A nerve conduction study (NCS), also called a nerve conduction velocity (NCV) test–is a measurement of the speed of conduction of an electrical impulse through a nerve. NCS can determine nerve damage and destruction.
During the test, the nerve is stimulated, usually with surface electrode patches attached to the skin. Two electrodes are placed on the skin over the nerve. One electrode stimulates the nerve with a very mild electrical impulse and the other electrode records it. The resulting electrical activity is recorded by another electrode. This is repeated for each nerve being tested.
The nerve conduction velocity (speed) is then calculated by measuring the distance between electrodes and the time it takes for electrical impulses to travel between electrodes.
A related procedure that may be performed is electromyography (EMG). An EMG measures the electrical activity in muscles and is often performed at the same time as NCS. Both procedures help to detect the presence, location, and extent of diseases that damage the nerves and muscles.
Reasons for the procedure
Nerve conduction velocity is often used along with an EMG to differentiate a nerve disorder from a muscle disorder. NCS detects a problem with the nerve whereas an EMG detects whether the muscle is functioning properly in response to the nerve’s stimulus.
Diseases or conditions that may be evaluated with NCS include, but are not limited to, the following:
- Guillain-Barré syndrome. A condition in which the body’s immune system attacks part of the peripheral nervous system. The first symptoms may include weakness or tingling sensations in the legs.
- Carpal tunnel syndrome. A condition in which the median nerve, which runs from the forearm into the hand, becomes pressed or squeezed at the wrist by enlarged tendons or ligaments. This results in pain and numbness in the fingers.
- Charcot-Marie-Tooth disease. A hereditary neurological condition that affects both the motor and sensory nerves. One characteristic is weakness of the foot and lower leg muscles.
- Herniated disk disease
- Chronic inflammatory polyneuropathy and neuropathy.
- Sciatic nerve problems
- Pinched nerves
- Peripheral nerve injury
- Nerve conduction studies may also be performed to identify the cause of symptoms such as numbness, tingling, and continuous pain.
There may be other reasons for your doctor to recommend NCS.
What you can expect?
- Your doctor will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
- Generally, no fasting or sedation is required prior to the procedure.
- Normal body temperature must be maintained before and during the procedure, as low body temperature slows nerve conduction.
- Notify your doctor of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
- Dress in clothes that permit access to the area to be tested or that are easily removed.
- Stop using lotions or oils on your skin for a few days before your procedure.
- Based on your medical condition, your doctor may request other specific preparation.
During the procedure
A nerve conduction velocity procedure may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor’s practices.
The NCS is performed by a neurologist (a doctor who specializes in brain and nerve disorders), although a technologist may also perform some portions of the test.
Generally, a NCS procedure follows this process:
- You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the procedure.
- If you are asked to remove clothing, you will be given a gown to wear.
- You will be asked to sit or lie down for the test.
- A neurologist will locate the nerve(s) to be studied.
- A recording electrode will be attached to the skin over the nerve with a special paste and a stimulating electrode will be placed at a known distance away from the recording electrode.
- The nerve will be stimulated by a mild and brief electrical shock given through the stimulating electrode.
- You may experience minor discomfort for a few seconds.
- The stimulation of the nerve and the detected response will be displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves).
After the procedure
The paste used to attach the electrodes will be removed from your skin.After the test, you may return to your previous activities, unless your doctor advises you differently. Your doctor may instruct you to avoid strenuous activities for the rest of the day.Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.
The voltage of the electrical pulses used during NCS is considered very low.
There may be risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.
Certain factors or conditions may interfere with the results of NVC test, such as damage to the spinal cord, severe pain before the test, and body temperature.
Tell your doctor if you have a cardiac defibrillator or pacemaker, as precautions may need to be taken.
The speed of nerve conduction is related to the diameter of the nerve and the degree of myelination (a myelin sheath is a type of “insulation” around the nerve). A normally functioning nerve will transmit a stronger and faster signal than a damaged nerve. This is kind of like an electric wire with rubber or plastic insulation around it. The larger the wire or electric cable and the better the insulation, the more consistent and stronger the signal.
In general, the range of normal conduction velocity will be approximately 50 to 60 meters per second. However, the normal conduction velocity may vary from one individual to another and from one nerve to another.
Abnormal results may be caused by some sort of neuropathy (damage to the nerve) that can result from a contusion or traumatic injury to a nerve. Various diseases can also cause the impulses to slow down.