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.
Ultimately, conduction velocities are specific to each individual and depend largely on an axon's diameter and the degree to which that axon is myelinated, but the majority of 'normal' individuals fall within defined ranges. [1]
Nerve impulses are extremely slow compared to the speed of electricity, where the electric field can propagate with a speed on the order of 50–99% of the speed of light; however, it is very fast compared to the speed of blood flow, with some myelinated neurons conducting at speeds up to 120 m/s (432 km/h or 275 mph)[ citation needed ].
| Type | Erlanger–Gasser classification | Diameter | Myelin | Conduction velocity | Associated muscle fibers |
|---|---|---|---|---|---|
| α | Aα | 13–20 μm | Yes | 50–60 m/s [2] [3] | Extrafusal muscle fibers |
| γ | Aγ | 5–8 μm | Yes | 4–24 m/s [4] [5] | Intrafusal muscle fibers |
Different sensory receptors are innervated by different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers, and nociceptors and thermoreceptors by type III and IV sensory fibers.
| Type | Erlanger–Gasser classification | Diameter | Myelin | Conduction velocity | Associated sensory receptors |
|---|---|---|---|---|---|
| Ia | Aα | 13–20 μm | Yes | 80–120 m/s [6] | Responsible for proprioception |
| Ib | Aα | 13–20 μm | Yes | 80–120 m/s | Golgi tendon organ |
| II | Aβ | 6–12 μm | Yes | 33–75 m/s | Secondary receptors of muscle spindle All cutaneous mechanoreceptors |
| III | Aδ | 1–5 μm | Thin | 3–30 m/s | Free nerve endings of touch and pressure Nociceptors of neospinothalamic tract Cold thermoreceptors |
| IV | C | 0.2–1.5 μm | No | 0.5–2.0 m/s | Nociceptors of paleospinothalamic tract Warmth receptors |
| Type | Erlanger–Gasser classification | Diameter | Myelin | Conduction velocity |
|---|---|---|---|---|
| preganglionic fibers | B | 1–5 μm | Yes | 3–15 m/s |
| postganglionic fibers | C | 0.2–1.5 μm | No | 0.5–2.0 m/s |
| Nerve | Conduction velocity [2] [3] |
|---|---|
| Median sensory | 45–70 m/s |
| Median motor | 49–64 m/s |
| Ulnar sensory | 48–74 m/s |
| Ulnar motor | 49+ m/s |
| Peroneal motor | 44+ m/s |
| Tibial motor | 41+ m/s |
| Sural sensory | 46–64 m/s |
Normal impulses in peripheral nerves of the legs travel at 40–45 m/s, and those in peripheral nerves of the arms at 50–65 m/s. [7] Largely generalized, normal conduction velocities for any given nerve will be in the range of 50–60 m/s. [8]
Nerve conduction velocity is just one of many measurements commonly made during a nerve conduction study (NCS). The purpose of these studies is to determine whether nerve damage is present and how severe that damage may be.
Nerve conduction studies are performed as follows: [8]
Although conduction velocity itself is not directly measured, calculating conduction velocities from NCS measurements is trivial. The distance between the stimulating and receiving electrodes is divided by the impulse latency, resulting in conduction velocity. NCV = conduction distance / (proximal latency-distal latency)
Many times, Needle EMG is also performed on subjects at the same time as other NCS procedures because they aid in detecting whether muscles are functioning properly in response to stimuli sent via their connecting nerves. [8] EMG is the most important component of electrodiagnosis of motor neuron diseases as it often leads to the identification of motor neuron involvement before clinical evidence can be seen. [9]
Typically, the electrodes used in an EMG are stuck to the skin over a thin layer of gel/paste. [8] This allows for better conduction between electrode and skin. However, as these electrodes do not pierce the skin, there are impedances that result in erroneous readings, high noise levels, and low spatial resolution in readings. [10]
To address these problems, new devices are being developed, such as 3-dimensional electrode arrays. These are MEMS devices that consist of arrays of metal micro-towers capable of penetrating the outer layers of skin, thus reducing impedance. [10]
Compared with traditional wet electrodes, multi-electrode arrays offer the following: [10]
Baseline nerve conduction measurements are different for everyone, as they are dependent upon the individual's age, sex, local temperatures, and other anthropometric factors such as hand size and height. [2] [11] It is important to understand the effect of these various factors on the normal values for nerve conduction measurements to aid in identifying abnormal nerve conduction study results. The ability to predict normal values in the context of an individual's anthropometric characteristics increases the sensitivities and specificities of electrodiagnostic procedures. [2]
