Nerve conduction study

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Nerve conduction study
Nerve conduction velocity.jpg
Nerve conduction study
Purposeevaluate the motor and sensory nerves

A nerve conduction study (NCS) 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 (physical medicine and rehabilitation physicians), and neurologists who subspecialize in electrodiagnostic medicine. In the United States, neurologists and physiatrists receive training in electrodiagnostic medicine (performing needle electromyography (EMG and NCSs) 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.

Contents

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. [1] [2] 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. [3] [4] [5] [1]

Nerve conduction studies are also used for evaluation of paresthesias (numbness, tingling, burning) and/or weakness of the arms and legs. [6] 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. [6]

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. [6] Patient should be advised to avoid applying lotions or creams to the skin, as these substances can interfere with electrode conductivity. [6] [7] [8] 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. [9]

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.

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. [10] These electrodes record the nerve's electrical response and are referred to as surface recording electrodes. [10] 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. [10] 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. [10]
  2. Stimulation: An electrical impulse is administered to the targeted nerve via the stimulating electrode, resulting in a "propagated nerve action potential (NAP)." [10] This electrical stimulation may be slightly painful, so practitioners should warn patients. [10]
  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 (CMAP) produced by "activation of muscle fibers" in the "target muscle supplied by the nerve." [10] 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. [10]

Parameters measured

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. [6] [7] [8] 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:

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 (neuropraxia), nerve crush injury (axonotmesis), and nerve transactional injury (neurotmesis). [11] 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. [6] Some of the common disorders that nerve conduction studies can diagnose are:

Types of studies

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 (CMAP) 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 (since these are subtracted out).[ citation needed ]

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 (SNAP) amplitude, sensory latency, and conduction velocity. The SNAP amplitude (in microvolts) represents a measure of the number of axons conducting between the stimulation site and the recording site. Sensory latency (in milliseconds) 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.

Sensory NCS: An example screenshot showing the results of a sensory nerve conduction velocity study of the right median nerve. Sensory neurography median nerve example.png
Sensory NCS: An example screenshot showing the results of a sensory nerve conduction velocity study of the right median nerve.

F-wave study

F-wave study uses supramaximal stimulation of a motor nerve and recording of action potentials from a muscle supplied by the nerve. This is not a reflex, per se, in that the action potential travels from the site of the stimulating electrode in the limb to the spinal cord's ventral horn and back to the limb in the same nerve that was stimulated. The F-wave latency can be used to derive the conduction velocity of the nerve between the limb and spine. In contrast, the motor and sensory nerve conduction studies evaluate conduction in the segment of the limb. F waves vary in latency and an abnormal variance is called "chrono dispersion". Conduction velocity is derived by measuring the limb length, D, in millimeters from the stimulation site to the corresponding spinal segment (C7 spinous process to wrist crease for median nerve). This is multiplied by two as it goes to the cord and returns to the muscle (2D). 2D is divided by the latency difference between mean F and M and 1 millisecond subtracted (F-M-1). The formula is .

H-reflex study

An h-reflex study uses stimulation of a nerve and recording the electrical reflex discharge from a muscle in the limb. This also evaluates conduction between the limb and the spinal cord. Still, in this case, the afferent impulses (those going toward the spinal cord) are in sensory nerves, while the efferent impulses (those coming from the spinal cord) are in motor nerves. This process cannot be changed.

Repetitive nerve stimulation

Patient risk and complications

Nerve conduction studies are beneficial to diagnose certain diseases of the nerves of the body. The test is not invasive, but can be painful due to the electrical shocks administered during the test. The shocks are associated with a low amount of electric current, so they pose minimal risk to the patients. Still, there is technically the risk of "bodily injury from electrical shock". [11] There is limited risk and complications studied in regards to NCS and thus no published absolute contraindications. [11] [13] However, relative risks should be considered based on patient history and physical. [11] [13] Of particular note are implanted electrical devices such as cardiac pacemakers or defibrillators or other implanted stimulators such as deep brain stimulators or spinal cord stimulators. [11] [13] Theoretically, delivering electricity through the body may affect systems in the body that depend on electrical signals, such as the heart and brain. [11] Patients are encouraged to tell the examiner before the study if they have such devices, but their existence in the patient does not prevent them from having the study performed. [13] Below are some special precautions and considerations regarding these devices and pregnancy.

