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International 10-20 Electrode Placement System for Sleep Educational Objectives
In order to perform consistent testing for electroencephalographic (EEG) recordings , a system was developed which would describe the locations of electrodes on the human skull. A great deal of research was conducted in the creation of the system. The 10-20 System of Electrode Placement is a method used to describe the location of scalp electrodes. These scalp electrodes are used to record the EEG using a machine called an electroencephalograph.
The EEG is a recording of brain activity. Each recording is the reflection of the activity of thousands of neurons in the brain. The pattern of activity changes with the level of a person's consciousness and brain activity. The EEG is used to record brain activity for many purposes including sleep research and to help in the diagnosis of brain disorders.
Important features which needed to be considered in the development of the 10 - 20 system include:
As a result of this joint effort of neurological societies worldwide, the
"
International 10-20 Electrode Placement System"
The system used for
sleep monitoring
is essentially the same, with one exception; Known, skull landmarks, such as the bridge of the nose, were used to develop reference points. Interestingly, cadavers were used for land marking to ensure proper and consistent recommendations for EEG locations. Therefore, the International 10-20 electrode placement is founded on the relationship of a "skull measured" electrode location and the corresponding cortical anatomy. The distinction between skull landmarks and electrode placement sites should be clearly marked, and is easily mistaken. When viewing the skull it is important to double check. Anatomic landmarks are used for measurement purposes only and an electrode is NOT placed here. Beginners should use two different color markers, one for electrode locations, one for anatomic locations. Why is the system termed 10-20?
The 10-20 system is based on the relationship between the location of an electrode and the underlying area of cerebral cortex.
Note: the smaller the number, the closer the position of the electrode site to the midline or axis of the skull. EEG electrodes are placed on the scalp at 10% and 20% of a measured distance. If a measurement was made around your skull, with a tape measure, (i.e. - a hat size measurement) a circumference distance would be determined. In the normal adult this measurement is approximately 55-60 cm.
For example 10% of this measurement or 5.5 or 6.0 cm would be used to determine precise locations around the skull. Holding the tape measure around the skull, a grease pencil mark would be placed every 6 cm, from a landmark starting point to determine electrode locations. An anatomic landmark is needed to know where to start with the measurements.
The "10" and "20" refer to the 10% or 20% inter-electrode distance. Why are percentages used? The skull may be different from patient to patient. A pediatric or adolescent head size is smaller than an adult. A traumatic accident to the skull may have occurred in the patients history creating an out of proportion condition. How about congenital abnormalities to the skull and scalp?
A percentage relationship measured on the skull surface will effectively locate the internal brain lobes. The internal brain lobes are labeled, for the most part, based on the first letter of the lobe, although 'C' is used and there isn't a central lobe of the brain.
Skull dimensions are measured accordingly in centimeters and then electrode site distances are converted with the 10% and 20% factors. Fifty percent (50%) is used frequently, but is a composite of 10%, 20%, & 20%. Remember, once a landmark is located, it should be used consistently and regularly to locate electrode sites.
Review What does the "International 10-20 Electrode Placement System" help to create?
An important feature of the international 10-20 system is that a patient tested in one facility, will be recorded accurately and precisely, the same as, in another totally different facility, providing the 10-20 system is used. The following measurements are required, and should be used religiously. Practice is the Key! Short cuts should be avoided, due to the variety of patients. Clinically, a practitioner would need to do 10-50 patients to feel comfortable with the measurement system. Each electrode position and anatomic location, and the rationale behind the measurements will then be evident. A short period of time or absence from using the measuring system, will often cause confusion, and send you back to this lesson.
By the way, it's always better to write down the distances and percentages when first starting. Good Luck !! The following topics are covered in this lesson, and should be completed in this order.
The Anterior to Posterior Measurement The system listed below is a "modified" 10-20 placement system for sleep. The use of this system is to determine the location of electrodes, in relationship to the cortical areas, and to reliably and precisely use 'anatomic landmarks' to locate these electrode positions from individual to individual. Use a china marker and
measuring tape. Be sure to make your
marks
perpendicular
to the
direction of your tape measure. Anterior to Posterior Measurement
Record your total (for this example use - 36
cm)
This will be the 'first' mark of the
central electrode, abbreviated as
Cz or C.
