A
nerve conduction study (NCS) is a test commonly used to evaluate the function, especially the ability of
electrical conductionElectrical conduction is the movement of electrically charged particles through a transmission medium . The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is,...
, of the motor and
sensory nerveSensory nerves are nerves that receive sensory stimuli, such as how something feels and if it is painful.They are made up of nerve fibers, called sensory fibers .Sensory neurons are neurons that are activated by sensory input ,...
s of the
human bodyThe human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 10 trillion cells, the basic unit of life...
.
Nerve conduction velocity (NCV) is a common measurement made during this test. The term NCV often is used to mean the actual test, but this may be misleading since velocity is only one measurement in the test suite.
NerveA nerve is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
conduction studies are used mainly for evaluation of paresthesias (
numbnessParesthesia is a sensation of tingling, pricking, or numbness of a person's skin with no apparent long-term physical effect. It is more generally known as the feeling of "pins and needles" or of a limb "falling asleep"...
, tingling, burning) and/or weakness of the arms and legs.
A
nerve conduction study (NCS) is a test commonly used to evaluate the function, especially the ability of
electrical conductionElectrical conduction is the movement of electrically charged particles through a transmission medium . The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is,...
, of the motor and
sensory nerveSensory nerves are nerves that receive sensory stimuli, such as how something feels and if it is painful.They are made up of nerve fibers, called sensory fibers .Sensory neurons are neurons that are activated by sensory input ,...
s of the
human bodyThe human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 10 trillion cells, the basic unit of life...
.
Nerve conduction velocity (NCV) is a common measurement made during this test. The term NCV often is used to mean the actual test, but this may be misleading since velocity is only one measurement in the test suite.
Purposes
NerveA nerve is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
conduction studies are used mainly for evaluation of paresthesias (
numbnessParesthesia is a sensation of tingling, pricking, or numbness of a person's skin with no apparent long-term physical effect. It is more generally known as the feeling of "pins and needles" or of a limb "falling asleep"...
, tingling, burning) 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. Some of the common disorders which can be diagnosed by nerve conduction studies are:
- Peripheral neuropathy
Peripheral neuropathy is the term for damage to nerves of the peripheral nervous system, which may be caused either by diseases of the nerve or from the side-effects of systemic illness....
- Carpal tunnel syndrome
Carpal tunnel syndrome , or median neuropathy at the wrist, is a medical condition in which the median nerve is compressed at the wrist, leading to paresthesias, numbness and muscle weakness in the hand. Night symptoms and waking at night is a characteristic of established carpal tunnel syndrome...
- Ulnar neuropathy
Ulnar nerve entrapment is a condition where the ulnar nerve becomes trapped or pinched due to some physiological abnormalities.-Symptoms:The symptoms of ulnar nerve entrapment depend on where the nerve is being compressed....
- Guillain-Barré syndrome
Guillain–Barré syndrome is an acute inflammatory demyelinating polyneuropathy , an autoimmune disorder affecting the peripheral nervous system, usually triggered by an acute infectious process. It is included in the wider group of peripheral neuropathies...
- Facioscapulohumeral muscular dystrophy
Facioscapulohumeral muscular dystrophy , which is also known as Landouzy-Dejerine, is an autosomal dominant form of muscular dystrophy that initially affects the skeletal muscles of the face , scapula and upper arms...
- Spinal disc herniation
A spinal disc herniation , informally and misleadingly called a "slipped disc", is a medical condition affecting the spine, in which a tear in the outer, fibrous ring of an intervertebral disc allows the soft, central portion to bulge out...
Description
The nerve conduction study consists of the following components:
- Motor NCS
- Sensory NCS
- F-wave study
- H-reflex
The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibers in their innervating nerves...
study
Motor NCS
Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the recording site is measured. This value is called the
latencyLatency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured.-Packet-switched networks:...
and is measured in milliseconds (ms). The size of the response - called the amplitude - is also measured. Motor amplitudes are measured in millivolts (mV). By stimulating in two or more different locations along the same nerve, the NCV across different segments can be determined. Calculations are performed using the distance between the different stimulating
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s and the difference in latencies.