Cardiovascular devices

Current literature and studies lack sufficient evidence to indicate that electrodiagnostic studies, such as NCS, "pose a safety hazard" to patients with cardiac pacemakers and implanted cardiac defibrillators (ICDs). [13] However, there exists the "theoretical concern that electrical impulses of nerve conduction studies " could be pick up by sensory mechanism with the devices. [13] This could result in causing the device to malfunction, stop working, or alter the programming. [13] The American Association of Neuromuscular & Electrodiagnostic Medicine has stated that despite these concerns, "no immediate or delayed adverse effects have been reported with routine NCS." [13] Some general rules to avoid possible interference are listed below.

Technique considerations

  • "15 cm (6 inches) separation" is recommended "between the stimulator and any wires, intravenous (IV lines) or catheters." [11]
  • "Stimulating the brachial plexus on the same side as a pacemaker or internal cardiac defibrillator" should be avoided [11] or with "extreme caution if it is necessary" to do. [13]
  • "Electrodes should not be placed in a manner where they read a response across the heart" [11]
  • While performing NCS of the neck, avoid the locations of "carotid sinus and vagus nerve" as "stimulating these could affect the rhythm of the heart." [11]

Contraindications

  • Patients who have an external cardiac pacemaker. [11] [13] External cardiac pacemakers, particularly the external pacing wires, "can be electrically sensitive to NCS stimulations" [11] and "present a serious potential hazard of electrical injury to the heart." [13]
  • Patient's who have a central venous catheter. They pose a possible "risk of generating a stimulus to the heart." [11] It has been studied and thus determined that "peripheral IV lines are not considered to be problematic" [11] [13]

Deep brain stimulators

Due to the typical lead placement of deep brain stimulators from the "subclavicular area to the lateral posterior neck" and then to the "occipital area", there is a "theoretical risk of introducing electrical current through the leads" which could transmit "directly into the brain" and through the cervical nerve roots. [13] The safety of performing NCS on patients with a DBS device has not been studied. [13] Physicians should weigh the risks and benefits of an NCS in these patients on a case-by-case basis. [13]

Pregnancy

The American Association of Neuromuscular & Electrodiagnostic Medicine has stated that there are "no known contraindications" that "exist from performing needle EMG or NCS on pregnant patients." [13] There have been no reported instances of any complications from the procedure or associated problems when "performed during pregnancy" in the current literature. [13]

See also

Related Research Articles

In neuroscience, an F wave is one of several motor responses which may follow the direct motor response (M) evoked by electrical stimulation of peripheral motor or mixed nerves. F-waves are the second of two late voltage changes observed after stimulation is applied to the skin surface above the distal region of a nerve, in addition to the H-reflex which is a muscle reaction in response to electrical stimulation of innervating sensory fibers. Traversal of F-waves along the entire length of peripheral nerves between the spinal cord and muscle, allows for assessment of motor nerve conduction between distal stimulation sites in the arm and leg, and related motoneurons (MN's) in the cervical and lumbosacral cord. F-waves are able to assess both afferent and efferent loops of the alpha motor neuron in its entirety. As such, various properties of F-wave motor nerve conduction are analyzed in nerve conduction studies (NCS), and often used to assess polyneuropathies, resulting from states of neuronal demyelination and loss of peripheral axonal integrity.

<span class="mw-page-title-main">Lambert–Eaton myasthenic syndrome</span> Autoimmune disorder causing muscular weakness

Lambert–Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder characterized by muscle weakness of the limbs. It is also known as myasthenic syndrome, Eaton–Lambert syndrome, and when related to cancer, carcinomatous myopathy.

Clinical neurophysiology is a medical specialty that studies the central and peripheral nervous systems through the recording of bioelectrical activity, whether spontaneous or stimulated. It encompasses both research regarding the pathophysiology along with clinical methods used to diagnose diseases involving both central and peripheral nervous systems. Examinations in the clinical neurophysiology field are not limited to tests conducted in a laboratory. It is thought of as an extension of a neurologic consultation. Tests that are conducted are concerned with measuring the electrical functions of the brain, spinal cord, and nerves in the limbs and muscles. It can give the precise definition of site, the type and degree of the lesion, along with revealing the abnormalities that are in question. Due to these abilities, clinical neurophysiology is used to mainly help diagnose diseases rather than treat them.

