The Nerve Conduction Velocity (NCV) test is a non-invasive diagnostic procedure used to assess the health and function of the body’s peripheral nerves. This test measures how effectively electrical signals travel along a nerve pathway, providing neurologists with quantifiable data about nerve damage or dysfunction. When a patient experiences symptoms like unexplained numbness, tingling, or muscle weakness, an NCV test is often ordered to help pinpoint the cause. It evaluates the integrity of the nerve structure outside of the brain and spinal cord.
Defining Nerve Conduction Studies
Nerve conduction studies measure two fundamental properties of nerve function: the speed of the signal and the strength of the response. The speed, or velocity, reflects the efficiency of the nerve’s insulation, known as the myelin sheath, which surrounds the axon. Myelin allows the electrical impulse to “jump” along the nerve fiber, dramatically increasing the transmission rate. NCV is calculated by measuring the distance between two points on the nerve and dividing it by the time the electrical impulse takes to travel that distance.
The second measurement is amplitude, which represents the magnitude of the electrical signal generated by the nerve’s response. Amplitude provides information about the number of functional nerve axons successfully transmitting the signal. A healthy nerve with intact axons produces a strong, high-amplitude signal. Conversely, a reduced amplitude suggests damage or loss to the nerve’s core fiber, the axon itself. NCV studies thus distinguish between two primary types of nerve damage: injury to the myelin sheath and injury to the axon.
The Procedure: What Patients Experience
Preparation for an NCV test focuses on ensuring accurate electrical readings. Patients should avoid applying lotions, creams, or oils to the skin, as these interfere with the recording electrodes. Maintaining a normal body temperature is also important because cold skin naturally slows down nerve conduction velocity, potentially leading to inaccurate results. Loose-fitting clothing is recommended to allow easy access to the nerves being tested, typically in the arms or legs.
During the procedure, small surface electrodes, resembling adhesive patches, are placed on the skin over the nerve or muscle being examined. A separate stimulating electrode applies a very mild, brief electrical impulse to the nerve at one point. This sensation is often described as a quick, mild shock or a tingling feeling, which causes a reflexive twitch in the muscle supplied by that nerve. While the sensation can be startling, it is not painful and is over quickly.
The recording electrodes then measure the resulting electrical activity further down the nerve pathway. The neurologist repeats this process at different points along the nerve to determine the speed and strength of the signal across various segments. The entire test duration is variable, depending on how many nerves are assessed, but generally takes between 15 minutes to an hour. The electrical voltage used is very low, and the test is considered safe.
Clinical Applications and Diagnoses
A physician orders an NCV test when damage to the peripheral nervous system is suspected. The test is effective at localizing the site of a nerve injury, determining if the problem is focal or widespread. A common application is diagnosing focal entrapment neuropathies, such as Carpal Tunnel Syndrome (CTS) in the wrist. In CTS, the median nerve is compressed, and the NCV test shows a distinct slowing of the electrical signal precisely as it passes through the carpal tunnel.
NCV studies are also used to evaluate generalized peripheral neuropathy, a condition often associated with diabetes or toxic exposures that affects multiple nerves, typically starting in the feet and hands. Another condition diagnosed is Guillain-Barré Syndrome, an autoimmune disorder where the immune system attacks the myelin sheath of peripheral nerves. The test can also help investigate radiculopathy, which is irritation or compression of a nerve root near the spine, often caused by a herniated disc.
Interpreting the Results
The interpretation of NCV results allows neurologists to differentiate between the two main types of nerve pathology. If the conduction velocity is significantly slower than normal, it points toward damage to the myelin sheath, a process called demyelination. This occurs because the myelin insulation, which enables the rapid jumping of the signal, is compromised. This pattern is seen in demyelinating conditions like Guillain-Barré Syndrome.
Conversely, if the conduction velocity remains near normal but the amplitude of the electrical response is low, it suggests damage or loss of the axon itself, known as axonal injury. Fewer intact axons mean fewer fibers are available to carry the electrical signal, resulting in a weak response. Axonal damage is characteristic of severe nerve injuries or certain forms of peripheral neuropathy.
The NCV test is frequently performed in combination with an Electromyography (EMG) test to gain a complete picture of the neuromuscular system. While NCV assesses the nerve’s ability to transmit a signal, the EMG component involves inserting a fine needle electrode into the muscle to evaluate its electrical activity and response. The combination of NCV and EMG results helps localize the problem, determining if the issue lies in the nerve, the connection between nerve and muscle, or the muscle tissue itself.