Basics of Electroneurodiagnostic Tests:
What Are They, and What Happens During Them?

Please click on the test that you would like to read about:

EEG (Electroencephalography)

Evoked Potentials (SEP, SSEP, AEP, BAEP, AER, VEP, VER, PVEP, FVEP)

EMG and/or NCS (Electromyography, Nerve Conduction Study, NCV, Nerve Conduction Velocity Testing

Sleep Study (Polysomnography; other studies include MSLT, Nap Study, CPAP Titration) 

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EEG / Electroencephalography

What is an EEG?

What does it measure?

Is there any preparation?

How is it done?

Is it painful?

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What is an EEG?

An EEG is a test that takes a look at the electrical activity produced by your brain. Sound pretty weird? Let me explain further. 

For those of your familiar with an EKG (or ECG, to be very correct), an EEG is no reach of imagination. Instead of putting sticky things on your chest and / or arms, wires are pasted onto your scalp. A recording is then run for about 20 - 30 minutes, depending on the laboratory you go to, and the purpose of the test. Tests may occasionally be run longer than this; either because the physician asked for an extended study, or because of protocols established in an individual laboratory. 

Physicians rarely do EEGs, although it does happen. They are almost always run by technologists who have been trained in the art of placing and pasting the wires (electrodes), as well as some training in how to read the actual test. The technologist is in no way obligated to give you the results of your test!! Strictly speaking, most laboratories do not allow technologists to do this, although it is done, occasionally. 

An EEG is a pain free procedure; there are no shots and no shocks. There may be a rough sensation in different areas of the head as the person is prepared to take the test; this is usually transitory, and most people don't notice it. The technologist cannot see what you are thinking, and if you go to sleep, they cannot tell what you are dreaming. The technologist cannot tell if the person being tested is crazy! 

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What does an EEG actually measure?

An EEG records the activity produced spontaneously by the brain. As such, it is recording the function of the brain, just as the EKG records the electrical function of the heart. As such, an EEG cannot tell what you think, or if you go to sleep, what you dream. 

Is it painful?

You get no shocks and no shocks. The test is not painful. 

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Is there any preparation?


If you are an inpatient in a hospital:

The patient (or you) may be asked to wash your hair and leave it clean and dry, putting no chemicals back into it whatsoever. This should be done, of course, only if the person (patient) has the ability to do so. Caffeine and sweets should be avoided, e.g.: coffee, tea, soda, or candy. It is possible that the doctor may ask you to stay awake the night before the test. The patient (or you) are encouraged to eat on a normal schedule if able, and of course, consistent with the doctors orders. Medications should be taken as ordered by the doctor.

If you are an out patient:

The patient will be asked to wash their hair, and leave it clean and dry, putting no chemicals back in it after doing this! No caffeine should be had before the test. This means no coffee, no tea, and no cola of any kind. No sweets should be imbibed; this means candy, and no soda (there are several kinds of soda which are so high in sugar and / or caffeine content, they may keep the patient awake). The patient may also be sleep deprived. Usually, this means that the patient will be asked to stay up until 12:00 midnight, and arise at 4:00 a.m. Some doctors may ask for 24 hours of no sleep prior to the test; no sleep deprivation may be ordered. If in doubt, get a little less sleep than usual. 
Sedation may or may not be given before the test. There are legal issues about this which are state and hospital dependent; I will not talk about this here. The standard sedation used is Chloral Hydrate, which is usually given in a syrup form. It can also be given by capsule. If sedation is given, the patient must be accompanied, especially leaving from the laboratory; the patient will probably not be permitted to leave without an escort!!!

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What happens during an EEG? How is it done?

