Nerve conduction tests are performed to evaluate the ability of nerves to conduct impulses and the speed thereof.
Nerve conduction studies can indicate the extent of nerve damage and the part of the nerve that is affected.
Why is it performed?
The peripheral nerve is the structure which carries the message from the brain and spinal cord to the muscle and also contains the nerves which transfer sensations from the skin to the brain. Almost all peripheral nerves are mixed, in that they contain both motor fibers (to the muscles) and sensory fibers (from the skin to the brain).
A good example of a peripheral nerve is the ulnar nerve, which can be felt in a groove at the back of the elbow. This is the nerve that causes the well-known 'funny bone' sensation.
Nerves may be damaged in all sorts of ways. The most common, though, is local damage at a particular spot. A typical example of this is carpal tunnel syndrome: because a particular nerve has to run through a special tunnel on its way to the hand, the nerve can become squashed in the tunnel.
This results in two main problems: sensory, in which the patient complains of pins and needles, and motor, which is when the thumb muscles become weak and thin. There are many examples of this kind of problem (generally in the forearm and lower leg), where, because of a particular anatomical feature like running through a tunnel, the nerve is prone to develop injury. Nerves may also be damaged by toxins and allergic-type conditions.
Nerves are rather like electrical wires: the inside bit (the 'copper wire') is called the axon and carries the actual impulse. The outside bit of the nerve, which acts like the insulation of an electrical wire, is called the myelin, and helps to ensure that the nerve impulse travels at great speed along the nerve.
Typically, damage to the axon is severe and recovery is often poor, taking months. On the other hand, damage to the myelin is usually repaired quickly, leading to fast recovery.
Common toxins associated with extensive nerve injury include diabetes (the mechanism is complicated and not well understood), alcohol, and medicines, e.g. some anti-HIV drugs.
By and large, nerve conduction tests are done for two reasons. Firstly, if the patient has a problem with one hand or foot, usually pins and needles, numbness and sometimes weakness, the conduction studies might confirm a focal problem with the nerve - often because of what is called the entrapment syndrome.
This sort of problem may get better with splints, but often requires surgery in order to relieve the pressure on the nerve.
Secondly, if the patient has abnormal sensation in both feet or all their limbs it is often due to a more generalized nerve problem, and nerve conductions may be useful to confirm that this is the case and to point out which part of the nerve is injured, the axon or the myelin.
How is it performed?
When a conduction study is done, the usual procedure is to put an electrode on a hand muscle and to stimulate the nerve at the wrist and elbow. This allows for the determination of the size of the muscle response and to check the speed of the nerve impulse. This can be very useful where surgery is considered, since it is often a very precise way of establishing exactly where the nerve is being squashed.
Another aspect of nerve conduction studies frequently includes the use of electromyography (EMG), during which the neurologist puts a fine needle into the muscle and looks at the waves emitted by the muscle fibers.
This can be helpful in situations where the nerve is not readily accessible to nerve conduction studies, for example in the case of a slipped disc squashing a nerve in the neck; it is also an important way to determine the extent of nerve injury.
Risks & limitations:
Nerve conduction studies are very accurate in the right setting, i.e. the diagnosis of a peripheral nerve problem. They are however less useful in the assessment of non-specific pain in the arm (but can at least rule out focal nerve compressions) and are somewhat useful in the common situation of a pinched nerve in the neck or lower back.
(They usually don't contribute much, unless there is substantial doubt about the cause of pain).
Is it painful?
Nerve conduction consists of a series of shocks. These are often quite uncomfortable, but are over with quickly and have no after-effects. When EGM is required, the needles used do cause a bit of pain - they are inserted into the muscle, and cause an unpleasant ache.