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• Glossary

The brain and spinal cord are both parts of the central nervous system. However, there is another part of the nervous system - called the peripheral nervous system - that includes the large nerves in the arms and legs. These can also be studied using specialised electrical testing methods.

EMG is short for electromyography, which means measuring the electrical activity of muscles. However, the term also covers nerve conduction studies - testing the electrical function of nerves in the limbs. So when your doctor tells you that he or she is sending you for an EMG, they may mean either or both of these tests. The clinical neurophysiologist will decide whether both tests or one or other are needed for your particular problem, and if both are required they will both usually be done at the same visit.

Nerve conduction studies
In many ways, the large nerves in the arms and legs are like electrical cables and they function by means of electrical impulses travelling along them. They can be tested using quite simple methods.

The nerves in the limbs can be classified as motor or sensory. A motor nerve is one that runs down to a muscle and controls its movement. A sensory nerve is one that carries sensation to the nerve roots and spinal cord. Many individual nerves have both motor and sensory components. Nerve conduction studies will normally involve the testing of both motor and sensory nerves.

Testing for trapped nerves
The most frequent reason for requesting nerve conduction studies is to look for evidence of a trapped nerve. Carpal tunnel syndrome is the type of nerve entrapment most frequently seen in clinical neurophysiology departments and it will be used here as an example to explain these tests.

The carpal tunnel is a small canal in the bones at the wrist. A major nerve - the median nerve - runs through it into the hand. The motor part of the nerve supplies muscles which move the thumb, while the sensory part of the nerve carries sensation from the thumb and the index and middle fingers and part of the ring finger. The median nerve shares the carpal tunnel with tendons running down from the muscles in the forearm to the fingers and space in the tunnel is quite limited. It is quite easy for the median nerve to become compressed (squashed) as it passes through this narrow passage. When this happens it is called carpal tunnel syndrome - or CTS for short.

Nerve conduction studies for carpal tunnel syndrome
Before starting treatment for carpal tunnel syndrome, it is important to be sure that this is the correct diagnosis. Other conditions, such as pressure on the nerves in the neck from a disc or arthritis, can cause similar symptoms that may be confused with carpal tunnel syndrome. Nerve conduction testing can usually confirm the diagnosis.

There are two parts to nerve conduction studies - testing the motor nerve and testing the sensory nerve.

Testing motor nerves
The median nerve begins at the top of the arm, travels down the forearm and through the carpal tunnel to divide into a number of smaller branches in the hand. If the motor part of the nerve is stimulated, an electrical impulse will travel down the nerve and produce an electrical signal that can be recorded quite easily from the skin over the large fleshy muscle at the base of the thumb. This can be done with stick-on electrodes or with small felt pads soaked in salt solution, usually mounted in a plastic holder and kept in place with a strip of Velcro.

The nerve is stimulated with a small electrical pulse to the skin, again commonly given through felt pads soaked in salt solution. The electrical pulses are very brief and feel like a sharp tapping sensation on the skin. Most people do not find this too uncomfortable.

When the nerve is stimulated, the electrical signal in the muscle appears very rapidly - within a few milliseconds. The time taken for the signal to appear is measured on the EMG machine. If a motor nerve is stimulated at two different points along its course and the time taken for the response to appear in the muscle is measured on each occasion, this information together with the distance between the two points can be used to calculate the speed at which the electrical impulse is travelling along the nerve. This is referred to as the conduction velocity. It is quite fast, usually 50-60 metres per second.

The median nerve is usually stimulated at the wrist and further up the arm where it passes in front of the elbow, with the responses being recorded each time over the thumb muscle. In people with carpal tunnel syndrome there is a delay in the appearance of the response in the thumb muscle when the median nerve is stimulated at the wrist. This makes sense, because the nerve is squashed at the wrist and the test shows that the signals are slowed down passing through this abnormal section of the nerve. The extent to which the signals are delayed as they pass through the carpal tunnel may give an indication of how badly the nerve has been affected.

Testing sensory nerves
Sensory nerves can be studied using similar methods. In carpal tunnel syndrome, the sensory branches of the median nerve to the fingers can be tested by stimulating them with small electrical pulses. These are delivered through ring electrodes, which fit around the fingers. They may be made of metal or of Velcro soaked in salt solution. It is also possible to stimulate the sensory nerves in the palm as they run up towards the carpal tunnel.

As the electrical signals travel along the nerve towards the spinal cord and brain, they can be recorded as they pass the wrist. This is done by placing felt pads or stick-on electrodes on the skin over the nerve. The speed at which the signals are moving is called the sensory conduction velocity. As with motor nerves, these signals normally move very quickly, at about 50-60 metres per second.

In carpal tunnel syndrome, because of the delay in the signals passing through the compressed section of the nerve at the wrist, their speed becomes reduced - perhaps to only 30-40 metres per second or less. As the compression of the nerve becomes more severe, the speed of the sensory nerve impulses becomes slower and slower and the signals become weaker until they eventually disappear altogether.

Other motor and sensory nerves can be tested with the same kind of procedure. The ulnar nerve, which runs from the top of the arm into the hand, can be tested by stimulating the little finger. The sural nerve in the leg is stimulated as it runs along the back of the calf and is recorded behind the ankle.

Although most nerve conduction studies are done on nerves in the arms and legs, it is quite possible to test nerves in other parts of the body. The facial nerve, for example, passes in front of the ear and divides into branches supplying the face. If it is stimulated just in front of the ear, electrical signals can normally be recorded quite easily from over the facial muscles. Checking whether these signals are still powerful enough to be recorded and whether they are delayed can be helpful in deciding how severely the nerve is affected in conditions such as Bell’s palsy. This is a relatively common disorder in which one side of the face becomes weak because of an abnormality in the facial nerve.

Testing for other types of nerve disorders
Nerve conduction testing is also used to test for and evaluate a whole range of other nerve disorders. If a limb is injured this technique can be used to test for nerve damage. The studies can give valuable information about which nerves are involved and how severely they have been injured.

Nerve conduction studies are also used in the diagnosis of peripheral neuropathies. This is a group of conditions in which, instead of a single nerve being involved, there is a generalised abnormality of the nerves in the limbs. There are many different varieties of peripheral neuropathy and both motor and sensory nerves may be affected. In the majority of cases, no specific cause can be found. However, it is sometimes linked with some common medical conditions such as diabetes, or chronic kidney disease. Heavy alcohol use, vitamin deficiency and certain drugs may also cause peripheral neuropathy. For this reason, nerve conduction studies will be just one part of the assessment of peripheral neuropathy - along with a detailed medical history and examination, and blood tests.

Nerve conduction studies in these cases may show several types of abnormality - slowing of nerve conduction or a decrease in the size of the electrical signals or both. The exact pattern of these abnormalities will help to classify the type of peripheral neuropathy.

Small fibre testing
Small fibre neuropathy is a particular type of peripheral neuropathy that affects the smallest nerve fibres. The methods described above cannot be used to test these microscopic fibres, so a number of other methods have been developed. For example, some of these small nerve fibres carry information about temperature. The ability to detect very small changes in temperature - warmer or cooler - of a metal plate touching the skin of the palm or the sole of the foot can give a good indication of whether these nerve fibres are functioning normally. This is called thermal threshold testing.