Normal 'adult' values for conduction velocities are typically reached by age 4. Conduction velocities in newborns and toddlers tend to be about half the adult values. [1]
Nerve conduction studies performed on healthy adults revealed that age is negatively associated with the sensory amplitude measures of the Median, Ulnar, and Sural nerves. Negative associations were also found between age and the conduction velocities and latencies in the Median sensory, Median motor, and Ulnar sensory nerves. However, conduction velocity of the Sural nerve is not associated with age. In general, conduction velocities in the upper extremities decrease by about 1 m/s for every 10 years of age. [2]
Sural nerve conduction amplitude is significantly smaller in females than males, and the latency of impulses is longer in females, thus a slower conduction velocity. [2]
Other nerves have not been shown to exhibit any gender biases.[ citation needed ]
In general, the conduction velocities of most motor and sensory nerves are positively and linearly associated with body temperature (low temperatures slow nerve conduction velocity and higher temperatures increase conduction velocity). [1]
Conduction velocities in the Sural nerve seem to exhibit an especially strong correlation with the local temperature of the nerve. [2]
Conduction velocities in both the Median sensory and Ulnar sensory nerves are negatively related to an individual's height, which likely accounts for the fact that, among most of the adult population, conduction velocities between the wrist and digits of an individual's hand decrease by 0.5 m/s for each inch increase in height. [2] As a direct consequence, impulse latencies within the Median, Ulnar, and Sural nerves increases with height. [2]
The correlation between height and the amplitude of impulses in the sensory nerves is negative. [2]
Circumference of the index finger appears to be negatively associated with conduction amplitudes in the Median and Ulnar nerves. In addition, people with larger wrist ratios (anterior-posterior diameter : medial-lateral diameter) have lower Median nerve latencies and faster conduction velocities. [2]
Amyotrophic lateral sclerosis is a progressive and inevitably fatal neurodegenerative disease affecting the motor neurons. [9] Because ALS shares many symptoms with other neurodegenerative diseases, it can be difficult to diagnose properly. The best method of establishing a confident diagnosis is via electrodiagnostic evaluation. To be specific, motor nerve conduction studies of the Median, Ulnar, and peroneal muscles should be performed, as well as sensory nerve conduction studies of the Ulnar and Sural nerves. [9]
In patients with ALS, it has been shown that distal motor latencies and slowing of conduction velocity worsened as the severity of their muscle weakness increased. Both symptoms are consistent with the axonal degeneration occurring in ALS patients. [9]
Carpal tunnel syndrome (CTS) is a form of nerve compression syndrome caused by the compression of the median nerve at the wrist. Typical symptoms include numbness, tingling, burning pains, or weakness in the hand. [12] [13] CTS is another condition for which electrodiagnostic testing is valuable. [12] [14] However, before subjecting a patient to nerve conduction studies, both Tinel's test and Phalen's test should be performed. If both results are negative, it is very unlikely that the patient has CTS, and further testing is unnecessary. [13]
Carpal tunnel syndrome presents in each individual to different extents. Measurements of nerve conduction velocity are critical to determining the degree of severity. [14] [15] These levels of severity are categorized as: [12] [13]
One common electrodiagnostic measurement includes the difference between sensory nerve conduction velocities in the pinkie finger and index finger. In most instances of CTS, symptoms will not present until this difference is greater than 8 m/s. [12] [13]
Guillain–Barré syndrome (GBS) is a peripheral neuropathy involving the degeneration of myelin sheathing and/or nerves that innervate the head, body, and limbs. [7] This degeneration is due to an autoimmune response typically initiated by various infections.
