Electrodiagnostic Testing
Electrodiagnostic tests measure the electrical activity of muscles and nerves. By measuring the electrical activity, they can determine if there is nerve damage, the cause of the damage, and if the damaged nerves are responding to treatment.
Two common electrodiagnostic tests are:
What is an NCS Test?
A nerve conduction study measures how electrical impulses move along a nerve; the two main parameters your physician will look at are velocity and amplitude of nerve signals: these will better characterize your disease and give clues to the causes. It is often followed by an electromyography (EMG) test, to exclude or detect muscle disorders.
A healthy nerve conducts signals with greater speed and strength than a damaged nerve. The speed of nerve conduction (i.e., nerve conduction velocity) is influenced by the myelin sheath—the insulating coating that surrounds the nerve. Whereas the amplitude (i.e., how “strong” is the signal) gives clues on how many functioning axons are still present in that nerve; damaged nerves have lower amplitude than healthy nerves when axons, rather than myelin, are damaged. Most neuropathies are, in fact, caused by damage to the nerve’s axon rather than damage to the myelin sheath surrounding the nerve. The nerve conduction study test is used to distinguish between demyelinating and axonal neuropathies, a relevant information that helps the physician narrow the diagnostic possibilities.
Purpose of an NCS Test
This test is used to diagnose nerve damage or dysfunction and confirm a particular diagnosis. As stated above, it can usually differentiate injury to the nerve fiber (axon) from injury to the myelin sheath surrounding the nerve, which is useful in diagnostic and therapeutic strategies.
How is an NCV Test Done?
During the test, flat electrodes are placed on the skin at intervals over the nerve that is being examined. A low intensity electric current is introduced to stimulate the nerves.
The velocity at which the resulting electric impulses are transmitted through the nerves is determined when images of the impulses are projected on a computer screen. If a response is much slower than normal, damage to the myelin sheath is implied. If the nerve’s response to stimulation by the current is decreased (i.e., reduced amplitude of the potential seen on the computer screen), but with a relatively normal speed of conduction, damage to the nerve axon is implied.
Does an NCV Test Hurt?
There is generally minimal discomfort with the test because the electrical stimulus is small and usually is minimally felt by the patient.
What is an EMG Test?
An electromyography (EMG) is a test that measures the electrical activity of a muscle. The test provides information about the muscle itself and shows how well it receives stimulation from the nerve. A nerve conduction study (NCS) test is performed before starting EMG testing.
Purpose of an EMG Test
An EMG is often used to evaluate unexplained muscle weakness, twitching or paralysis, and to find the causes of numbness, tingling and pain. It can determine whether a muscle disorder begins in the muscle itself (called myopathy) or is caused by a nerve disorder (i.e., the muscle is not activated properly due to lack of information coming from peripheral nerves).
How is an EMG Test Done?
In an EMG, a physician inserts a very fine needle, which serves as an electrode, through the skin into the muscle. With the electrode in place, the patient is asked to slowly contract the muscle—for example, by bending the arm—with gradually increasing force, while the electrical activity is being recorded. The activity can be displayed visually on a computer screen or played audibly through a speaker. The results can provide information about the ability of the muscle to respond to nerve stimulation.
Is an EMG Test Painful?
The patient may feel some discomfort, similar to an injection, when the needle or needles are inserted. Afterward, the examined muscle may feel tender or sore for a few days, and there may be a small bruise.