An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light flash or a pure tone. Different types of potentials result from stimuli of different modalities and types. Evoked potential is distinct from spontaneous potentials as detected by electroencephalography (EEG), electromyography (EMG), or other electrophysiologic recording method. Such potentials are useful for electrodiagnosis and monitoring that include detections of disease and drug-related sensory dysfunction and intraoperative monitoring of sensory pathway integrity.

<span class="mw-page-title-main">Functional electrical stimulation</span> Technique that uses low-energy electrical pulses

Functional electrical stimulation (FES) is a technique that uses low-energy electrical pulses to artificially generate body movements in individuals who have been paralyzed due to injury to the central nervous system. More specifically, FES can be used to generate muscle contraction in otherwise paralyzed limbs to produce functions such as grasping, walking, bladder voiding and standing. This technology was originally used to develop neuroprostheses that were implemented to permanently substitute impaired functions in individuals with spinal cord injury (SCI), head injury, stroke and other neurological disorders. In other words, a person would use the device each time he or she wanted to generate a desired function. FES is sometimes also referred to as neuromuscular electrical stimulation (NMES).

<span class="mw-page-title-main">Electromyography</span> Electrodiagnostic medicine technique

Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyograph detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated. The signals can be analyzed to detect abnormalities, activation level, or recruitment order, or to analyze the biomechanics of human or animal movement. Needle EMG is an electrodiagnostic medicine technique commonly used by neurologists. Surface EMG is a non-medical procedure used to assess muscle activation by several professionals, including physiotherapists, kinesiologists and biomedical engineers. In computer science, EMG is also used as middleware in gesture recognition towards allowing the input of physical action to a computer as a form of human-computer interaction.

Electroneuronography or electroneurography (ENoG) is a neurological non-invasive test used to study the facial nerve in cases of muscle weakness in one side of the face. The technique of electroneuronography was first used by Esslen and Fisch in 1979 to describe a technique that examines the integrity and conductivity of peripheral nerves. In modern use, ENoG is used to describe study of the facial nerve, while the term nerve conduction study is employed for other nerves.

Axonotmesis is an injury to the peripheral nerve of one of the extremities of the body. The axons and their myelin sheath are damaged in this kind of injury, but the endoneurium, perineurium and epineurium remain intact. Motor and sensory functions distal to the point of injury are completely lost over time leading to Wallerian degeneration due to ischemia, or loss of blood supply. Axonotmesis is usually the result of a more severe crush or contusion than neurapraxia.

Intraoperative neurophysiological monitoring (IONM) or intraoperative neuromonitoring is the use of electrophysiological methods such as electroencephalography (EEG), electromyography (EMG), and evoked potentials to monitor the functional integrity of certain neural structures during surgery. The purpose of IONM is to reduce the risk to the patient of iatrogenic damage to the nervous system, and/or to provide functional guidance to the surgeon and anesthesiologist.

<span class="mw-page-title-main">Nerve conduction velocity</span> Speed at which an electrochemical impulse propagates down a neural pathway

In neuroscience, nerve conduction velocity (CV) is the speed at which an electrochemical impulse propagates down a neural pathway. Conduction velocities are affected by a wide array of factors, which include age, sex, and various medical conditions. Studies allow for better diagnoses of various neuropathies, especially demyelinating diseases as these conditions result in reduced or non-existent conduction velocities. CV is an important aspect of nerve conduction studies.

Proximal diabetic neuropathy, also known as diabetic amyotrophy, is a complication of diabetes mellitus that affects the nerves that supply the thighs, hips, buttocks and/or lower legs. Proximal diabetic neuropathy is a type of diabetic neuropathy characterized by muscle wasting, weakness, pain, or changes in sensation/numbness of the leg. It is caused by damage to the nerves of the lumbosacral plexus.

