Nerve conduction study


A nerve conduction study is a medical diagnostic test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. These tests may be performed by medical specialists such as clinical neurophysiologists, physical therapists, physiatrists, and neurologists who subspecialize in electrodiagnostic medicine. In the United States, neurologists and physiatrists receive training in electrodiagnostic medicine as part of residency training and, in some cases, acquire additional expertise during a fellowship in clinical neurophysiology, electrodiagnostic medicine, or neuromuscular medicine. Outside the US, clinical neurophysiologists learn needle EMG and NCS testing.

Purpose and indications

Nerve conduction studies along with needle electromyography measure nerve and muscle function, and may be indicated when there is pain and/or weakness in any extremity which could indicate spinal nerve compression or some other neurologic injury or disorder. Spinal nerve injury does not cause neck, mid back pain or low back pain, and for this reason, evidence has not shown EMG or NCS to be helpful in diagnosing causes of axial lumbar pain, thoracic pain, or cervical spine pain.
Nerve conduction studies are also used for evaluation of paresthesias and/or weakness of the arms and legs. The type of study required is dependent in part by the symptoms presented. A physical exam and thorough history also help to direct the investigation.

Preparation and procedure

Patients typically do not require special preparation before undergoing an NCS and should take their medications and eat normally prior to the examination. Patient should be advised to avoid applying lotions or creams to the skin, as these substances can interfere with electrode conductivity. The test is non-invasive and can be performed in an outpatient clinic or hospital setting.
The nerve conduction study is often combined with needle electromyography. The Department of Health and Human Services Inspector General recently identified the use of NCSs without a needle electromyography at the same time a sign of questionable billing.
The nerve conduction study consists of the following components:

Equipment

Below is a general list of equipment used during an NCS, but it may not include everything an NCA practitioner may use.
  • "Electrodiagnostic machine with stimulator"
  • "Surface electrodes"
  • * Types include "wire ring, disposable pads, or standard bar."
  • "Ultrasound gel"
  • "Alcohol prep pads"
  • "4x4 gauze"
  • "Adhesive bandages"

    Technique

  1. Electrode placement: Surface electrodes are strategically placed on the skin over the nerve being tested and on a muscle it supplies or further along the path of that same nerve. These electrodes record the nerve's electrical response and are referred to as surface recording electrodes. A ground electrode is then placed on the limb being studied between the recording electrodes and the mapped areas of stimulation from the stimulation electrode. To decrease outside electrical interference and improve the quality of the recording, gel is usually placed between the electrode and the skin and, depending on the type of electrode used, the electrodes may be held in position with medical tape.
  2. Stimulation: An electrical impulse is administered to the targeted nerve via the stimulating electrode, resulting in a "propagated nerve action potential." This electrical stimulation may be slightly painful, so practitioners should warn patients.
  3. Recording: The NAP is then detected and recorded by the surface recording electrode placed distally either along the same nerve pathway and through a compound muscle action potential produced by "activation of muscle fibers" in the "target muscle supplied by the nerve." The time taken for the NAP to travel from the stimulation point through the "fastest axons" to cause a CMAP in the targeted muscle and the "size of the response" is recorded.

    Parameters measured

  • Latency: the time delay between the electrical stimulation and the beginning of the nerve response, i.e. saltatory conduction. This value is usually 0.1 msec or less. There are two types of latency taken into account during the study: onset latency and peak latency. Onset latency is the time it takes for the electrical stimulus to trigger an action potential in the nerve. Peak latency is a representation of the time delay for the signal to travel down the "majority of the axons" in the nerve. Onset latency is measured before the upstroke of the waveform, and peak latency is measured at the "peak of the waveform amplitude." The myelination of the nerves being tested determines the values of these two latencies.
  • Conduction Velocity: How fast a triggered action potential propagates down the axon of a nerve. How fast or slow the conduction velocity is dictated by the structural integrity of the myelin sheath. It is calculated by “dividing the change in distance by the change in time."
  • Amplitude: The “maximum voltage difference between two points.” The amplitude indicates different properties of the nerve depending on the type of study being performed.
  • Duration: The measurement of the beginning and end of the waveform graphed during the NCS.
  • Area: The amount of space taken up under the curves created by the waveforms graphed during the NCS. Amplitude and duration contribute to the area's value.
  • Temporal Dispersion: It is the “range of conduction velocities of the fastest and slowest nerve fibers.” The NCS waveform becomes wider with nerve stimulation toward the nerve's origin compared to further down the nerve, with the area under the waveform staying constant. This is seen when the “slower fibers conduction” reaches the “recording electrode later than faster fibers.”

