Nerve conduction velocities

Author(s): Anthony Chiodo, MD, Michael Wheaton, MD

Originally published:09/09/2015

Last updated:09/09/2015

1. OVERVIEW AND DESCRIPTION

Fundamentals

Nerve Conduction Studies (NCS) are a method to perform an extracellular recording of the intracellular process of neuron or muscle depolarization and action potential propagation along the axon or muscle fiber1. It is done by recording the action potentials (APs) for all of the axons that compromise a peripheral nerve. This recording of nerve signal transmission occurs at the surface of the skin using specialized electrodes. The course of the specific peripheral nerve being studied is identified using anatomical landmarks and knowledge of neuroanatomy. A stimulator electrode consisting of a cathode and anode is used to deliver a variable strength and duration electrical stimulus. A stimulus with more than sufficient strength to cause all of the axons to depolarize and propagate an AP and is termed supramaximal stimulation and is used for most NCS1. The depolarization and AP propagation is detected by a recording electrode (referred to as G1) and the reference electrode (G2)1. Several pieces of information are recorded about the resulting action potential. The distance (in centimeters) between the cathode of the stimulating electrode and the G1 electrode is recorded and is most commonly a set distance. The time that elapses (in milliseconds) between stimulus and depolarization is called the signal response latency. Both the time to initial depolarization (onset response latency) and the peak AP depolarization (peak response latency) at the G1 electrode are recorded. These two parameters, distance and time, allow for a velocity (meters/second) to be calculated. The signal amplitude (measured in micro- or millivolts) represents the maximal AP depolarization and voltage gradient of all of the depolarized fibers of a peripheral nerve or muscle fibers as measured at the G1 electrode, representing the number of axons or muscle fibers present. A ground electrode is used to help minimize electrical interference from the equipment, ambient surroundings and the patient.

Motor NCS can be recorded by stimulating a mixed sensorimotor or pure motor nerve and recording on a muscle this nerve innervates. Sensory NCS can be recorded by stimulating a sensorimotor or pure sensory nerve and recording at a separate location on the same nerve. Normal values for each NCS are determined based on recordings and calculations in asymptomatic subjects and are provider-specific.

The summation of the motor neuron APs, transmission through the neuromuscular junction and subsequent muscle fiber AP is termed the Compound Motor Action Potential (CMAP). The CMAP includes information about the number of afferent motor axons the nerve contains, the diameter and amount of myelin of those axons, the function of the neuromuscular junction (NMJ), and the number of excitable muscle fibers. The signal amplitude reflects the total number of motor neurons and muscle fibers that depolarize. Only the onset response latency is recorded for CMAPs and reflects the myelin quality of the largest diameter (fastest) axons contained in the nerve (1). To calculate a conduction velocity of the CMAP, two stimulation sites (one proximal and one distal) need to be recorded. This is done because the G1 electrode is placed on the muscle belly and the response latencies include the time it takes for transmission across the NMJ and action potential propagation through the muscle. The CMAP conduction velocity can be calculated along any proximal region of nerve using this calculation:

((distal – proximal distance) / (distal – proximal response latency)) (1).

The NMJ transmission time and muscle AP will be constant between the two sites and subtracted out, allowing calculation of the conduction velocity of the segment of motor nerve between the two stimulation sites. For this reason, at least two sites of stimulation are used for most motor NCS, but only one for most sensory NCS.

The summation of the sensory neuron APs of a peripheral nerve is termed the Sensory Nerve Action Potential (SNAP). In antidromic recording, the stimulus is delivered in the opposite of the physiologic direction (in sensory NCS, this is proximal to distal). Orthodromic recording measures AP propogation in the normal physiological direction and is distal to proximal for sensory NCS. The more common method of recording SNAPs is antidromic due to larger signal amplitudes and ease of recording2. The G1 electrode is placed over the skin along the course of the nerve and the G2 electrode is placed 3-4 cm distal to G1 along the same course. In antidromic stimulation, the stimulating electrode is then placed proximal to G1 along the nerve with the cathode closer than the anode to G1. Similar to a CMAP, signal amplitude reflects the total number of neurons that depolarize; however, SNAP amplitudes are several orders of magnitude smaller and are measured in microvolts rather than millivolts. Both the onset response latency and peak response latency are useful for SNAPs2. The peak response latency is helpful because it is a more reliable and reproducible, particularly given the overall small amplitudes of a SNAP and reflects a variety of diameters of sensory nerve axons1. The onset response latency reflects the AP propagation time for the largest diameter (fastest) sensory axons and is used to calculate the SNAP conduction velocity. The distance between cathode and G1 electrode is used for the numerator and the onset response latency is used as the denominator.

Proximal responses that are commonly evaluated are F-responses and H-reflex. The H-reflex is a mono-synaptic reflex arc, resulting from a submaximal stimulation of the sensory fibers of the tibial nerve behind the knee at low intensity and high duration. The sensory fibers synapse in the spinal cord fires the S1 motor neurons with resultant stimulation of the soleus muscle from which the recording electrode obtains the motor signal. In contrast, the F-response is generated from a supramaximal antidromic stimulation of motor neurons. After synapsing with interneurons, an orthodromic motor nerve AP returns resulting in stimulation of the muscle from which the signal is obtained. F-responses are commonly obtained from ulnar, median, fibular (peroneal), and tibial nerves.

Common Errors Affecting Accuracy

Decreased temperature leads to delayed Na+ channel inactivation and prolonged depolarization, which results in a larger voltage change and an increase in CMAP and SNAP signal amplitudes. The delay in Na+ channel inactivation will delay AP propagation and saltatory conduction, thus delaying onset response latency and conduction velocities1. These effects tend to be more prominent in SNAPs than CMAPs.

