Études de conduction nerveuse

What are nerve conduction studies?

Nerve conduction studies give doctors information about how well and how fast the nerves in your body send waves of electricity (electrical impulses). This test can be used to check for various different types of problems with the peripheral nervous system. The peripheral nervous system includes all the nerves in your body apart from those in your brain and within the spinal cord itself. Nerves in the brain and spinal cord make up the central nervous system.

Nerve conduction studies are used for a wide variety of reasons including:

• To assess nerve damage following an injury.

• To check for damage to nerves caused by certain conditions (peripheral neuropathy).

• To check for 'trapped' nerves - conditions such as syndrome du canal carpien.

• To monitor changes to a known damaged nerve's function over time.

Very small electrical shocks made by a machine are used to mimic the electrical pulses made by nerves. By attaching a small device that is able to detect electricity or supply electricity (an electrode) to the skin, the nerve can be stimulated with a very small electrical pulse. If the nerve is attached to a muscle, the muscle will contract in response to the electrical signal.

To test sensory nerves, the electrodes are usually attached to the fingers or toes with another electrode either at the ankle or wrist. When the electrical pulse is applied to the fingers or toes the sensory nerve carries the electrical signal. The electrode at the wrist or ankle detects the electrical impulse when it reaches that point.

Nerve conduction velocity

© Private individual, CC BY-SA 3.0, via Wikimedia Commons

The electrodes are connected to a machine which generates the impulses and detects them. It can measure the time taken for the impulse to travel in the nerve from the first electrode to the second. This information, plus the distance between the two electrodes, can be used to work out the speed at which the impulse is travelling along the nerve. This is referred to as the conduction velocity. It is quite fast - usually, 50-60 metres per second.

Nerve conduction studies can also be used to measure whether the size of the electrical impulses decreases as they travel along the nerve.

There are many sources of potential error. This is because of the way the test is performed and the difficulty interpreting the test results. Studies show that the accuracy is also affected depending on which nerves are being tested.

In this test, several electrodes are attached to your skin with tape or a special paste. The electrodes can be stick-on or made of small felt pads soaked in salt water solution. Felt electrodes are usually mounted in a plastic holder and kept in place with a strip of Velcro®.

A pulse-emitting electrode is placed directly over the nerve to be tested. If the nerve controls a muscle, a recording electrode is placed over the muscles under control of that nerve. Several quick electrical pulses are given to the nerve. The electrical pulses are very brief and feel like a sharp tapping sensation on the skin. Many people find this unpleasant with a slight numbness or tingling sensation but fortunately it does not last very long.

The time it takes for the muscle to contract in response to the electrical pulse is recorded. The speed of the response is called the conduction velocity.

If the nerve being tested is a sensory nerve, the recording electrode is placed in a position that will record the impulses going back towards the brain. Other nerves of the body may be studied for comparison.

How long does a nerve conduction study take?

Nerve conduction tests may take from 15 minutes to 1 hour or more, depending on how many nerves and muscles are studied.

Usually very little. Your local hospital should give you specific information about any preparation necessary.

It is also helpful if your hands and feet are as warm as possible. If they are cold this may slow down the waves of electricity (electrical impulses).

You may be asked to avoid hand lotions or creams.

Loose-fitting clothing that can be rolled up to above the elbows and knees is very helpful.

Bracelets, rings and watches will usually be removed for tests on the hands, and socks or tights removed for investigation on the feet.

While it may sound alarming to have an electrical impulse applied to your skin, the amount of electricity that passes through you is very small.

Most people tolerate the test very well and have no side-effects after the test. Some people may have some mild discomfort or tingling which may last for a few hours after the test.

Nerves act a bit like electrical cables. They use electrical impulses to allow communication between the brain and all the other parts of the body. The brain can send signals, in the form of electrical impulses via the spinal cord to the peripheral nervous system.

Peripheral nerves can be 'motor' nerves, which means they are attached to muscles and cause the muscles to contract. They can be 'sensory' nerves, which means they are attached to special body sensors which detect things like heat, pressure, touch, etc. Or they can be mixed nerves, which means they have both a motor and a sensory part. Motor nerves use electrical signals to make muscles move. Sensory nerves send information about the environment back to the brain in the form of electrical impulses.

To work well, most nerves need to be surrounded by a special substance called myelin. Myelin provides a form of insulation for nerves, helping to keep the electrical impulses within the nerve fibre. If nerves are damaged the electrical signal often moves slower through the nerve fibre. The speed of the nerve impulse is one of the things that can be detected by nerve conduction studies.