Two primary classifications exist: demyelinating (Schwann cell damage) and axonal (direct nerve fiber damage). [7] [16] Each of these then branches into additional sub-classifications depending on the exact manifestation. In all cases, however, the condition results in weakness or paralysis of limbs, the potentially fatal paralysis of respiratory muscles, or a combination of these effects. [7]
The disease can progress very rapidly once symptoms present (severe damage can occur within as little as a day). [7] Because electrodiagnosis is one of the fastest and most direct methods of determining the presence of the illness and its proper classification, nerve conduction studies are extremely important. [16] Without proper electrodiagnostic assessment, GBS is commonly misdiagnosed as polio, West Nile virus, tick paralysis, various toxic neuropathies, CIDP, transverse myelitis, or hysterical paralysis. [7] Two sets of nerve conduction studies should allow for proper diagnosis of Guillain–Barré syndrome. It is recommended that these be performed within the first 2 weeks of symptom presentation and again sometime between 3 and 8 weeks. [16]
Electrodiagnostic findings that may implicate GBS include: [3] [7] [16]
Lambert–Eaton myasthenic syndrome (LEMS) is an autoimmune disease in which auto-antibodies are directed against voltage-gated calcium channels at presynaptic nerve terminals. Here, the antibodies inhibit the release of neurotransmitters, resulting in muscle weakness and autonomic dysfunctions. [17]
Nerve conduction studies performed on the Ulnar motor and sensory, Median motor and sensory, Tibial motor, and Peroneal motor nerves in patients with LEMS have shown that the conduction velocity across these nerves is actually normal. However, the amplitudes of the compound motor action potentials may be reduced by up to 55%, and the duration of these action potentials decreased by up to 47%. [17]
At least half the population with diabetes mellitus is also affected with diabetic neuropathy, causing numbness and weakness in the peripheral limbs. [18] Studies have shown that the Rho/Rho-kinase signaling pathway is more active in individuals with diabetes and that this signaling activity occurs mainly in the nodes of Ranvier and Schmidt-Lanterman incisures. [18] Therefore, over-activity of the Rho/Rho-kinase signaling pathway may inhibit nerve conduction.
Motor nerve conduction velocity studies revealed that conductance in diabetic rats was about 30% lower than that of the non-diabetic control group. In addition, activity along the Schmidt-Lanterman incisures was non-continuous and non-linear in the diabetic group, but linear and continuous in the control. These deficiencies were eliminated after the administration of Fasudil to the diabetic group, implying that it may be a potential treatment. [18]
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.
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.
Carpal tunnel syndrome (CTS) is a nerve compression syndrome associated with the collected signs and symptoms of compression of the median nerve at the carpal tunnel in the wrist. Carpal tunnel syndrome is an idiopathic syndrome but there are environmental, and medical risk factors associated with the condition. CTS can affect both wrists.
The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
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.
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, 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.
Tarsal tunnel syndrome (TTS) is a nerve compression syndrome or nerve entrapment syndrome causing a painful foot condition in which the tibial nerve is entrapped as it travels through the tarsal tunnel. The tarsal tunnel is found along the inner leg behind the medial malleolus. The posterior tibial artery, tibial nerve, and tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles travel in a bundle through the tarsal tunnel. Inside the tunnel, the nerve splits into three segments. One nerve (calcaneal) continues to the heel, the other two continue on to the bottom of the foot. The tarsal tunnel is delineated by bone on the inside and the flexor retinaculum on the outside.
Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired autoimmune disease of the peripheral nervous system characterized by progressive weakness and impaired sensory function in the legs and arms. The disorder is sometimes called chronic relapsing polyneuropathy (CRP) or chronic inflammatory demyelinating polyradiculoneuropathy. CIDP is closely related to Guillain–Barré syndrome and it is considered the chronic counterpart of that acute disease. Its symptoms are also similar to progressive inflammatory neuropathy. It is one of several types of neuropathy.
Ulnar tunnel syndrome, also known as Guyon's canal syndrome or Handlebar palsy, is ulnar neuropathy at the wrist where it passes through the ulnar tunnel. The most common presentation is a palsy of the deep motor branch of the ulnar nerve causing weakness of the interosseous muscles. Ulnar tunnel syndrome is usually caused by a ganglion cyst pressing on the ulnar nerve, other causes include traumas to the wrist and repetitive movements, but often the cause is unknown (idiopathic). Long distance bicycle rides are associated with transient alterations in ulnar nerve function. Sensory loss in the ring and small fingers is usually due to ulnar nerve entrapment at the cubital tunnel near the elbow, which is known as cubital tunnel syndrome, although it can uncommonly be due to compression at the wrist.