    Results and interpretation

The interpretation of nerve conduction studies is complex and requires the expertise of health care practitioners such as clinical neurophysiologists, medical neurologists, physical therapists, or physiatrists. NCS results provide information on whether a nerve conducts electrical signals at a normal speed and strength. Abnormalities in latency, amplitude, conduction velocity or temporal dispersion can indicate:
  • Demyelination Injury: A condition where the protective myelin sheath of the nerve is damaged, but the axon of the nerve is not. The destruction of the myelin’s insulation properties disrupts saltatory conduction indicated by a decrease in the conduction velocity and increase in the temporal dispersion in a NCS. The latency may also be prolonged in this condition.
  • Axonal Injury: An injury to the nerve where the axons are damaged, and the myelin may become damaged in the process as well. A reduction in the amplitude of the nerve conduction waveform may indicate damage to the axons of a nerve. Conduction velocity and distal latency might be mildly slower if the damage affects the “ largest and the fast conducting axons.”
  • Conduction Block: It occurs when action potentials fail to propagate down the nerve. This is usually due to an extensive loss of myelin that saltatory conduction no longer works, and thus, no signal can be transmitted. A conduction block is apparent on an NCS through a significant drop in amplitude of over 50% “across the area of injury.”

    Applications and clinical significance

The utilization of an NCS, understanding of its parameters, and interpretation of the results can help clinicians diagnose different types of nerve injuries, such as nerve compression injury, nerve crush injury, and nerve transactional injury. Abnormal parameters in multiple nerves or across all nerves in a given limb or multiple limbs may indicate damage to multiple nerves, polyneuropathy, or generalized nerve disease or damage, generalized peripheral neuropathy. Some of the common disorders that nerve conduction studies can diagnose are:

Motor NCS

Motor NCS are obtained by stimulating a nerve containing motor fibers and recording at the belly of a muscle innervated by that nerve. The compound muscle action potential is the resulting response and depends on the motor axons transmitting the action potential, the status of the neuromuscular junction, and muscle fibers. The CMAP amplitudes, motor onset latencies, and conduction velocities are routinely assessed and analyzed. As with sensory NCS, conduction velocity is calculated by dividing distance by time. In this case, however, the distance between two stimulation sites is divided by the difference in onset latencies of those two sites, providing the conduction velocity in the segment of the nerve between the two stimulation sites. This method of calculating conduction velocity avoids being confounded by time spent traversing the neuromuscular junction and triggering a muscle action potential.

Sensory NCS

Sensory NCS is performed by electrical stimulation of a peripheral nerve while recording the transmitted potential at a different site along the same nerve. Three main measures can be obtained: sensory nerve action potential amplitude, sensory latency, and conduction velocity. The SNAP amplitude represents a measure of the number of axons conducting between the stimulation site and the recording site. Sensory latency is the time that it takes for the action potential to travel between the stimulation site and the recording site of the nerve. The conduction velocity is measured in meters per second. It is obtained by dividing the distance between the stimulation site and the recording site by the latency: Conduction velocity = Distance/Latency.
Image:Sensory neurography median nerve example.png|thumb|360px|alt=An example screenshot showing the results of a sensory nerve conduction velocity study|Sensory NCS: An example screenshot showing the results of a sensory nerve conduction velocity study of the right median nerve.