Sensory NCS
Sensory NCS are performed by electrical stimulation of a peripheral nerve and recording from a purely-sensory portion of the nerve, such as on a finger. Like the motor studies, sensory latencies are on the scale of milliseconds. Sensory amplitudes are much smaller than the motor amplitudes, usually in the microvolt (μV) range. The sensory NCV is calculated based upon the latency and the distance between the stimulating and recording electrode.
F-wave study
F-wave study uses supramaximal stimulation of a motor nerve and recording of
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
s from a muscle supplied by the nerve. This is not a
reflexA reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses...
, per se, in that the
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
travels from the site of the stimulating electrode in the limb to the spinal cord's anterior horn cell 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 nerve between the limb and spine, whereas 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 in millimeters from the stimulation site to the corresponding spinal segment (C7 spinous process to wrist crease for median nerve). This is multiplied by 2 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 2D/(F-M-1).
H-reflex study
H-reflex study uses stimulation of a nerve and recording the reflex electrical discharge from a muscle in the limb. This also evaluates conduction between the limb and the spinal cord, but in this case, the afferent impulses (those going towards 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.
Small-pain-fibers method
In 1998 a small-pain-fibers (spf-NCS) method was cleared by the FDA. This method uses an electrical stimulus with a neuroselective frequency to determine the minimum voltage causing conduction. Rather than comparing the data with population averages on a bell-shaped curve, which at best has about 65% sensitivity, the patient is his own control. In a three year LSU Pain Center study it was found that the nerve requiring the greatest voltage to cause conduction of the A-delta (Fast Pain) fibers identified nerve root pathology with 95% sensitivity. Besides being painless, the test is fast. A new version, uses a potentiometer to objectively measure the amplitude of the action potential at a distant site along the nerve being tested. The previous version relied on the patient reporting a sensation when the nerve fired. The spf-NCS does not require myelin loss to detect function change, so velocity is not measured.
Interpretation of nerve conductions
The interpretation of nerve conduction studies is complex, but in general, different pathological processes result in changes in latencies, motor and/or sensory amplitudes, or slowing of the conduction velocities to differing degrees. For example, slowing of the NCV usually indicates there is damage to the
myelinMyelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system...
. Another example, slowing across the wrist for the motor and sensory latencies of the
median nerveThe median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....
indicates focal compression of the median nerve at the wrist, called
carpal tunnel syndromeCarpal tunnel syndrome , or median neuropathy at the wrist, is a medical condition in which the median nerve is compressed at the wrist, leading to paresthesias, numbness and muscle weakness in the hand. Night symptoms and waking at night is a characteristic of established carpal tunnel syndrome...
. On the other hand, slowing of all nerve conductions in more than one limb indicates generalized sick nerves, or generalized
peripheral neuropathyPeripheral neuropathy is the term for damage to nerves of the peripheral nervous system, which may be caused either by diseases of the nerve or from the side-effects of systemic illness....
. People with
diabetes mellitusDiabetes mellitus —often referred to simply as diabetes—is a condition in which the body either does not produce enough, or does not properly respond to, insulin, a hormone produced in the pancreas. Insulin enables cells to absorb glucose in order to turn it into energy...
often develop generalized peripheral neuropathy.
Patient risk
Nerve conduction studies are very helpful to
diagnoseDiagnosis is the identification of the nature of anything, either by process of elimination or other analytical methods...
certain
diseaseA disease or medical condition isan abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs...
s of the nerves of the body. The test is not invasive, but can be a little painful due to the
electrical shocksAn electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere ....
. However, the shocks are associated with such a low amount of electrical current that they are not dangerous to anyone. Patients with a permanent
pacemakerA pacemaker is a medical device which uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...
or other such implanted stimulators such as
deep brain stimulatorsDeep brain stimulation is a surgical treatment involving the implantation of a medical device called a brain pacemaker, which sends electrical impulses to specific parts of the brain...
or
spinal cord stimulatorA spinal cord stimulator , also known as a dorsal column stimulator, is an implantable medical device used to treat chronic neurological pain. An electric impulse generated by the device produces a tingling sensation that alters the perception. The device is implanted into the epidural space...
s must tell the examiner prior to the study. This does not prevent the study, but special precautions are taken.
The nerve conduction study is sometimes combined with
electromyographyElectromyography is a technique for evaluating and recording the activation signal of muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these...
.
Other special nerve conduction studies that are occasionally performed include double stimuli and repetitive stimulation.
External links