Specialized Nerve Tests: NCV and SSEP
The spinal cord is the thick, whitish bundle of nerve tissue that extends from the lowest part of the brain down through the spinal column. In humans, spinal nerves branch off from the spinal cord to various parts of the body. There are 31 pairs of spinal nerves which are grouped according to where they leave the spinal column. There are eight pairs of neck (cervical) nerves, 12 pairs of upper back (thoracic) nerves, five pairs of lower back (lumbar) nerves, five pairs of pelvis (sacral) nerves and one pair of tail bone (coccygeal) nerves. It is important for the doctors to detect any abnormality in these nerves' function when dealing with spine problems. The specialized nerve tests like EMG, NCV and SSEP are designed to diagnose any abnormality in the functioning of these nerves. These studies investigate the functioning of the nervous system, while imaging studies like the CT scans or MRI scans look at the structure or anatomy of a particular body part or organ.
The nerves in our bodies are very similar to electric cables. The brain and spinal cord send electrical signals through the nerves to different muscles. The muscles, in turn, have specialized mechanism to properly understand the electrical signals and act accordingly, thus moving different parts of the body. In the 1950s scientists created a machine that was capable of detecting the electrical signals passing through the nerves to the muscles by placing sensitive detectors called electrodes at specific areas of the human body. These electrical signals could be recorded onto a TV screen just like the EKG and provided useful information about the functioning of specific nerves. We will discuss some of these techniques in the following sections.
What is EMG?
EMG, or Electromyography is a technique for evaluating and recording the electrical activity produced by muscles. EMG is performed using a machine called the electromyograph, to produce a record called an electromyogram. An electromyogram detects the tiny amount of electricity generated by muscle cells when they are activated by the nerves connected to them. Specialists analyze these signals to detect medical abnormalities in the spinal cord, nerves and the muscles that are connected to specific nerves.
There are two types of EMG: intramuscular EMG and surface EMG. The intramuscular EMG is the most common. It involves inserting a needle electrode through the skin into the muscle that is being studied. Surface EMG (SEMG) involves placing electrodes on the skin over the muscle to detect the electrical activity of the muscle. It is not used as often because it provides less useful information than the intramuscular EMG. These are evaluated by a physician who specializes in physical medicine or neurology and is trained to interpret the findings.
What are the uses of EMG?
EMGs can detect abnormal muscle electrical activity in many diseases and conditions, including inflammation of muscles, pinched nerves, damage to nerves in the arms and legs, disc herniation, and degenerative diseases such as muscular dystrophy, Lou Gehrig's disease and Myesthenia gravis, among others. The EMG helps doctors distinguish between muscle conditions that begin in the muscle and nerve disorders that cause muscle weakness.
What is a NCV test?
NCV, or Nerve Conduction Velocity, is an electrical diagnostic test that provides information about abnormal conditions in the nerves. Nerves are stimulated with small electrical impulses by one electrode while other electrodes detect the electrical impulse "down-stream" from the first electrode. The resulting nerve impulses are then measured and the outcome is interpreted by the physician. The NCV test is often done at the same time as the EMG in order to exclude or detect both nerve and muscle conditions.
What are the uses of NCV?
While interpreting NCV, the distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission. Slower than normal speed could indicate nerve damage from direct trauma, diabetic or peripheral neuropathy, viral nerve infection or nerve entrapment diseases like the Carpal Tunnel Syndrome among other conditions.
What is SSEP?
SSEP, or Somatosensory Evoked Potential, is a test showing the electrical signals of sensation going from the body to the brain and spinal cord. The signals show whether the nerves that connect to the spinal cord are able to send and receive sensory information like pain, temperature and touch. When ordering electrical tests to diagnose spine problems, SSEP is combined with EMG and NCV tests. Clinical usefulness of SSEP is mainly to assess the patient for generalized disorders of the nervous system like multiple sclerosis (MS) etc.
How is SSEP different from EMG?
SSEP is typically used to monitor the nervous system during complex spine surgeries to add an extra layer of protection to the sensitive neural structures. During spine surgery, EMG is used to show if a nerve is being irritated or pinched. It is used to monitor nerve output to the muscles in procedures where screws are placed in the middle or lower part of the spine. SSEP is used to double check whether the sensory part of the nerve is working correctly.
What can you expect during these tests?
You begin by changing into a hospital gown so that your arms and legs are exposed. The test starts with NCV test. You will lie down on a gurney or bed in a private room, and the physician will then place some electrodes on the back of one of your hands and circular clamps around two of your fingers. The clamps do not pinch. The doctor will then measure the distance from the clamps to a certain point on the inside of your arm, and mark that spot with a pen. Then, using a device that looks like a large pager with two knobs sticking out on one end, he/she will touch your arm with the two knobs at the top of your wrist, where he/she had made the mark, and begin to send electricity into your arm. The pulses will be quite faint at first, and he will gradually increase the strength of the current until your hand jumps up and down on the bed. Based on where your physician thinks your spine problem is, the NCV may be performed on one, two or all four of your extremities.
After the NCV, the physician will determine if you need to have the EMG. For an EMG, a needle electrode will be inserted through the skin and into the muscle. This electrode will be attached to a computer. The needles used are very thin, about the size of an acupuncture needle. The doctor will move the needle up and down in the muscle. At this time, a static sound can be heard coming from the computer. This is the sound of your muscles firing up. The doctor then may ask you to tense and relax certain muscles so he/she can see their response to the electrode.
For an SSEP test, detection electrodes will be glued to particular spots on your scalp, neck and back. A small generator will be used to create tiny electrical impulses that are used to stimulate nerves in the wrist or the ankle. While the impulses are usually not painful, they may cause your thumb or toe to twitch a little, which is normal. Responses to the electrical stimulation will be recorded through the electrodes using special equipment.
After the procedures the electrodes will be removed and you will be able to put your clothes and shoes back on.
How long do the tests take?
The tests can take between 20 and 90 minutes depending on whether you are getting an EMG alone or an EMG combined with NCV. You can do any of your normal activities like eating, driving and/or exercising before and immediately after the tests. There are no lasting side effects of any of these tests.
How should you prepare for the tests?
Please inform the physician if you are taking any blood thinners (Plavix, Coumadin, aspirin, etc), have a pace-maker implanted or have been diagnosed with a bleeding disorder. You should take a bath or shower before the test to remove extra oil from the skin. Avoid applying body lotions or skin oils on the day of the test.
What is the benefit of these tests?
The specialized electrical nerve tests are the only studies that directly assess the proper functioning of the body's nervous system. Therefore, regardless of the uncomfortable sensation, the EMG/NCV is an extremely beneficial and informative test. One of the most satisfying things about the test is that the attending physician can tell you their findings right after the testing is complete. There is no waiting for blood test results or for the radiologist to read the scan. You will walk out of the test with information that will help you to understand whatever is going on to warrant the test. Additionally, a detailed report is also sent to your treating physician to help him diagnose the cause of your symptoms and offer the best treatment plan to provide you relief from them.