Routine Clinical Electromyography

Chapter 2 Routine Clinical Electromyography



NERVE CONDUCTION STUDIES


There are three types of NCS that are used in clinical practice: motor, sensory, and mixed NCS. The motor fibers are assessed indirectly by stimulating a nerve while recording from a muscle and analyzing the evoked compound muscle action potential (CMAP), also referred to as the motor response or the M wave (M for motor). The sensory fibers are evaluated by stimulating and recording from a nerve and studying the evoked sensory nerve action potential (SNAP), also referred to as the sensory response. Mixed NCSs are less commonly used and assess directly the sensory and motor fibers in combination by stimulating and recording from a mixed nerve and analyzing the evoked mixed nerve action potential (MNAP).



Stimulation Principles and Techniques


Percutaneous (surface) stimulation of a peripheral nerve is the most widely used nerve conduction technique in clinical practice. The output impulse is a rectangular wave with a duration of 0.1 or 0.2 ms, although this may be increased up to 1 ms in order to record a maximal response. Two different types of percutaneous (surface) electric stimulators are used: both are bipolar having a cathode (negative pole) and anode (positive pole). The first type is a constant voltage stimulator that regulates voltage output so that current varies inversely with the impedance of the skin and subcutaneous tissues. The second type is a constant current stimulator that changes voltage according to impedance, so that the amount of current that reaches the nerve is specified within the limits of skin resistance. In bipolar stimulation, both electrodes are placed over the nerve trunk. As the current flows between the cathode and anode, negative charges accumulate under the cathode depolarizing the nerve, and positive charges gather under the anode hyperpolarizing the nerve.


With bipolar stimulation, the cathode should be, in most situations, closer to the recording site. If the cathode and anode of the stimulator are inadvertently reversed, anodal conduction block of the propagated impulse may occur. This is due to hyperpolarization at the anode that may prevent the depolarization that occurs under the cathode from proceeding past the anode. In situations where it is intended for the volley to travel proximally (such as with F wave or H reflex recordings), the bipolar stimulator is switched and the cathode is placed more proximally.


Supramaximal stimulation of nerves that results in depolarization of all the available axons is a paramount prerequisite to all NCS measurements. To achieve supramaximal stimulation, current (or voltage) intensity is slowly increased until it reaches a level where the recorded potential is at its maximum. Then, the current should be increased an additional 20–30% to ensure that the potential does not increase in size further (Figure 2-1). Stimulation via a needle electrode deeply inserted near a nerve is used less often in clinical practice. This is usually reserved for circumstances where surface stimulation is not possible, such as in deep-seated nerves (e.g., sciatic nerve or cervical root stimulation).




Recording Electrodes and Techniques


Surface electrodes are most often used for nerve conduction recordings. Surface recording electrodes are often made as small discs that are placed over the belly of the muscle or the nerve (Figure 2-2). The advantages of surface recording are that the evoked response is reproducible and changes only slightly with the position of the recording electrode. Also, the size (amplitude and area) of the response is a semiquantitative measure of the number of axons conducting between the stimulating and recording electrodes.



With motor conduction studies, the active recording electrode is placed over the belly of the muscle that correlates with the endplate zone. This ensures that muscle activity at the moment of depolarization is recorded as soon as the nerve action potential has arrived at the endplate. Ring electrodes are convenient to record the antidromic sensory potentials from hand digital nerves over the proximal and distal interphalangeal joints (Figure 2-3). These ring electrodes could act as stimulation points with orthodromic recording from hand digits.



Needle recording is also possible but is less popular and reserved for situations where the recording sites are deep-seated muscles or nerves. Needle recordings are also useful to improve the recording from small atrophic muscles or a proximal muscle not excitable in isolation. In contrast to surface recording, needle electrode recording registers only a small portion of the muscle or nerve action potentials and the amplitude of the evoked response is extremely variable and highly dependent on the exact location of the needle. Hence, amplitude and area measurement are not reproducible which renders this technique not clinically valuable such as in assessing conduction block or estimating the extent of axonal loss (see below).



