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What is an Evoked Potential?
An evoked potential is a close relative of an EEG. The difference is that, while an EEG records whatever activity the brain generates, any evoked potential gives the person being tested a stimulus, and then records the brains response to that stimulus. There are three kinds of stimuli that are used in everyday, clinical testing. They are auditory, visual, and tactile (also called somatosensory).
Whereas an EEG measures an actual running strip of the brains activity, the evoked potential displays a "snapshot" of the brain waves. The actual measurement takes place on the wave form. What we do is measure the time it takes from the actual stimulus (of whatever organ), until the time that the electrical response generated by the bodies reaction to the stimulus reaches the brain. That response is seen in the form of the electrical potential which is then generated by the brain in response. Phew! Let's try to reword this.
Put very simply, an Evoked Potential (which, in the old nomenclature, was called an Evoked Response), is a method measuring how fast an electrical impulse makes it from point A to B in your brain. We know how long it is supposed to take, but we would like to see how long it takes in the person being tested.
As an example, let us suppose that you live in Chicago. We know how long it takes to get from downtown Chicago to Ohare Airport. But every day is slightly different - that is like thinking about every person that one does. As an example, if their is a slowdown because of a part of the road which narrows, this would be one type of abnormality which we can see.
Now that we have the English version written, using an Auditory Evoked Potential as an example. A click is delivered to the persons ear. The computer is set to record the arrival of that stimulus, and, and this case, the following 10 - 20 milliseconds. However, that signal is extremely small, and so repeated clicks are delivered in very fast trains. As this happens, the computer records the acceptable wave forms, and then averages each response into the one or ones before it. By doing this, random noise is filtered out, and a "pure" response is thus produced. That moment that the click occurs, until the moment that the stimulus hits the inner ear, is in the range of about 1.1 - 1.75 milliseconds.
There are several types of evoked potentials, and each one has it's own characteristic waves that are measured in order to produce a result.
As to the question whether there is any pain associated with this test. There are no shots. The visual and auditory tests (evoked potentials can be ordered singly, such as only a visual, or in any combination, including all as a complete battery) are actually quite boring, although somewhat annoying.
Yes. The patient will be asked to wash their hair, and leave it clean and dry, putting no chemicals back in it after doing this! If you are only receiving a visual evoked potential, it is all right to have coffee, tea, or sweets; the visual evoked potential can have the effect of being very boring, almost sedentary in a way. It is only very slightly more exciting than watching the test pattern on a television screen.
Outside of that, it is best not to be too awake for the other evoked potentials, the technologist needs the person being tested to relax. 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 (their are several kinds of soda which are so high in sugar and / or caffeine content, they may keep the patient awake). There is no need to be sleep deprived for this test, in fact, if the person is taking a visual evoked potential, sleep deprivation is advised against; the test can have a hypnotic effect.
IMPORTANT! Bring any devices such as a hearing aid or glasses, whether they are reading, driving, or everyday glasses.
Let the technologist know on arriving if you are wearing or currently using a pacemaker, TENS unit, or indwelling nerve stimulator. Although these devices will probably not prevent the test from being done, the knowledge of their existence will help the technologist avoid extra work.
All evoked potentials start the same way, the technologist begins as if they were setting up for an EEG.
To begin, it is customary for the head to then be measured, with the technologist marks 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 on the scalp and head.
Auditory Evoked Potential (BAEP, BAER, ABR)
Typically, only 4 wires are pasted on a persons scalp. There will be 1 at the very top center of the persons head, there will be a second wire usually elsewhere on the scalp; typically toward the front of the head, about the mid line of the head. Two more wires are pasted on, one for each ear. Sometimes they are placed on the earlobe, sometimes, behind the ear or earlobe, sometimes on the bone just behind the ear. (For the initiated, the placements are A1, Cz, and A2. A1 and A2 may be located at M1 and M2 instead; here the difference is made between the earlobe, and the Mastoid bone.)
NOTE that some laboratories use something called ear insert electrodes. These are what looks like an earplug covered in gold foil; and they are inserted just like earplugs. The type of wires / electrodes used, their placement and application, and sometimes in the number of wires is laboratory dependent.