<span class="mw-page-title-main">Rheobase</span>

Rheobase is a measure of membrane potential excitability. In neuroscience, rheobase is the minimal current amplitude of infinite duration that results in the depolarization threshold of the cell membranes being reached, such as an action potential or the contraction of a muscle. In Greek, the root rhe translates to "current or flow", and basi means "bottom or foundation": thus the rheobase is the minimum current that will produce an action potential or muscle contraction.

<span class="mw-page-title-main">Hereditary motor and sensory neuropathy</span> Medical condition

Hereditary motor and sensory neuropathies (HMSN) is a name sometimes given to a group of different neuropathies which are all characterized by their impact upon both afferent and efferent neural communication. HMSN are characterised by atypical neural development and degradation of neural tissue. The two common forms of HMSN are either hypertrophic demyelinated nerves or complete atrophy of neural tissue. Hypertrophic condition causes neural stiffness and a demyelination of nerves in the peripheral nervous system, and atrophy causes the breakdown of axons and neural cell bodies. In these disorders, a patient experiences progressive muscle atrophy and sensory neuropathy of the extremities.

Repetitive nerve stimulation is a variant of the nerve conduction study where electrical stimulation is delivered to a motor nerve repeatedly several times per second. By observing the change in the muscle electrical response (CMAP) after several stimulations, a physician can assess for the presence of a neuromuscular junction disease, and differentiate between presynaptic and postsynaptic conditions. The test was first described by German neurologist Friedrich Jolly in 1895, and is also known as Jolly's test.

Electroanalgesia is a form of analgesia, or pain relief, that uses electricity to ease pain. Electrical devices can be internal or external, at the site of pain (local) or delocalized throughout the whole body. It works by interfering with the electric currents of pain signals, inhibiting them from reaching the brain and inducing a response; different from traditional analgesics, such as opiates which mimic natural endorphins and NSAIDs that help relieve inflammation and stop pain at the source. Electroanalgesia has a lower addictive potential and poses less health threats to the general public, but can cause serious health problems, even death, in people with other electrical devices such as pacemakers or internal hearing aids, or with heart problems.

Somatosensory evoked potential is the electrical activity of the brain that results from the stimulation of touch. SEP tests measure that activity and are a useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex. SEP components include a series of positive and negative deflections that can be elicited by virtually any sensory stimuli. For example, SEPs can be obtained in response to a brief mechanical impact on the fingertip or to air puffs. However, SEPs are most commonly elicited by bipolar transcutaneous electrical stimulation applied on the skin over the trajectory of peripheral nerves of the upper limb or lower limb, and then recorded from the scalp. In general, somatosensory stimuli evoke early cortical components, generated in the contralateral primary somatosensory cortex (S1), related to the processing of the physical stimulus attributes. About 100 ms after stimulus application, additional cortical regions are activated, such as the secondary somatosensory cortex (S2), and the posterior parietal and frontal cortices, marked by a parietal P100 and bilateral frontal N140. SEPs are routinely used in neurology today to confirm and localize sensory abnormalities, to identify silent lesions and to monitor changes during surgical procedures.

Clinical Electrophysiological Testing is based on techniques derived from electrophysiology used for the clinical diagnosis of patients. There are many processes that occur in the body which produce electrical signals that can be detected. Depending on the location and the source of these signals, distinct methods and techniques have been developed to properly target them.

Electromyoneurography (EMNG) is the combined use of electromyography and electroneurography This technique allows for the measurement of a peripheral nerve's conduction velocity upon stimulation (electroneurography) alongside electrical recording of muscular activity (electromyography). Their combined use proves to be clinically relevant by allowing for both the source and location of a particular neuromuscular disease to be known, and for more accurate diagnoses.

<span class="mw-page-title-main">Neuromechanics</span> Interdisciplinary field

Neuromechanics is an interdisciplinary field that combines biomechanics and neuroscience to understand how the nervous system interacts with the skeletal and muscular systems to enable animals to move. In a motor task, like reaching for an object, neural commands are sent to motor neurons to activate a set of muscles, called muscle synergies. Given which muscles are activated and how they are connected to the skeleton, there will be a corresponding and specific movement of the body. In addition to participating in reflexes, neuromechanical process may also be shaped through motor adaptation and learning.

