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).

Figure 2-1 Supramaximal stimulation of a peripheral nerve during a motor nerve conduction study (median nerve stimulating at the wrist while recording abductor pollicis brevis). With a subthreshold stimulus of 3.6 mA (top response), none of the fibers were stimulated and no response was evoked. With a higher stimulus of 8.6 mA (second response), few fibers are stimulated and a low-amplitude compound muscle action potential (CMAP) is recorded. A further increase in the current to 12.4 mA (third response) reveals a larger CMAP that is still submaximal. A 17.4 mA stimulus results in a maximal CMAP (fourth response). This can only be confirmed after increasing the stimulus intensity by 20–25% (supramaximal stimulus of 22 mA, fifth response) and evoking a CMAP that is identical to the maximal CMAP. With maximal and supramaximal stimulations, all nerve fibers are stimulated.

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.

Figure 2-2 Belly-tendon recording of compound muscle action potential. The settings are for peroneal motor conduction study recording the extensor digitorum brevis and stimulating distally at the ankle. Note that the active electrode (G1) is over the belly of the muscle while the reference electrode (G2) is over the tendon. The ground electrode is placed nearby.

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.

Figure 2-3 Ring electrodes used in recording sensory nerve action potential (SNAP). The settings are for antidromic median SNAP recording index finger.

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.

Recording Procedure

A prepulse preceding the stimulus triggers the sweep on a storage oscilloscope. A stimulus artifact occurs at the beginning of the sweep, and serves as an indicator of the time when the shock occurred from which point latencies are measured. Digital averaging is a major improvement in recording low-amplitude responses by eliminating artifacts and noise. Signals time-locked to the stimulus summate at a constant latency and appear as an evoked potential, distinct from the background noise. The signal-to-noise ratio increases in proportion to the square root of the trial number. For example, four trials give twice as big a response as a single stimulus, whereas nine trials give three times the amplitude. Modern instruments digitally indicate the latency and amplitude when the desired spot on the waveform is marked.

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).

Figure 2-4 Antidromic and orthodromic median sensory nerve action potentials. The response in (A) is antidromic, i.e., stimulating the wrist and recording the index finger. The response in (B) is orthodromic, i.e., stimulating the index and recording at the wrist. Note that the peak latencies are comparable while the antidromic response is much larger in amplitude than its orthodromic counterpart.

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):

Figure 2-5 Antidromic median sensory nerve action potential stimulating wrist while recording middle finger revealing commonly measured parameters including a shock (stimulation) artifact.

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:

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):

5. Conduction velocity. This is a computed measurement of the speed of conduction and is expressed in meters per second (m/s). Measurement of conduction velocity allows the comparison of the speed of conduction between different nerves and subjects, irrespective of the length of the nerve and the exact sites of stimulations. Also, motor conduction velocity reflects the pure speed of the largest motor axon and, in contrast to distal and proximal latencies, does not include any neuromuscular transmission time nor propagation time along the muscle membrane. Motor conduction velocity is calculated after incorporating the length of the nerve segment between distal and proximal stimulation sites. The nerve length is estimated by measuring the surface distance along the course of the nerve and should be more than 10 cm to improve the accuracy of surface measurement. Motor conduction velocity is calculated as follows:

Figure 2-6 Median motor nerve conduction study revealing commonly measured parameters following distal (top) and proximal (bottom) stimulations.

Mixed Nerve Conduction Studies

Mixed NCSs are done by stimulating and recording from nerve trunks with sensory and motor axons. Often, these tests are done by stimulating a nerve trunk distally and recording more proximally, since the reverse is often contaminated by large CMAP that obscures the relatively low-amplitude MNAPs. In situations where the nerve is deep (such as the elbow or knee), the MNAP may be very low in amplitude or unelicitable, due to considerable tissue interposing between the nerve and recording electrode. Hence, these studies are not popular in clinical practice and are restricted to evaluating mixed nerves in distal nerve segments, such as in the hand or foot during the evaluation of carpal tunnel syndrome and tarsal tunnel syndrome, respectively (Figure 2-7).

Figure 2-7 Mixed nerve action potential. This is a median palmar response stimulating the mixed median nerve in the palm and recording orthodromically the median nerve at the wrist.

Segmental Stimulation in Short Increments

Routine NCSs are often sufficient to localize the site of lesion in entrapment neuropathies. However, during the evaluation of a focal nerve lesion, inclusion of the unaffected segments in conduction velocity calculation dilutes the effect of slowing at the injured site and decreases the sensitivity of the test. Therefore, incremental stimulation across the shorter segment helps localize an abnormality that might otherwise escape detection. More precise localization requires moving the stimulus in short increments along the course of the nerve while keeping the recoding site constant. This procedure is often labeled inching (or actually centimetering) since 1 cm increment is a common distance measurement. The large per-step increase in latency more than compensates for the inherent measurement error associated with stimulating multiple times in short increments. The analysis of the waveform usually focuses on sudden changes in latency values or abrupt drop in amplitude.

The inching technique is particularly useful in assessing patients with carpal tunnel syndrome, ulnar neuropathies at the elbow or wrist, or peroneal neuropathy at the fibular neck. For example, with stimulation of a normal median nerve in 1 cm increments across the wrist, the latency changes approximately 0.16–0.21 ms per cm from midpalm to distal forearm. A sharply localized latency increase across a 1 cm segment indicates a focal abnormality of the median nerve.

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).

Figure 2-8 The effect of temperature on antidromic median sensory nerve action potential stimulating at the wrist and recording the index finger. Response obtained with a skin palm temperature of (A) 33.5∞C and (B) 29.5∞C. Note that cool limb results in a SNAP with slower onset and peak latencies and higher amplitude.

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.