EEG with Extracranial Foramen Ovale Electrodes
An EEG with Extracranial Foramen Ovale electrodes (EFOs) can be used to find out what part
of your brain your seizures are coming from. EFOs are special EEG wires that are put through
the skin to record brain waves. The purpose of this handout is to help explain what will happen
before, during, and after the test.
What is an EFO?
An EFO electrode is a very thin wire about the
thickness of a hair that goes through the skin
and is closer to your brain. It records more
information than a scalp electrode. EFOs are
placed through the skin as a pair. One will be
on the left side of the face and one will be on
the right side of the face. The tip of each
electrode is then guided towards the bottom of
the skull near a small opening called the
foramen ovale. The electrode does not go into
your brain. Although EFOs are more sensitive
to brain waves than scalp electrodes, you will
still need both kinds to help find the seizure
focus. Both types of electrodes will be placed
on the day you are admitted. The EFOs will be
put in first. This takes place in an x-ray room.
How are EFOs Placed?
The electrode wire is threaded into a
hollow, thin needle. This needle will
carry the wire to its target at the bottom
of the skull. The point of skin entry is in
front of the ear. In the picture, the ear
has been covered up by a sterile towel.
The needle shaft can be seen entering the
skin at the arrow point. The wire is so
fine that it cannot be seen. The doctor
then pushes the needle along with the
wire toward the target. The target is a
small opening in the skull base called the
foramen ovale. It lies about 2 inches in
from the surface of the skin. Once both
EFOs are in place, the needles are taken out,
and you return to your hospital room.
We have done this test for hundreds of
patients and have had no complications yet.
This does not mean that there are no risks. It
means that the risks are low, but not zero.
The doctor will discuss these risks with you.
Before The Electrodes Are Placed
When the date of your test is known, you
will be asked to have labs drawn so that the
neurosurgeon knows that you are safe to
have the test. Do not eat breakfast or wear
make-up on the day of admission. Do not
take aspirin or ibuprofen-like products
for 1 week before the procedure. Please
be in Admissions by 7:30 a.m. You will go
to your hospital room. Later, you will be
taken to x-ray. When you arrive in the x-ray
room, the neurosurgeon will explain how the
wires are placed, discuss what the risks and
benefits are, and ask you to sign a consent
If you are reading this Health Facts for You
as a parent of a minor or guardian of the
person who is having this test, please be
prepared to go with your child or dependent
adult to x-ray where you will be asked to
sign the consent form. When you sign this
form, it means that you agree to have the
electrodes put in.
During the Placement
You will be helped onto the x-ray table and
asked to lie down. The x-ray camera will be
positioned over your head. The doctor will
wash your face in front of your ears with
iodine and mark a spot for the wires to go in.
The doctor will numb the skin on your
cheeks with local anesthesia. Using the
x-ray pictures as a guide, the wire (mounted
on the carrier needle) will be pushed into
place. The needle will then be taken out and
the wire taped to your cheek. Although we
use local anesthesia, there may still be pain
when the needle goes in. The doctor will
tell you when to expect it. The pain quickly
fades after the electrodes are in.
After the Placement
When both the left and the right EFO
electrodes are in place, x-rays will be taken
for your record. You will then be taken to
your hospital room where scalp electrodes
will be attached and all the wires will be
hooked up to the EEG machine. Feel free to
ask for Tylenol® if you have any pain from
the EFOs. Report any swelling, redness, or
numbness to your nurse right away. The
EFOs will stay in until you are ready to go
home. An EEG technologist will then
remove the electrode wires in your room.
What if the EFO Electrodes Don't Reveal Where the Seizures are Coming From?
Should this happen, your doctor may wish to discuss setting up a second test using other types of
electrodes. The reason we begin with EFO electrodes is because they are tolerated the best and
Your health care team may have given you this information as part of your care. If so, please use it and call if you
have any questions. If this information was not given to you as part of your care, please check with your doctor. This
is not medical advice. This is not to be used for diagnosis or treatment of any medical condition. Because each
person’s health needs are different, you should talk with your doctor or others on your health care team when using
this information. If you have an emergency, please call 911. Copyright © 6/2016 University of Wisconsin Hospitals
and Clinics Authority. All rights reserved. Produced by the Department of Nursing HF#4271