How do you measure?
As the saying goes - all roads lead to Rome!
However you get there though - one thing is for sure:
The end result should be the same.
The ACNS guidelines state the following:
All 21 electrodes and placements recommended by the International Federation of Clinical Neurophysiology should be used. The 10-20 System is the only one officially recommended by the IFCN.
For the full guideline: click here.
A quick search on YouTube and you’ll find multiple examples; however, whichever method you choose:
Accuracy is important.
And skipping the measurement for compliant patients is not considered the standard of care; as well as, is outside the guidelines for this medical procedure.
Here's a quick road map:
1) Find your patient’s nasion, inion and pre-auricular points:
2) Using centimeters measure from the nasion to the inion.
3) Calculate 10% and 20% (quick tip: move the decimal over for 10% and double that for 20%). For example, a measurement of 35 cm becomes 3.5 cm for the 10% mark and then 7.0 cm for the 20% mark.
4) Do the same thing from the pre-auricular to pre-auricular - now you have your first cross at CZ.
5) Next is the circumference and frankly, the most challenging part of the measurement. Measure around the circumference of the head - each location will be 10% from each other. However, you have to start somewhere: using the nose as the midline make a cross on the forehead and put 1/2 of 10% on one side (5%) for Fp1 and 1/2 on the other for Fp2.
6) Then continue around using 10% for all of your marks. Be sure to make them intersect with the marks made in Step 1 and 2. Also use those earlier step marks as a guide to cross F7/F8 and T5/T6.
7) The last 4 steps all use 25% - measure from Fp1 to O1 (using the C3 location as a guide). Repeat on the other side.
8) Now the only remaining locations that do not have a cross are F3, FZ, and F4 in the front and P3, PZ, and P4 in the back.
9) Referencing the same photo and still using 25% - measure from F7 thru FZ to F8. Half the measurement at FZ and then cross 25% the distance for F3 and F4. Repeat the same steps for T5 thru PZ to T6.
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Roya Tompkins, MS, REEG/EP T, RPSGT