Measure 20% forward from the mark
of Cz to mark a 'Frontal' bone electrode site
( F )
; And now measure an additional 20% forward to mark the Frontal Pole "zero" ( Fpz ) Measure 20% back from the mark of Cz to locate the Parietal Pole ( P), Then an additional 20% back to mark the Occipital location "O"
Note: the
'O' location
should not be confused with the Inion
location.
2nd Check point: Horizontal
china marks will be on the forehead at
10%
up
Measure from
the Tragus of the ear - Pre-auricular point of an ear to Pre-auricular
point of the other ear and record your total.
(This sample - 40 cm) Make an anterior to posterior mark at 50% of the total. (20 cm). Notice that this is the second mark of Cz and you should visualize a plus sign. "
+
" From Cz; Measure 20% (8 cm) in each direction towards the original pre auricular point. Make an anterior - posterior mark on each hemisphere of the skull. This will be the first mark of C3 (left side), and C4 (right side) of the skull. From this point make an additional mark 20% towards the pre auricular point. T3 (left side), and T4 (right side)
Note: Normally these
markings or measurements will be identical on each hemisphere. On occasion
the anatomical structure of the head and skull may prove otherwise.
Check point: Five marks are now on the pre-auricular axis. Two anterior - posterior marks on each hemisphere, C3 and C4 and one mark each at 10% up from the tragus, T3 and T4 Other Marks Make a plus on the forehead as follows...... As you look at the patient , there should be a horizontal mark on the forehead. It is the 'Fp' line which is 10% up from the previous A- P measurements. Make a cross or plus ( + ) at the 10% line, up from the Nasion. From behind the patient, tilt the head backward, and place a vertical mark,in line with the center vertical axis of the nose, to visulize a " +" on the forehead. This will be your true FPz. {The only non measured mark}
Circumferential Measurement Notice: to make an accurate "hat size" measurement, wrap the tape measure, horizontally around the head, assuring that the tape is aligned with the previous horizontal marks. Hold the tape measure, on the forehead, at Fpz and wrap the tape towards the ear through the 'T' electrode site, then through the 'O' electrode site, then continue the same way around the other side of the head until you again reach Fpz
{Fpz, T, O, T and back to Fpz} If these landmarks are not used, the circumferential measurement may be incorrect or appear to be tilted. The tape measure should visually appear level around the head. If it does not, then perhaps one of the previous measurements were incorrect. Remember to use a consistent marking technique, either above or below the tape measure. Record your total circumference. (example, 60 cm) Use one half of the total to mark the center of the back of skull. (example 30cm) Measure from Fpz to "O" and make a vertical mark through the horizontal mark at "O". This is your true Oz and a " + " will be complete.
Final Frontal pole and Occipital Locations
At this point
you will note that Fpz and Oz are marked with vertical lines and represent
the front and rear center of the skull. Ten sites are used for EEG, therefore the distance between FP1 and Fp2 would be 10%. The distance between O1 and O2 = 10%. These four (4) sites are used for sleep studies.
If the skull
circumference is 60 cm, then 10% = 6.0cm. Fpz is the
center of the frontal bone, therefore a mark 5% on either side would =
10%. Extend the horizontal mark through the vertical marks to show three " + " marks. Repeat this procedure for occipital sites O1 and O2. Make vertical marks 5% (3 cm) on each side of Oz. Extend the horizontal mark to show three " + " marks.
Check Point: Look at the markings you have
completed so far. Cz, Fpz, FP1, FP2, Oz, O1, and O2. Final "Central" Measurements
To mark the final cross sections ( "
+ "
) for C3 (and C4)...... through the first central mark on the left side,
then back to O1 and record your total. (example, 25 cm). (You are
measuring the distance of 10% up from the front to 10% up from the inion.) Half of the
total (i.e., 12.5 cm) is where the first mark of C3 intersects with this
second mark. This will be your
true C3. Measure from FP2 through the first central mark back to O2 and record your total. Half of the total, is where the first mark of C4 intersects with the second mark, will be your true C4.
The modified 10-20 system for sleep is completed and all sleep montage sites are " + ".
The exam for this lesson and a lesson evaluation must be completed for credit to be awarded! ||
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placement
of electrodes was developed. 
Measure
from the Nasion (the bridge of the nose)
Note: