a myelin sheath the entire length of the axon until just before the terminal hillock, which lacks myelin and forms the neuromuscular junction (NMJ).
Myelin serves multiple purposes in the nerve. Without myelin, current would be able to flow in all directions and attenuate due to the resistance of the surrounding tissues. If too much current is lost, the stimulus weakens and will not further propagate the action potential. Additionally, by insulating the nerve axons, myelin allows for faster conduction because current is maintained in the myelinated portions. Small breaks in the myelin, termed the “nodes of Ranvier,” are heavily concentrated with sodium and potassium channels that maintain membrane potential and can trigger action potentials. This type of conduction is termed “saltatory conduction” (Fig. 70–3).
Once an action potential reaches the terminal axon, the speed slows due to the lack of myelination. The terminal axon has an increased number of calcium channels. When depolarized, calcium floods into the terminal axon. Calcium influx promotes fusion of vesicles filled with acetylcholine (ACh) to the membrane, which release into the synaptic cleft (see Fig. 70–2). At the NMJ, two types of potentials exist—endplate potentials (EPP) and miniature endplate potentials (MEPP). MEPPs occur once every 5 seconds at the motor endplate due to spontaneous quanta release. EPPs occur with any depolarization of the endplate transmembrane potential. When multiple EPPs occur, calcium lingers for 200 ms so that if a second action potential occurs, increased quantity of ACh can be released. This phenomenon is termed “facilitation.” Once released, ACh binds to ACh receptors in the postsynaptic membrane, which leads to opening of voltage-gated sodium channels, resulting in depolarization of the motor cell, calcium release from the sarcoplasmic reticulum, and eventual muscle contraction (Fig. 70–4).
Figure 70–4
Innervation of skeletal muscle. (A) Motor units include a motor neuron and the group of muscle fibers that are innervated by its branches. (B)The neuromuscular junction. Acetylcholine release from motor neuron terminals stimulates nicotinic receptors in the muscle membrane, producing an excitatory postsynaptic potential. (C) Endplate potentials. Acetylcholine in a single presynaptic vesicle (quantum) evokes a miniature endplate potential. Action potential in a motor neuron triggers the release of many quanta, and miniature endplate potentials summate to exceed the threshold for the action potential in the muscle fiber. (Reproduced with permission from General Physiology. In: Kibble JD, Halsey CR, eds. Medical Physiology: The Big Picture New York, NY: McGraw-Hill; 2014.)
Another important concept to understand at the NMJ is the safety factor. The safety factor is the difference in voltages between the threshold level and the final magnitude of an EPP. The safety factor allows for the membrane to stay depolarized above threshold, despite the depletion of quanta. If the EPP ever falls below the safety factor, no further ACh can be released from the terminal axon. Such is the case in NMJ disorders such as myasthenia gravis.
A sound understanding of the machine used for EDX studies is essential for every electromyographer. Most systems consist of a stimulator, amplifier, and computer with monitor and speakers (Fig. 70–5). The stimulator has both a cathode (negative terminal) and an anode (positive terminal). It is connected to the base unit that communicates with the computer setup. A constant-voltage or constant-current stimulus can be given at different intensities and frequencies. The responses are recorded either through electrodes on the skin for NCSs or from the needle for the EMG portion of the test. There is an active electrode (G1), reference electrode (G2), and a ground. These electrodes connect to the differential amplifier. The amplifier takes the signals seen by the G1 and G2 electrodes, subtracts the common waveforms, and then sends the signal to the filters. This process is termed “common-mode rejection.”
In EDX studies, action potentials from muscles and nerves are viewed and interpreted as waveforms. These waves have a frequency (cycles per time or hertz) and amplitude that varies with time. Compound muscle action potentials (CMAPs) and sensory nerve action potentials (SNAPs) are the summation of all waveforms that occur after stimulation. The CMAPs are primarily low-frequency waveforms, and the SNAPs are high-frequency waveforms.
Filters help to remove the waveforms that are not related to the particular study of interest (motor, sensory, or externally produced). The high-frequency filters are also known as “low-pass filters” because they allow the low-frequency waveforms to pass on to the analog-to-digital converter. The low-frequency filters are conversely known as “high-pass filters” because they allow the high-frequency waveforms to pass through. Frequency settings for filters are also important to understand in order to optimally view denervation potentials and evaluate motor unit action potentials (MUAPs) during the needle electrode examination (NEE). Common motor filter settings are 2–5 Hz (low) to 10 kHz (high). Typical sensory filter setting are 5–10 Hz (low) to 2–3 kHz (high). Needle EMG settings typically range from 10–20 Hz (low) to 10 kHz (high). The notch filter is used to remove the common-mode (60 Hz) signal that comes from most electrical devices.
It is also helpful to understand the changes that occur when filter settings are altered. When the low-frequency filter is increased, fewer of the low-frequency waveforms pass through. As a result, the peak latency is reduced in SNAPs and the onset latency is reduced in CMAPs because the high-frequency waveforms peak quicker. Similarly, when the high-frequency filter is lowered, fewer high-frequency waveforms pass through, resulting in longer peak and onset latencies. Increasing the low-frequency filter or lowering the high-frequency filter generates smaller amplitudes because fewer waveforms are allowed through to summate.
Once the signal passes through the selected filters, it goes on to the converter, which converts the signal from analog to digital. On many machines, the analog sound does not require conversion. The video, however, is converted to a digital signal that is then displayed on the monitor and saved on the computer. When measurements are entered, the computer can generate conduction velocities, onset or peak latencies, and amplitudes, which all assist the electromyographer in his or her interpretation of the data.
Along with the amplifier and filters, the EMG machine has a number of other functions that can be adjusted. “Gain” is the increase in amplitude of an action potential as a result of electrical amplification. “Sensitivity” is similar but refers to the display resolution of the amplified signal. When the sensitivity is decreased, the waveforms will appear larger on the screen. The “sweep” refers to the number of milliseconds displayed per unit on the screen. Increasing the sweep speed may decrease the onset latency. The optimal sweep speed is 2 ms per division, which allows for clinical accuracy of latency readings of 0.1 ms.
