Text of the book “Diseases and injuries of the peripheral nervous system”

Nerve conduction velocity

(
summary
) is an important aspect of nerve conduction research.
This is the speed at which an electrochemical impulse travels down a nerve pathway. Conduction velocity is affected by a wide range of factors, including age, gender and various diseases. Research allows for better diagnosis of various neuropathies, especially demyelinating diseases since these conditions lead to decreased or absent conduction velocity. Salt conductivity

Content

• 1 Normal conduction velocities
• 2 Test Methods 2.1 Nerve Conduction Studies 2.1.1 Micromachined 3D Electrode Arrays

Normal conduction velocities

Ultimately, conduction velocities vary from person to person and depend largely on the axon's diameter and the degree of myelination of that axon, but most "normal" people fall within certain limits.[1]

Nerve impulses are very slow compared to the speed of electricity, where the electric field can travel at speeds on the order of 50–99% of the speed of light;
however, this is very fast compared to the speed of blood flow, with some myelinated neurons conducting at speeds of up to 120 m/s (432 km/h or 275 mph). Types of motor 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. Types of sensory fibers

Types of autonomic efferent fibers

Peripheral nerves

Normal impulses in the peripheral nerves of the legs travel at 40–45 m/s and 50–65 m/s in the peripheral nerves of the arms.[7] In general, the normal conduction velocity for any given nerve will be in the range of 50–60 m/s. [8]

What to expect

NCV testing is usually performed on an outpatient basis, i.e. does not require an overnight hospital stay.

Before the procedure, the doctor will ask the patient to do the following:

• remove clothing, jewelry, glasses, hair clips and other metal objects that may affect the test results.
• wear a hospital gown.
• sit or lie down, depending on which part of the body is being tested.

Once the person is ready, the doctor will locate the nerves and attach a recording electrode to the skin over the nerves.

The second electrode will be placed a short distance apart. This second electrode delivers a mild and short-lived electrical shock to stimulate the nerve. This should not be painful, but may cause minor discomfort. There will be no pain after the test is completed.

The stimulation and neural response will appear on the monitor for recording by the physician.

A person may need to undergo an EMG test after an NCV test. During an EMG test, needles are inserted into muscles and a person stretches the muscles to test their function. This can be uncomfortable and cause soreness and bruising in the needle areas.

Test methods

Nerve conduction studies

Nerve conduction velocity is just one of many measurements commonly taken during a nerve conduction study (NCS). The purpose of these studies is to determine whether nerve damage is present and how severe it may be.

Nerve conduction studies are performed as follows:[8]

• Two electrodes are attached to the patient's skin over the nerve being tested.
• Electrical impulses are sent through one electrode to stimulate the nerve.
• The second electrode records the impulse sent along the nerve as a result of stimulation.
• The time difference between stimulation from the first electrode and capture by the subsequent electrode is known as latency. Nerve conduction delays are typically on the order of milliseconds.

Although conduction velocity itself is not directly measured, calculating conduction velocity from NCS measurements is trivial. The distance between the stimulating and receiving electrodes is divided by the pulse delay, resulting in conduction velocity.

Many times, Needle EMG is also performed on subjects at the same time as other NCS procedures as they help determine whether the muscles are functioning properly in response to stimuli sent through their connecting nerves.[8] EMG is a critical component of the electrodiagnosis of motor neuron diseases, as it often leads to the identification of motor neuron involvement before clinical evidence becomes available.[9]

Micromachined 3D Electrode Sets

Typically, electrodes used in EMG are glued to the skin on top of a thin layer of gel/paste.[8] This ensures better conductivity between the electrode and the skin. However, because these electrodes do not pierce the skin, there is resistance which results in erroneous readings, high noise levels, and low spatial resolution in readings.[10]

To solve these problems, new devices such as three-dimensional electrode arrays are being developed. These are MEMS devices that consist of arrays of metal microtowers that can penetrate the outer layers of the skin, thereby reducing impedance.[10]

Compared to traditional wet electrodes, multi-electrode arrays offer the following:[10]

• The electrodes are approximately 1/10 the size of standard wet surface electrodes.
• Electrode arrays can be created and scaled to cover areas of virtually any size.
• Reduced resistance
• Improved Signal Strength
• Higher amplitude signals
• Providing better tracking of nerve impulses in real time

How to prepare

The doctor performing the NCV test will explain the procedure and give the person an opportunity to ask questions.