Recording Settings and Filters


Filters are set in the recording equipment to reject low- and high-frequency electrical noise. Low-frequency (high-pass) filters exclude signals below a set frequency, while high-frequency (low-pass) filters exclude signals above a certain frequency. Filtering results in some loss or alteration of the signal of interest. For instance, as the low-frequency filter is reduced, more low-frequency signals pass through, and the duration of the recorded potential increases slightly. Likewise, as the high-frequency filter is lowered, more high-frequency signals are excluded, and the amplitude of the recorded potential usually decreases. Hence, all potentials should be obtained with standardized filter settings, and only compared to normal values collected using the same filter settings. The recommended low and high filter settings for motor conduction studies are 10 Hz and 10 kHz, respectively. The high-frequency filter is set lower for sensory nerve conduction studies than for motor nerve conduction since high-frequency noise (>10 kHz) commonly obscures high-frequency sensory potentials. For sensory conduction studies, the low- and high-frequency filters settings are typically 20 Hz and 2 kHz.


The amplifier sensitivity determines the amplitude of the potential. Overamplification of the response truncates the response, which results in false measurements of evoked response amplitude and area, while underamplification prevents accurate measurements of the takeoff point from baseline. Typically, sensory studies are recorded with a sensitivity of 10–20 μV/division and motor studies with a sensitivity of 2–5 mV/division.




Sensory Nerve Conduction Studies


Sensory NCSs are performed by stimulating a nerve while recording the transmitted potential from the same nerve at a different site. Hence, SNAPs are true nerve action potentials. Antidromic sensory NCSs are performed by recording potentials directed toward the sensory receptors while orthodromic studies are obtained by recording potentials directed away from these receptors. Sensory latencies and conduction velocities are identical with either method, but SNAP amplitudes are higher in antidromic studies and, hence, more easily obtained without the need for averaging techniques. Since the thresholds of some motor axons are similar to those of large myelinated sensory axons, superimposition of muscle action potentials may obscure the recorded antidromic SNAPs. These volume-conducted muscle potentials often occur with mixed nerve stimulation or may result from direct muscle co-stimulations. Fortunately, SNAPs can still be measured accurately in most cases because the large-diameter sensory fibers conduct 5–10% faster than motor fibers. This relationship may change in disease states that selectively affect different fibers. In contrast to the antidromic studies, the orthodromic responses are small in amplitude, more difficult to obtain, and might require averaging techniques (Figure 2-4).



SNAPs may be obtained by (1) stimulating and recording a pure sensory nerve (such as the sural and radial sensory responses), (2) stimulating a mixed nerve while recording distally over a cutaneous branch (such as the antidromic median and ulnar sensory responses), or (3) stimulating a distal cutaneous branch while recording over a proximal mixed nerve (such as the orthodromic median and ulnar sensory studies). The active recording electrode (G1) is placed over the nerve and the reference electrode (G2) is positioned slightly more distal with antidromic recordings or slightly more proximal with orthodromic techniques. The distance between G1 and G2 electrodes should be fixed (usually at about 3–4 cm), since it has a significant effect on SNAP amplitude. The SNAP is usually triphasic with an initial small positive phase, followed by a large negative phase and a positive phase. Several measurements may be recorded with sensory NCSs (Figure 2-5):








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Sensory conduction velocity may also be calculated after a distal and a proximal stimulation and measurement. For example, the median sensory SNAPs are obtained at the wrist and elbows and the conduction velocity is measured as follows:



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Motor Nerve Conduction Studies


Motor NCS is performed by stimulating a motor or mixed peripheral nerve while recording the CMAP from a muscle innervated by that nerve. The CMAP is the summated recording of synchronously activated muscle action potentials. The advantage of this technique is a magnification effect based on motor unit principles: Stimulation of each motor axon results in up to several hundred muscle action potentials with this number depending on the innervation ratio (number of muscle fibers per axon) of the examined muscle.


A belly-tendon recording is a typical electrode placement to obtain a CMAP: a pair of recording electrodes are used with an active lead (G1) placed on the belly of the muscle and a reference lead (G2) on the tendon (see Figure 2-2). Both active and reference electrodes locations are an essential determinant of the CMAP size, shape, and latency. The propagating muscle action potential, originating near the motor point and under G1, gives rise to a simple biphasic waveform with an initial large negative phase followed by a smaller positive phase. With incorrect positioning of the active electrode away from the endplate, the CMAP will show an initial positive phase that corresponds to the approaching electrical field of the impulses from muscle fibers toward the electrode. Similar initial positivity is also recorded with a volume-conducted potential from distant muscles activated by anomalous innervation or by accidental spread of stimulation to other nerves.


