After the history of the patient is taken and the physical examination is performed, most of the studies begin with the nerve conduction studies (NCSs). The needle electromyography (EMG) examination is done after the nerve conduction studies are completed, because the findings on the NCSs are used in the planning and interpretation of the needle EMG.
Motor, sensory, or mixed nerve conduction studies can be performed giving important information on the underlying nerve pathology.
- MOTOR NERVE CONDUCTION STUDIES (MNCS) are performed to assess functional status of the motor fibers of the peripheral nerves. Motor responses are in the range of several millivolts (mV), as opposed to sensory and mixed nerve responses, which are in the microvolt (μV) range. For motor conduction studies, the gain is usually set at 2 to 5 mV per division. Recording electrodes are placed over the muscle of interest. The active recording electrode is placed over the center of muscle belly, the reference electrode is placed distally over the tendon. The stimulator then is placed over the nerve that supplies the muscle, with the cathode placed closest to the recording electrode. The duration of the electrical pulse is generally set to 0.2 ms (but 0.5 ms or 1 ms pulse durations are sometimes used, if necessary), and most nerves require a current in the range from 20 to 50 mA to achieve supramaximal stimulation. As current is slowly increased from a baseline 0 mV by 5-10 mA increments, more of the underlying nerve fibers are brought to action potential, and subsequently more muscle fiber action potentials are generated. The recorded potential, known as the compound muscle action potential(CMAP), represents the summation of all underlying individual muscle fiber action potentials. The CMAP is a biphasic potential with an initial negativity (upward deflection from the baseline). For each stimulation site, the latency (ms), amplitude (mV), duration (ms), and area (automatic calculation) of the CMAP (mV) are measured. A motor nerve conduction velocity (MNCV) can be calculated after two sites have been stimulated, and it is measured in meter per second (m/s). The MNCV is a measure of the speed of the fastest conducting motor axons, which calculated by dividing the distance traveled by the nerve conduction time. The distal motor latency (MDL) is more than a conduction time along the motor axon. It includes the conduction time along the distal motor axon to the neuromuscular junctions (NMJs), the NMJ transmission time and the muscle depolarization time. Therefore, to calculate a true motor conduction velocity two stimulation sites must be used (one distal and one proximal), and the MNCV calculated dividing the distance between two stimulation points by the difference between latencies does not include the NMJ or the muscle.
- SENSORY NERVE CONDUCTION STUDIES (SNCS) are performed to assess functional status of the sensory fibers of the peripheral nerves. In contrast to motor conduction studies, in which CMAP reflects conduction along the motor nerve, NMJ, and muscle fibers, only nerve fibers are assessed in sensory conduction studies. Because most sensory responses are very small (usually in the range between 1 to 50 μV), technical factors and electrical noise assume more importance. For sensory conduction studies, the gain is usually set at 10 to 20 μV per division. Pair of recording electrodes are placed in a line over the nerve at an interelectrode distance of 2-3 cm with the active electrode placed closest to the stimulator. Recording ring or surface disk electrodes can be used to test sensory nerves in the fingers. For sensory studies, an electrical pulse of 100 or 200 μs (0.1 s, 0.2 s) in duration is used, and most nerves require a current in the range from 10 to 30 mA to achieve supramaximal stimulation. Sensory fibers usually have a lower threshold of stimulation than do motor fibers. As in motor studies, the current is slowly increased from a baseline of 0 mA, usually in 3 to 5 mA increments, until the recorded sensory potential is maximized. This potential, the sensory nerve action potential(SNAP), is a compound potential that represents the summation of all the individual sensory fiber action potentials. SNAPs are usually biphasic or triphasic potentials. For each stimulation site, the onset latency (ms), peak latency (ms), duration (ms), and amplitude (μV) are usually measured. The sensory nerve conduction velocity (SNCV) can be calculated with one stimulation alone by taking the measured distance between the stimulator and the active recording electrode and dividing by the onset latency. No NMJ or muscle time needs to be subtracted by using two stimulation sites. The SNCV is measured in meter per second (m/s). Sensory conduction studies can be performed using either antidromic (stimulating toward the sensory receptors) or orthodromic (stimulating away from the sensory receptors) techniques. Latencies and conduction velocities should be identical with either method, although the amplitude generally is higher in antidromically conducted potentials.