To prepare, people may need to do the following:

• Stop using lotions, oils, perfumes or other products on your skin for a few days before the test.
• Tell your doctor about all medications you currently take, including pain relievers and over-the-counter supplements.
• Dress in loose clothing or easy-to-remove layers.
• Tell your doctor if they have a pacemaker or heart defibrillator.

The NCV test does not require sedation or fasting, although some people may need to take additional precautions depending on their existing health problems.

Causes of conduction velocity deviations

Anthropometric and other individual factors

Basic measurements of nerve conduction are different for everyone, as they depend on age, gender, local temperatures and other factors. anthropometric factors such as hand size and height.[5][11] It is important to understand the influence of these various factors on normal nerve conduction measurements to assist in identifying abnormal nerve conduction findings. The ability to predict normal values ​​in the context of a person's anthropometric characteristics increases the sensitivity and specificity of electrodiagnostic procedures.[5]

Age

Normal “adult” conduction velocity values ​​are usually achieved by 4 years of age. Conduction velocities in newborns and toddlers are typically about half of those in adults.[1]

Nerve conduction studies performed on healthy adults have shown that age is negatively associated with measures of sensory amplitude in the Median, Ulnar, and Sural nerves. Negative associations were also found between age and conduction velocity and latency in the median sensory, median motor, and ulnar sensory nerves. However, sural nerve conduction velocity is not associated with age. In general, conduction velocity in the upper extremities decreases by approximately 1 m/s for every 10 years.[5]

Sex

The amplitude of conduction along the sural nerve in women is significantly less than in men, and the latency of impulses is longer in women, so the conduction speed is lower.[5]

Other nerves do not show any gender bias.

Temperature

In general, the conduction velocities of most motor and sensory nerves are directly and linearly related to body temperature (low temperatures slow nerve conduction velocities and higher temperatures increase conduction velocities).[1]

Conduction velocities in the sural nerve appear to be particularly highly correlated with local nerve temperature.[5]

Height

Conduction velocities in both the median sensory and ulnar sensory nerves are negatively associated with human height, which likely explains the fact that among most of the adult population, conduction velocities between the wrist and fingers of a person decrease by 0.5 m/s per every inch of height increase.[5] As a direct consequence, the latency of impulses in the median, ulnar and sural nerves increases with altitude.[5]

The correlation between the height and amplitude of impulses in sensory nerves is negative.[5]

Hand factors

Index finger circumference is negatively associated with conduction amplitudes in the median and ulnar nerves. Additionally, individuals with a larger carpal ratio (antero-posterior diameter: medial-lateral diameter) have lower median nerve latency and higher conduction velocity.[5]

Medical conditions

Amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a progressive and inevitably fatal neurodegenerative disease that affects motor neurons.[9] Because ALS shares many symptoms with other neurodegenerative diseases, it can be difficult to diagnose correctly. The best method for establishing a definitive diagnosis is an electrodiagnostic evaluation. Specifically, motor nerve conduction studies should be performed in the median, ulnar, and peroneal muscles, as well as sensory nerve conduction studies in the ulnar and sural nerves.[9]

In ALS patients, distal motor latency and conduction velocity slowing were shown to worsen as their degree of muscle weakness increased. Both symptoms are consistent with axonal degeneration in patients with ALS.[9]

Carpal tunnel syndrome

Carpal tunnel syndrome (CTS) is a form of nerve compression syndrome caused by compression of the median nerve of the wrist. Typical symptoms include numbness, tingling, burning pain, or weakness in the arm.[12][13] CTS is another condition for which electrodiagnostic testing is valuable.[12][14] However, before subjecting a patient to nerve conduction studies, both the Tinel Test and the Phalen Test must be performed. If both results are negative, it is very unlikely that the patient has CTS and no further testing is required.[13]

Carpal tunnel syndrome affects each person to varying degrees. Nerve conduction velocity measurements are critical in determining severity.[14][15] These severity levels are divided into:[12][13]

• Mild CTS: long latency, very slight decrease in conduction velocity. There is no suspicion of axonal degeneration.
• Moderate CTS: Abnormal sensory conduction velocities and decreased motor conduction velocities. There is no suspicion of axonal degeneration.
• Severe CTS: lack of sensory responses and prolonged motor latency (decreased motor conduction velocity).
• Extreme CTS: lack of sensory and motor responses.