Whenever possible, the nerve is stimulated at two or more points along its course. Typically, it is stimulated distally near the recording electrode and more proximally to evaluate its proximal segment. Several measurements are evaluated with motor NCSs (Figure 2-6):










Physiologic Variabilities


Temperature. Nerve impulses propagate slower by 2.4 m/s or approximately 5% per degree Celsius as the limb cools from 38 to 29°C. Also, cooling results in a higher CMAP and SNAP amplitude and longer duration probably because of accelerated and slowed Na+ channel inactivation. Hence, a CMAP or SNAP with high amplitude and slow distal latency or conduction velocity should be highly suspicious of a cool limb (Figure 2-8).



To reduce this type of variability, skin temperature is measured with a plate thermistor that correlates linearly with the subcutaneous and intramuscular temperatures. If the skin temperature falls below 33 to 34°C, it is necessary to warm the limbs by immersion in warm water. Warming packs or a hydroculator can also be used, particularly in bedridden or intensive care unit patients. Adding 5% of the calculated conduction velocity for each degree below 33°C theoretically normalizes the result. However, such conversion factors are based on experience with healthy individuals and do not apply to patients with abnormal nerves.


Age. Nerve conduction velocities are slow at birth since myelination is incomplete. They are roughly one-half the adult value in full-term newborns and one-third that of term newborns in 23- to 24-week premature newborns. They reach adult values at 3–5 years. Then, motor and sensory nerve conduction velocities tend to slightly increase in the arms and decrease in the legs during childhood up to 19 years. With aging, conduction velocities slowly decline after 30–40 years of age, that the mean conduction velocity is reduced about 10% at 60 years of age.


Aging also causes a diminution in SNAP and CMAP amplitudes, which decline slowly after the age of 60 years. This affects SNAP amplitudes more prominently, that normal upper limb SNAP amplitude drops up to 50% by age 70, and lower limb SNAPs in healthy subjects above the age of 60 years are low in amplitude or unevokable. Hence, absent lower extremity SNAPs in the elderly must always be interpreted with caution, and are not necessarily considered abnormal without other confirmatory data.


Height and nerve segments. An inverse relationship between height and nerve conduction velocity suggests that longer nerves generally conduct slower than shorter nerves. For example, the nerve conduction velocities of the peroneal and tibial nerves are 7–10 m/s slower than the median and ulnar nerves. This cannot be explained entirely by the small reduction in temperature of the legs as compared with the arms. Possible factors to account for the length-related slowing include abrupt distal axonal tapering, progressive reduction in axonal diameter, or shorter internodal distances. For similar reasons, nerve impulses propagate faster in proximal than in distal nerve segments. Hence, adjustments of normal values must be made for individuals of extreme height, which is usually no more than 2–4 m/s below the lower limit of normal.


Anomalies. Anomalous peripheral innervations may mislead the electrodiagnostic physician and occasionally lead to erroneous diagnosis and treatment. There are several anomalous peripheral innervations that are important to recognize since they have a significant effect on NCS.


1. Martin-Gruber anastomosis. This is an anomalous connection between the median and the ulnar nerves in the forearm that usually consists of motor axons. Two or three communicating branches in the forearm leave the median nerve and join the ulnar nerve to innervate the ulnar-innervated intrinsic hand muscles, in particular the first dorsal interosseous muscle (the most common target), the hypothenar muscles (abductor digiti minimi), the thenar muscles (adductor pollicis, deep head of flexor pollicis brevis), or a combination of these muscles. Martin-Gruber anastomosis, also referred to as median-to-ulnar anastomosis in the forearm, is present in approximately 15–20% of the population, and is sometimes bilateral. This anomaly manifests during ulnar or median NCSs depending on where the anomalous fibers terminate.



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Aug 12, 2016 | Posted by in CARDIOLOGY | Comments Off on Routine Clinical Electromyography

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