A lack of supramaximal stimulation can lead to artificially low signal amplitudes2. Too intense of a stimulus can lead to co-stimulation of adjacent nerves through volume conduction of the electrical signal in tissues. This may lead to artificially high signal amplitude or faster conduction velocities. In addition, too intense of a stimulus will enlarge the area of depolarization, leading to nerve activation further distal to the cathode at the time of stimulation, leading to a decrease in the distal latency2.

Longer nerves, in general, have slower conduction velocities due to the smaller diameter of the nerve as it tapers distally. Taller individuals may have slowed conduction velocities due to more tapering of the nerve and the tendency of recording NCS distally in the limbs. Additionally, longer limbs tend to be cooler than shorter ones due to the physical limitations of euthermia resulting from arterial and venous flow1.

Anomalous innervations can also contribute to errors. Without prompt recognition, the presence of an accessory peroneal nerve or Martin-Gruber anastomosis, a median to ulnar crossover in the forearm, can lead to the false interpretation of normal findings as pathologic.

Technical errors such as incorrect electrode placement, measurement errors, poor instrument maintenance or the incorrect use of filters on the recording equipment can also negatively impact the accuracy of NCS findings.

Neuropathology – NCS Correlation

Nerve conduction studies are commonly used to evaluate for specific nerve injury or entrapment. Lesions of the motor or sensory axon typically results in loss of motor and sensory evoked amplitudes. Decreased distal latency and conduction velocity is only seen when there is severe large fiber loss. Lesions affecting myeling lead to conduction slowing and are demonstrated by prolonged distal latencies for distal entrapments. For proximal entrapments, loss of conduction velocity across that specific segment or conduction block across that segment can be demonstrated.  Another feature of diseases or injuries to myelin include conduction block. This is a drop in the evoked amplitude on stimulation proximal to the lesion, but a normal amplitude on distal stimulation. This indicates that some of the individual axon’s impulses fail to traverse the area of injury in spite of the fact that that nerve is still alive and viable and able to be stimulated distal to the site of entrapment. Temporal dispersion also occurs with demyelinating lesions. When some axons have slowed APs through a region and others conduct normally, the resultant waveform is a curve that is wider due to a larger difference between the fastest and slowest fibers. The spreading of action potential speeds also results in a lower amplitude, as there is not as many APs coming in at the exact peak time.

2. RELEVANCE TO CLINICAL PRACTICE

Nerve conduction studies are also useful in evaluating for generalized neuropathies. In axonal neuropathies, the primary abnormality will be the loss of sensory and motor evoked amplitude with proximal or distal testing. Prolongation of distal latency or decrease in conduction velocity will only occur after extensive large fiber loss and is only a secondary feature. In acquired demyelinating neuropathies, salient nerve conduction findings include the loss or prolongation of F-wave latencies and H-reflexes, prolonged distal latencies, decreased conduction velocities, and the presence of conduction block with proximal stimulation as well as temporal dispersion. In congenital demyelinating neuropathies, similar findings are seen but without the features of conduction block or temporal dispersion.

With the many ways that nerve conduction studies are used, the choice of tests to do on any one patient needs to be individualized. A history and physical examination prior to study will lead to a differential diagnosis. That differential diagnosis will then dictate what sensory and motor nerve conduction studies and proximal responses to evaluate. Doing a routine nerve conduction “screen” is not appropriate5.

3. CUTTING EDGE/CONCEPTS/EMERGING ISSUES

Motor unit number estimation is a nerve conduction technique that is used in research settings to allow the estimation of the number of nerve axons in a motor nerve. Although primarily used in ALS research, its application could extend to other motor axonal disorders such as post-polio syndrome and radiculopathy. It might also be useful in evaluating nerve recovery after surgical repair. There are multiple methods of this technique which are time consuming and challenging to complete. Further techniques might be more useful in the clinical setting to be able to follow patients over time4.

4. GAPS IN KNOWLEDGE/EVIDENCE BASED

There is a growing consensus about the need to come up with universal standard values for nerve conduction studies. Eliminating variance in how nerve conduction studies are conducted and reported will allow for reproducibility and longitudinal evaluation regardless of where a patient is studied. The concern over the quality of nerve conduction studies had heightened with the proliferation of hand held devices and mobile labs where large numbers of nerves are tested with lack of adherence to practices to avoid common errors in nerve conduction studies5.

REFERENCES

  1. Preston, D.C., Shapiro, B.E. Electromyography and Neuromuscular Disorders. Philadelphia, PA: Elsevier: 2005.
  2. Dumitru, D., Amato, A., Zwarts, M. Electrodiagnostic Medicine. Philadelphia, PA: Elsevier: 2001.
  3. Olney, R.K. (1999). Chapter 10. Consensus Criteria for the Diagnosis of Partial Conduction Block. Muscle Nerve 1999; 22: Supplement 8: S225-S229.
  4. Gooch Cl, et. Al. Motor unit number estimation: A technology and literature review. Muscle and Nerve 2014; 50: 884-893.
  5. AANEM Position Statement. Proper Performance and Interpretation of Electrodiagnostic Studies. 2014, available at https://www.aanem.org/getmedia/bd1642ce-ec01-4271-8097-81e6e5752042/Position-Statement_Proper-Performance-of-EDX_-2014.pdf . (date accessed 6/17/15).
  6. Sandin KJ, et. al. Clinical quality measures for electrodiagnosis in suspected carpal tunnel syndrome. Muscle and Nerve 2010; 41: 444-52.

Author Disclosure

Anthony Chiodo, MD
Department of Defense: Research Grants; NIDRR: Research Grant

Michael Wheaton, MD
Nothing to Disclose

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