One common electrodiagnostic measurement involves the difference between sensory nerve conduction velocities in the little finger and index finger. In most cases of CTS, symptoms do not appear until the difference exceeds 8 m/s.[12][13]

Guillain-Barre syndrome

Guillain-Barré syndrome (GBS) is a peripheral neuropathy associated with degeneration of the myelin sheath and/or nerves that innervate the head, body, and extremities.[7] This degeneration occurs due to an autoimmune response usually initiated by various infections.

There are two main classifications: demyelinating (damage to Schwann cells) and axonal (direct damage to nerve fibers).[7][16] Each of these then branches into additional subclasses depending on the exact manifestation. However, in all cases, the condition results in weakness or paralysis of the limbs, potentially fatal paralysis of the respiratory muscles, or a combination of these effects.[7]

Once symptoms appear, the disease can progress very quickly (serious damage can occur within a day).[7] Since electrodiagnosis is one of the fastest and most direct methods for determining the presence of disease and its correct classification, nerve conduction studies are of utmost importance.[16] Without proper electrodiagnostic evaluation, GBS is commonly misdiagnosed as Polio, West Nile Virus, Tick Paralysis, various Toxic Neuropathies, CIDP, Transverse Myelitis, or Hysterical Palsy.[7] Two sets of nerve conduction studies should allow the correct diagnosis of Guillain–Barré syndrome. It is recommended that they be carried out within the first 2 weeks of the onset of symptoms and again after 3-8 weeks.[16]

Electrodiagnostic findings that may indicate GBS include:[6][7][16]

• Complete conduction blocks
• Abnormal or absent F waves
• Amplitudes of attenuated action potential of complex muscles
• Long latency of motor neurons
• Severely slow conduction velocity (sometimes below 20 m/s)

Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease in which autoantibodies are directed against voltage-gated calcium channels at presynaptic nerve terminals. Here, antibodies inhibit the release of neurotransmitters, leading to muscle weakness and autonomic dysfunction.[17]

Nerve conduction studies performed on the ulnar motor and sensory, median motor and sensory, tibial, and peroneal motor nerves in patients with LEMS have shown that the conduction velocities of these nerves are in fact normal. However, the amplitudes of compound motor action potentials can be reduced by up to 55%, and the duration of these action potentials reduced by up to 47%.[17]

Peripheral diabetic neuropathy

At least half of the population with diabetes is also affected by diabetic neuropathy, causing numbness and weakness in the peripheral extremities.[18] Studies have shown that the Rho/Rho-kinase signaling pathway is more active in people with diabetes, and that this signaling activity occurs primarily in the nodes of Ranvier and the incisors of Schmidt-Lanterman.[18] Therefore, excessive activity of the Rho/Rho kinase signaling pathway may suppress nerve conduction.

Motor nerve conduction velocity studies showed that conduction in diabetic rats was approximately 30% lower than in nondiabetic controls. In addition, Schmidt-Lanterman incisor activity was intermittent and nonlinear in the diabetic group, but linear and continuous in the controls. These deficiencies were corrected following the introduction of Fasudil to the diabetic group, implying that it may be a potential treatment.[18]

Main points

• Nerves carry messages from the senses to the central nervous system and then to the muscles.
• Diffusion effects and Coulomb force work together to cause ions (like Na+, K+ and Cl-) to form a voltage across cell membranes.
• The change in cell membrane permeability due to electrical currents is called an action potential.
• Depolarization causes muscle contraction and can lead to contractions in adjacent muscle cells. The created wave of depolarization stimulates rhythmic contractions. An electrocardiogram (ECG) is a recording of tension in the heart.

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