Idiopathic ulnar neuropathy at the elbow is a condition where pressure on the ulnar nerve as it passes through the cubital tunnel causes ulnar neuropathy. The symptoms of neuropathy are paresthesia (tingling) and numbness primarily affecting the little finger and ring finger of the hand. Ulnar neuropathy can progress to weakness and atrophy of the muscles in the hand. Symptoms can be alleviated by the use of a splint to prevent the elbow from flexing while sleeping.
Acute motor axonal neuropathy (AMAN) is a variant of Guillain–Barré syndrome. It is characterized by acute paralysis and loss of reflexes without sensory loss. Pathologically, there is motor axonal degeneration with antibody-mediated attacks of motor nerves and nodes of Ranvier.
Ulnar neuropathy is a disorder involving the ulnar nerve. Ulnar neuropathy may be caused by entrapment of the ulnar nerve with resultant numbness and tingling. It may also cause weakness or paralysis of the muscles supplied by the nerve. Ulnar neuropathy may affect the elbow as cubital tunnel syndrome. At the wrist a similar neuropathy is ulnar tunnel syndrome.
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.
Nerve compression syndrome, or compression neuropathy, or nerve entrapment syndrome, is a medical condition caused by chronic, direct pressure on a peripheral nerve. It is known colloquially as a trapped nerve, though this may also refer to nerve root compression. Its symptoms include pain, tingling, numbness and muscle weakness. The symptoms affect just one particular part of the body, depending on which nerve is affected. The diagnosis is largely clinical and can be confirmed with diagnostic nerve blocks. Occasionally imaging and electrophysiology studies aid in the diagnosis. Timely diagnosis is important as untreated chronic nerve compression may cause permanent damage. A surgical nerve decompression can relieve pressure on the nerve but cannot always reverse the physiological changes that occurred before treatment. Nerve injury by a single episode of physical trauma is in one sense an acute compression neuropathy but is not usually included under this heading, as chronic compression takes a unique pathophysiological course.
Injuries to the arm, forearm or wrist area can lead to various nerve disorders. One such disorder is median nerve palsy. The median nerve controls the majority of the muscles in the forearm. It controls abduction of the thumb, flexion of hand at wrist, flexion of digital phalanx of the fingers, is the sensory nerve for the first three fingers, etc. Because of this major role of the median nerve, it is also called the eye of the hand. If the median nerve is damaged, the ability to abduct and oppose the thumb may be lost due to paralysis of the thenar muscles. Various other symptoms can occur which may be repaired through surgery and tendon transfers. Tendon transfers have been very successful in restoring motor function and improving functional outcomes in patients with median nerve palsy.
Multifocal motor neuropathy (MMN) is a progressively worsening condition where muscles in the extremities gradually weaken. The disorder, a pure motor neuropathy syndrome, is sometimes mistaken for amyotrophic lateral sclerosis (ALS) because of the similarity in the clinical picture, especially if muscle fasciculations are present. MMN is thought to be autoimmune. It was first described in the mid-1980s.
Carpal tunnel surgery, also called carpal tunnel release (CTR) and carpal tunnel decompression surgery, is a nerve decompression in which the transverse carpal ligament is divided. It is a surgical treatment for carpal tunnel syndrome (CTS) and recommended when there is constant (not just intermittent) numbness, muscle weakness, or atrophy, and when night-splinting no longer controls intermittent symptoms of pain in the carpal tunnel. In general, milder cases can be controlled for months to years, but severe cases are unrelenting symptomatically and are likely to result in surgical treatment. Approximately 500,000 surgical procedures are performed each year, and the economic impact of this condition is estimated to exceed $2 billion annually.
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.
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.
Ischemic monomelic neuropathy(IMN) is an uncommon vascular access complication in hemodialysis patients that manifests as multiple mononeuropathies without clinical ischemia. Ischemic monomelic neuropathy is most likely to affect patients who have had brachiocephalic vascular grafts, and it is characterized by symptoms of acute pain, numbness, and paresthesia in addition to motor weakness. The term "ischemic monomelic neuropathy" was first used in 1983 by Wilbourn, despite the fact that Bolton et al. had originally reported on it in 1979.
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