One of 3 stimulators types stimulators will be used.
These are placed on the bone behind the ear, and actually vibrate.
These are earplug like devices with a tubing sticking out of them which leads to a little box. A hole in the earplug delivers the sound.
This is by far the most common method.
The clicks will then become louder, they will be rapid. You will hear no other sounds through the stimulating device except the ones described. This will take several minutes, during which the person may hear the clicks stop and start several times, or not stop at all. The technologist will then switch ears, and perform the same test on the other side.
Depending on the reason the test was ordered, the person being tested may then notice that the sound gets a little softer, and the process is repeated, this may happen several times more, even to the point of not being able to accurately hear the sound. The speed of the clicks may also change during different parts of the test.
At the end of the test, the earphones will be taken off, the wires will be taken off, and the paste/glue will be cleaned from the scalp.
Visual Evoked Potentials (VEP)
Usually before beginning this test, the vision of the person being tested is tested. This is an acuity test, measuring how small of a shape/letter/number can the person correctly perceive. Again, this is laboratory dependent. At the end of the test, the diameter of the pupils may also be measured.
The minimum number of wires pasted to the scalp are 3. However, depending on the laboratory, and reason for the test, there may be as many as 10, although it is unusual to have that many on. Minimally though, there is 1 wire placed in the very back of the head, one in the front part of the head, and one at the vertex of the head. (For the initiated, Fz, Cz, and Pz.)
There may also be 2 wires placed directly beneath the eyes, or, in place of this, special "contacts" may be worn. The purpose of those wires/electrodes is to assess the function of the retina versus the function of the occipital cortex, where vision is processed. As always, the use of these wires or lack their of is laboratory dependent and/or test dependent (it is not necessary in all tests).
There are 4 types of visual stimulators. The choice of which to use is based on laboratory, why the test is ordered, and what the conditions are (For example, is the test being performed on an infant, is it done in the room of the person being tested, etc.)
The monitor used looks like a security camera monitor. It may be capable of color, some are not; again, this depends on the laboratory. This monitor presents a pattern to the person being tested, something which is visually simplistic; with high contrast in the pattern. The most common pattern is a checkerboard, usually black and white in color.
A strobe lamp/light of some kind, usually the flashes are very slow in presentation, about once per second.
A collection of L.E.D.s is used, much like that of a computer notebook screen, but smaller. Typically the stimulator is about 4" by 4" size, and the colors of the stimulus and red and black.
Depending on the laboratory, and what the technologist sees in the recording, more than one different types of stimulation may be used.
After the test, the wires are taken off; the paste/glue is then cleaned off.
Somatosensory Evoked Potentials (SEP, SSEP)
The number of wires applied to the scalp varies; again, according to the individual laboratory, and the reason for the test. Seeps are done both on the upper extremities and/or lower extremities. There are 2 different types of SEPs, routine and dermatomal. The first, or "upper" requires a minimum of 3 wires on the scalp, it is customary to apply 3 other leads; 1 on each shoulder, and 1 on the back of the neck. The second type is a "lower"; a minimum of 2 electrodes are applied to the scalp, usually 2 to 3 electrodes are applied to the low back.
The third and fourth types are upper and lower, but something called dermatomal stimulation is used.
The difference between the 2 major types of SEPs, (routine or dermatomal) from the perspective of the person being tested, is that the stimulator during the routine will only be located on the wrist in the upper extremities, or on the top, or side of the foot, or the back of the knee or the side of the leg by the knee.
During dermatomal stimulation, however, the stimulator will be moved to different parts of the body.
The stimulator used will deliver a pulsing shock, which is only enough to make, for example, your thumb twitch, or your toe twitch. Although annoying, most people get used to the sensation fairly fast. Most people do not find the procedure painful, and get used to the sensation of the stimulation fairly quickly.
There are 2 different types of stimulators:
This consists of 2 discs being held against the skin to deliver the stimulation. It is the one used in routine clinical use.
Usually almost exclusively for the operating room. The saving grace with this method is that there is less current used than that of the surface stimulators, and therefore less pain.
After the test, the wires are taken off; the paste/glue is then cleaned off.