Nerve conduction studies are often run at the same time as an EMG. They are used to test the speed (velocity) and size of the signal sent along a nerve.
What are nerve conduction studies?
A nerve conduction test gives doctors information about any potential problems with your peripheral nervous system (i.e. the nerves outside of the brain and spinal cord).
The brain sends an electrical impulse down the spinal cord and then along the peripheral nerves. This electrical impulse then either causes a muscle to contract (motor nerves) or is used to detect things like pressure and heat (sensory nerves). If a nerve is damaged, then the electrical impulse may travel slower down the nerve and the signal may not be as strong.
What are they used for?
Nerve conduction studies are used when nerve damage is suspected. This may be due to:
- Injury (such as fractures or lacerations).
- Conditions such as Diabetes.
- Trapped nerves (e.g. this is common in the neck or conditions such as carpal tunnel syndrome).
Nerve conduction studies can differentiate between damage to the myelin sheath of the nerve (surrounding fatty coating) or damage to the axon (central portion) of the nerve. This is due to the myelin sheath being an insulator of the impulse and so damage to this part of the nerve causes significant decrease in the nerve conduction velocity. If an impulse is considerably slower than normal, damage to the myelin sheath is present. If the nerve's response is decreased but at a normal speed, then damage to the nerve axon is present.
What happens during the test?
Electrodes are attached to the skin using tape, velcro or a special paste. One of these is a pulse emitting electrode which is positioned directly over the nerve which is being tested. Recording electrodes are then positioned over the muscle(s) that the nerve supplies.
A series of electrical impulses stimulate the nerve (from the pulse emitting electrode). The time between these impulses and the muscle contracting is recorded. The strength of the contraction is also registered by the recording electrodes over the muscle. If a sensory nerve is being tested, the recording electrode is placed on the nerve and records the time for the impulse to be returned back towards the brain.
The electrical impulses are not painful, but feel like tiny shock-like tapping sensations which may be uncomfortable. Testing may take just a few minutes if only one nerve is being tested. For a lot of nerves, the tests may take a lot longer. For comparison purposes the same nerves may be tested on the other side of the body.