The entire produce takes anywhere from 45 minutes to 1 1/2 hours for a standard, routine EEG. 
Before starting the technologist may first ask you about your medical history, with special attention paid to why that person is receiving an EEG. 
It is customary for the head to then be measured, with the technologist marking it as they go on. The measurements are done with a centimeter tape measure, the markings is done with a soluble marker, for instance a grease pencil; some laboratories use a water soluble marker. In all cases, the marks should wash from the skin/hair. Although the laboratory may not measure and mark the head, they will rub a gritty solution on different spots throughout the scalp and head. 
As the tech does this, they will begin to apply the electrodes, which are usually small metal cups placed on the skin. This is usually done one of 2 ways. The first, and most common, is with a special electrically conductive EEG paste. Something will be placed on top of the electrode, usually a small piece of paper, tape, a small piece of gauze or a small piece of cotton. The other way is by using a glue, which is a actually a cousin of Crazy Glue. If glue is used, after the electrodes are applied, a special gel is squirted through the hole in the top of the cup, it is an electrically conductive gel. 
After applying the electrodes, the person receiving the test will be asked to either lay or sit down. If the person had the electrodes applied while sitting in an easy chair or lying on a bed or cart, they will most probably remain their for the test. 
During the test, various procedures may be done. 

The technologist will first ask the person being tested to open, then close their eyes. They may do this several times. 
The person will be asked to breath deeply, in and out, as if blowing on a pin-wheel in order to constantly make it turn. Some technologists may simply ask the patient to breath deep and fast. This is usually done for 3 or 5 minutes; the length is usually determined by the technologist based on the information that they have gathered about that person's reason for taking the test. 
Hyperventilation should not be performed if the person being tested has one of the following problems: 

A current cardiac problem such as fairly recent heart attacks, chronic heart disease, etc. 

  1. A current pulmonary problem such as athsma or bronchitis.
  2. A current or recent trauma to the brain such as a stroke or brain surgery.
  3. Sickle Cell Anemia

If the person being tested can walk up 3 flights of stairs without stopping, this is a fairly good way of deciding that it is all right for the person to begin hyperventilation, unless the person being tested is confused, or their word is in someway suspect. 
The next procedure is called photic stimulation; this is commonly done in most, if not all EEGs. This is done by having a strobe light in front of the person's eyes, the light will be flashed at different frequencies; usually about 10 seconds of flashing for each frequency. In most laboratories, the person will be asked to open and close their eyes during this part of the test. 
There are several other procedures which the technologist might ask you to do. 

  1. You may be asked to open your eyes, and then clench and unclench your fists, or wiggling your fingers and/or toes, or possibly even to think of moving. 
  2. You may be asked to scan a detailed picture.
  3. You may be asked to do mental exercises, such as counting backwards out loud or adding numbers up.

After the test, which usually lasts no less than 20 minutes, the electrodes will be taken off, the hair / scalp cleaned, and the person sent home. 

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NCS, NCV, Nerve Conduction Study / EMG Electromyography

What is an Nerve Conduction Study and/or an EMG?

What do these tests measure?

Is there any preparation?

How are these test(s) done?

Are these tests painful?

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What is a Nerve Conduction Study and / or and Electromyography (NCS / EMG)?

These are actually 2 different tests, but they are usually done in tandem, although there will be instances when that is not so. The first test here, the NCS, is a measure of the functioning of the nerves in the periphery of the body. Whereas an Evoked Potential measures the function of the nerves in the arms/legs, or the eyes or the ear in relation to the brain, this test measure only the nerves in the arms or legs. There are rare instances where other nerves are used: the upper back, or the stomach / chest, for instance.

What Do These Tests Measure?

Nerve Conduction Studies

This test measures how long it takes for an electric, or nervous, impulse to get from point A in the body to point B. We know how long it is supposed to take, we want to see how long it takes in the individual who is being tested.

Again, we can use the analogy of the expressway. We know about how long, given optimal conditions, it takes to get from, say, downtown Chicago to O'Hare Airport. We also know that distance. By driving a car along that route, however, we can find out the actual time on that given day. If construction is being done along the way, and the highway narrows, the same amount of cars will take longer to get through than if the highway was open and clear. Likewise, if you start with many cars on the freeway, and several cars take exit ramps, then less cars arrive at the airport than were started with. Both of these scenarios are equivalent to different clinical conditions. The above also illustrates the general principles involved, plus the illustrates some of the common clinical problems.


This test measures the amount of electrical activity being generated by the nerve and or muscle. The test can see several types of abnormalities. Oversimplifying the subject, several types of activity can be recorded. The first, normal activity, is a "normal size" burst of activity and/or noise, which is quiet when the person being tested is relaxed, and noisy when the person flexes the muscle being tested can be recorded.

Is There Any Preparation?