Electrodiagnosis (EDX) is a method of medical diagnosis that obtains information about diseases by passively recording the electrical activity of body parts or by measuring their response to external electrical stimuli. The most widely used methods of recording spontaneous electrical activity are various forms of electrodiagnostic testing (electrography) such as electrocardiography (ECG), electroencephalography (EEG), and electromyography (EMG). Electrodiagnostic medicine is a medical subspecialty of neurology, clinical neurophysiology, cardiology, and physical medicine and rehabilitation. Electrodiagnostic physicians apply electrophysiologic techniques, including needle electromyography and nerve conduction studies to diagnose, evaluate, and treat people with impairments of the neurologic, neuromuscular, and/or muscular systems. The provision of a quality electrodiagnostic medical evaluation requires extensive scientific knowledge that includes anatomy and physiology of the peripheral nerves and muscles, the physics and biology of the electrical signals generated by muscle and nerve, the instrumentation used to process these signals, and techniques for clinical evaluation of diseases of the peripheral nerves and sensory pathways.

References

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  3. ↑ North American Spine Society (February 2013), "Five Things Physicians and Patients Should Question", Choosing Wisely: an initiative of the ABIM Foundation , North American Spine Society, archived from the original on 11 November 2013, retrieved 25 March 2013, which cites
  4. ↑ Sandoval, Alexius E.G. (November 2010). "Electrodiagnostics for Low Back Pain". Physical Medicine and Rehabilitation Clinics of North America. 21 (4): 767–776. doi:10.1016/j.pmr.2010.06.007. PMID   20977959.
  5. ↑ "National Guideline Clearinghouse | Diagnosis and treatment of degenerative lumbar spinal stenosis". 2014-03-25. Archived from the original on 2014-03-25. Retrieved 2024-11-11.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 Novello, Briana J.; Pobre, Thomas (2024), "Electrodiagnostic Evaluation of Peripheral Neuropathy", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID   33085316 , retrieved 2024-11-21
  7. 1 2 "Guidelines in electrodiagnostic medicine. American Association of Electrodiagnostic Medicine". Muscle & Nerve. 15 (2): 229–253. February 1992. doi:10.1002/mus.880150218. ISSN   0148-639X. PMID   1549146.
  8. 1 2 AANEM (March 2015). "Proper Performance and Interpretation of Electrodiagnostic Studies. [Corrected]". Muscle & Nerve. 51 (3): 468–471. doi:10.1002/mus.24587. ISSN   1097-4598. PMID   25676356.
  9. ↑ "Questionable Billing for Medicare Electrodiagnostic Tests" (PDF). Archived (PDF) from the original on 26 January 2022. Retrieved 11 July 2024.
  10. 1 2 3 4 5 6 7 8 Ahn, Suk-Won; Yoon, Byung-Nam; Kim, Jee-Eun; Seok, Jin Myoung; Kim, Kwang-Kuk; Kwon, Ki-Han; Park, Kee Duk; Suh, Bum Chun (2018-01-20). "Nerve conduction studies: basic principal and clinical usefulness". Annals of Clinical Neurophysiology. 20 (2): 71–78. doi: 10.14253/ACN.2018.20.2.71 .
  11. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Cuccurullo, Sara J., ed. (October 2019). Physical Medicine and Rehabilitation Board Review (4 ed.). New York, NY: Springer Publishing Company. doi:10.1891/9780826134578. ISBN   978-0-8261-3456-1.
  12. 1 2 3 4 5 6 Ahn, Suk-Won; Yoon, Byung-Nam; Kim, Jee-Eun; Seok, Jin Myoung; Kim, Kwang-Kuk; Kwon, Ki-Han; Park, Kee Duk; Suh, Bum Chun (2018-01-20). "Nerve conduction studies: basic principal and clinical usefulness". Annals of Clinical Neurophysiology. 20 (2): 71–78. doi:10.14253/ACN.2018.20.2.71.
  13. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 "Risk In Electrodiagnostic Medicine" (PDF). www.aanem.org. July 22, 2014 [December 16, 2009]. Retrieved November 6, 2024.
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