- MIXED NERVE CONDUCTION STUDIES are the nerve conduction studies in which the recorded potential reflects both motor and sensory fiber action potentials generated along the nerve. In the routine clinical practice, the median, ulnar, and distal tibial nerves are most often selected for examination. These mixed nerve studies are used most often in the electrodiagnosis of median nerve neuropathy at the wrist, ulnar neuropathy at the elbow, and tibial nerve neuropathy across the tarsal tunnel. During routine motor and sensory NCV studies, the largest and fastest fibers are not recorded (only Aβ). The largest fibers (Aα, Ia sensory fibers which supply the muscle spindles) are recorded only during mixed nerve studies, wherein the entire mixed nerve is stimulated and also recorded. For mixed nerve measurements, the settings are similar to those used for sensory conduction studies. During the studies the palms and soles are stimulated, the recording is over the median and ulnar nerves at wrist, while the tibial nerve is recorded above and posterior to the medial malleolus. The recorded potential, the mixed nerve action potential (MNAP), is a compound potential that represents the summation of all the individual sensory and motor fiber action potentials. MNAPs are biphasic or triphasic potentials. The onset latency (ms), peak latency (ms), duration (ms), amplitude (μV), and conduction velocity (m/s) are usually measured.
- LATE RESPONSES STUDIES are the nerve conduction studies in which the two recorded late responses are the F-wave and the H-reflex; both late responses are used routinely to study the more proximal nerve segments, such as the plexus or the roots (cervical, lumbosacral). The F-wave is a late motor response that occurs after the CMAP (also known as the direct motor M potential – “M wave”) using supramaximal stimulus. The F-wave derives its name from foot because it was first recorded from the intrinsic foot muscles. In the upper extremity, when the median or ulnar nerves are stimulated at the wrist, the F-wave usually occurs at a latency of 25 to 32 ms. In the lower extremity, when the peroneal or tibial nerves are stimulated at the ankle, the F-wave usually occurs at a latency of 45 to 56 ms. The F-wave is derived by antidromic travel up the nerve to the anterior horn cell, with backfiring of a small population of anterior horn cells, and orthodromic travel back down the nerve past the stimulation site to the muscle. The F-wave circuitry is pure motor, and the F-wave is a small CMAP representing 1% to 5% of the muscle fibers. Each F-wave varies slightly in latency, configuration, and amplitude (Fig. 2). Several measurements can be made on the F-wave including minimal (fastest) and maximal (slowest) latency and F-wave persistence. In general, normal chronodispersion is up to 4 ms in the upper extremities and up to 6 ms in the lower extremities. F-waves tend to have lower sensitivity for radiculopathy but can be useful in the assessment of polyneuropathy. The H-reflex derives its name from Paul Hoffmann, who originally described this reflex in 1918. The H-reflex is a true reflex with a sensory afferent, a synapse, and a motor efferent segment. The H-reflex can be routinely elicited only in the lower arm, by stimulating the tibial nerve in the popliteal fossa and recording from the soleus muscle. Some authors elicited the H-reflex in the upper arm, by stimulating the median nerve at the elbow and recording from the flexor carpi radialis (FCR) muscle. The circuitry of the H-reflex involves the Ia muscle spindles as sensory afferents and the α motor neurons and their axons as efferents. If a low submaximal stimulus with a long duration is applied to a nerve, it is possible to relatively selectively activate the Ia fibers. The triphasic H responses appear at a latency of 25 to 34 ms with similar morphology and without chronodispersity. The H-reflex is most commonly used to evaluate for an S1 radiculopathy or to distinguish from an L5 radiculopathy.
Fig. 2 F-waves recorded from the Abductor Digiti Minimi (ADM) muscle (minimal F-wave latency 24.8 ms, maximal F-wave latency 27.1 ms), superimposed mode– left and raster mode – right