Yes. The person being tested will be asked to shower or bathe before the test. After showering bathing (whichever you do!) the person will be asked NOT to apply any skin conditioner or oil. Medication should be taken as usual, unless specified by a physician. In specific cases, the physician may explicitly specify that certain drugs not be taken 24 hours before the test - this is usually for a specific type of EMG known as a Single Fiber Examination. It is not necessary to bring an escort; the person being tested may resume normal activities after leaving the testing area.

How Are These Tests Done?

First off, these tests can be performed if:

Pacemakers and T.E.N.S. units can create artifact during the test, which may help obscure the test results, or make it difficult to interpret. This is not always the case, however, and questions should be directed to the physician in attendance during the test.

Before any testing is done, a brief physical examination will be performed.  Some of the parts of the physical examination, as well as some of the questions being asked may be familiar to the person being tested. It is important, however, for the physician to be very familiar with the case, and to clearly understand the details needed to correctly assess the test. Some of the parts of the physical examination may be new to the person.

The order of the tests may differ; customarily, the nerve conduction study is done first, this does not always hold true. It may also be that only one test or the other will be done, but not both. The order and choice of tests is depends on the reason for the test, the protocol of the laboratory, as well as the physician directing the test.

The person being tested will be asked to disrobe, usually only that part of the body to be examined. For instance, if it is only the hand, then there is a chance that the person will not have to disrobe. (It is a good idea to wear loose fitting clothing to an examination.)

Nerve Conduction Studies

There are 4 distinct types of nerve conduction studies. The first is called a motor nerve conduction, the second is called a sensory nerve conduction, the third is referred to as an "F response", and the 4th is referred to as an "H reflex."

For all the nerve conduction testing, the person is customarily asked to lie down on a cot or bed; although there are situations in which the patient is allowed or even asked to sit upright.  Wires will be taped onto the body part to be tested.

Motor Nerves: In this test, 2 wires will be taped, one over the muscle being tested. As a for instance, let us say that the physician would like to test the median motor nerve, located at the wrist. The first wire is taped to the base of the thumb, the other to a bony prominence of the thumb further toward the tip of the fingers. A third wire is then placed, usually at the base of the thumb, further back towards the wrist. Single pulses of brief, small, electrical shocks are given. They should be enough to make the affected muscle twitch. Most people tolerate the test very well, it is better to try to take the test; most people find that the test is easier to take than they thought.

Sensory Nerves: This test is basically the same, although the placements of the electrodes are different; there may also be different electrodes used, depending on the body part being tested, and the protocol of the laboratory. For instance, instead of using small discs, special ring electrodes may be placed around the finger being tested. Also, the stimulation usually requires less current. A muscle twitch is usually absent from the test; although the patient will feel the stimulation.

F Response: To the person being tested, this test appears to be the same as the motor nerve test. In fact, it may be done in conjunction with the motor nerve, and the person being tested may not even know that a different test is being performed. The difference lies in the setting of the machine and what is being looked at by the technologist or physician. A third difference is that, during a motor stimulation, only 1 shock may be necessary, although this is rarely the case. During an F Response, more than 1 shock must be given. It may be as little as 3, or as much as 20 or even 40. The actual number of stimulations depends on what is seen while performing the test, what is being looked for in that particular case, the clarity of absence of the response, and, of course, the laboratory protocol.

H Relfex: Again, it appears very similar to the motor response. Again, the person being tested may not even realize that it is a different test being done. Again, the shocks may vary in number from 3 to 15. The number of shocks is determined based on the same criteria as that of an F Response. Although this test is by far mostly done on the back of the knee/leg, it may also be done on the arm. The H Reflex is done on both sides of the body, each side done 1 at a time.


A small, thin needle, sometimes described as a pin, is inserted into the muscle being tested. Once there, it will be held there momentarily, and then the doctor will move it slightly, several times. The number of "sticks" depends upon the number of muscles which need to be tested. Although, there is usually 1 stick per muscle, there are occasions in which this rule might be broken.

 Are these Tests Painful?

Nerve Conduction Studies

It depends on the person.  A sharp pulse is administered, it is actually a shock which is just strong enough to make a person's muscle(s) jump wherever simulation is applied.


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