00:01
When we're evaluating a new
onset patient with seizures,
or a patient with epilepsy,
critical to our
evaluation is the EEG,
or the electroencephalography.
00:11
This is a test to monitor the
electrical sensitivity of the brain
and thereby detect disorders
that may be arising from abnormal
electrical activity, a seizure.
00:22
We can use three types of EEGs to
evaluate patients with seizures.
00:27
We can use surface electrodes,
those are electrodes that are put out
on the surface of the head on the skin,
and they measure the electrical
activity outside of the brain,
outside of the skull,
the EEG activity within the brain
is really high in amplitude,
and as that activity moves
through the surfaces,
excuse me,
the tissue of the brain,
the skull that's around the brain
surface and the scalp that is dampened
and so the surface electrode is
a good window into the brain.
00:54
But there are some things going on that
we can't see with surface electrodes.
00:58
Sometimes then we'll
use cortical electrodes.
01:00
These are placed at the time of a
surgery when a patient has a craniotomy
to take off the skull.
01:06
Electrodes are placed on to
the surface of the brain.
01:10
This is a very sensitive
way to evaluate seizures
and precise foci of seizures
right on the surface of the brain.
01:17
But even then,
sometimes we can't detect a seizure
that may have a very deep focus.
01:22
And so the third type of EEG,
electrode,
we can use our depth electrodes.
01:26
These are inserted
deep into the brain
and they detect the deeper foci,
seizures that originating
from deep foci
within central
structures in the brain.
01:36
By far and away the most
common EEG is the scalp EEG
and we'll use cortical electrodes and
depth electrodes in selected cases
to really evaluate and interrogate
a specific seizure focus
prior to considering
a seizure surgery.
01:49
So what is EEG?
Let's take a look at it.
01:51
How does it work?
What are we looking at?
And how do we use it clinically,
and in vignettes to evaluate patients.
01:57
Well, this is what
the EEG looks like.
01:59
It's a bunch of squiggly
lines on a page.
02:02
And we can break those down
to look into certain areas of the brain
and see what's happening electrically.
02:09
When we look at the
numbers to the left,
the odd numbers are the
left side of the head.
02:14
So the ones, threes,
fives and sevens come from the left,
electrodes on the left side.
02:19
The even numbers of the
right side of the head,
the two's, fours,
sixes and eights
come from electrodes that are
on the right side of the head.
02:27
The left,
is the first left chain,
the first left lines are
the parasagittal chain,
you can see right along the
mid-line on the left side.
02:38
The next few four lines are
the right parasagittal chain,
that's that line
on the right side,
immediately adjacent
to the central sulcus.
02:48
Down below the next four lines
are the electrodes in
the left temporal chain.
02:52
This is looking at the temporal
cortices of the left side of the brain,
and then the right
temporal chain.
02:57
And then the very center,
the central sulcus is
the Z band or the Z line,
which is typically at the
very bottom of the EEG.
03:04
We can change how the electrodes
and how the lines are displayed,
but this is the most common
conventional way to evaluate an EEG.
03:12
And when we're
looking at the brain,
we start by looking at the front
and you can see the F for front.
03:18
And then we move back to
the back of the brain,
the frontal part of the brain,
the parietal part of the brain,
the occipital part of the brain,
and the temporal part of the brain,
we're looking at all of
the areas of the brain.
03:29
This is what a
seizure looks like.
03:32
So here we're looking
at normal activity
at the very beginning
of this EEG tracing,
and we start to see highly
synchronized discharges
coming from the
left temporal chain,
about midway through the recording,
and you can see that here,
the EEG begins very
scattered and disorganized,
there's low amplitude,
irregular activity,
which is normal of the brain.
03:56
All the neurons are firing
in many different directions.
03:59
And as a result,
that activity cancels out,
the amplitude is low,
and the pattern is irregular.
04:05
A seizure is a highly
synchronized area of brain.
04:08
When all the neurons are firing
in the same way at the same time.
04:13
And that shows up
as a discharge.
04:15
And you can see here
that driving discharge
beginning in the left temporal
chain early in the EEG.
04:21
That then spreads and we see in
the latter aspect of this EEG
spreading to the left
parasagittal chain,
as well as to the right
sides of the brain,
those even numbered EEGs.
04:33
If we move one slide forward,
now we see activity
all over the brain,
highly synchronized
spikes and slow waves,
that clonic phase where the
patient is jerking of this seizure.
04:45
And then if we move
further forward,
we see that there's
an abrupt cessation
and seizures are characterized
by abrupt cessation.
04:53
You can see all this motor activity
this really dark motor activity ceases.
04:57
And there is really low amplitude,
very quiet EEG after the seizure,
and that is that period
of post-ictal depression.
05:05
We see that in patients and this
is what it looks like on the EEG.
05:10
So in general,
when we're looking at the EEG
at each of those squiggly lines,
we're looking for
different frequencies.
05:17
When we look at the
brain at the EEG,
there are basically four
frequencies we can think about.
05:21
The first is the beta frequency.
05:23
This is a very high hertz
frequency greater than 13 hertz,
it's present in the
EEG of healthy people.
05:30
It can be caused by medications
like benzodiazepines,
or indicate muscle activity
or muscle artifact.
05:36
And when we were looking at that
seizure, at the very end,
we saw this very
dark, high amplitude,
very fast high
frequency activity.
05:44
And that was muscle activity
from the patient's jerking,
that clonic phase, the end of
the clonic phase of the seizure,
and that was beta activity.
05:53
Moving down in frequency,
alpha frequency activity typically
is in the range of eight to 13 hertz,
that's the frequency
with which you see it.
06:01
That's the number of bumps
that you count in between
one second on the EEG, and you can
see here about eight to 13 hertz
is in the alpha frequency.
06:11
This is the normal post your activity
in a relaxed and awake person.
06:15
And it's present when
their eyes are closed,
and it goes away when
they open their eyes
or when they're attending to something
or when they're under stress.
06:22
So when we're sitting with
our eyes closed, relaxed,
what we see on the
brain is alpha activity,
the posterior dominant rhythm,
the dominant relaxed
with rhythm in the brain.
06:32
As the brain calms are quiet,
so we see even slower activity.
06:37
Theta and Delta frequency
activity are slower waves.
06:41
The theta waves as you can see,
here, it's about four to seven hertz
or four to seven waves bumps
within that one
second of EEG tracing,
and delta is less than four
hertz in its frequency.
06:52
These slow waves
are seen in sleep,
and they're normal
in slow wave sleep,
they mean the brain is calm as
quiet as turned down as sleeping,
or in certain
pathologic conditions,
like patients who are
medicated who are sedated
on benzodiazepines,
or barbiturates,
or have another cause of
coma or encephalopathy.
07:14
So what's happening in the
brain when we see a seizure,
what's happening on
the EEG that we see?
Well, there are several different
patterns that we can see.
07:22
In between a seizure we can
see an interactional spike,
and that's what you see on
the surface EEG tracing here.
07:28
This is a highly
synchronized one single spike
that indicates an area of
potential seizure onset.
07:36
It doesn't mean a seizure,
but it means there's a reduced
frequency for this area of the brain
to develop into a seizure,
we can also see a silent period,
typically right after a seizure,
we may see an increased
frequency and interactive spikes.
07:50
But in between seizures,
the brain may be silent, and we may
not see that inter-ictal activity.
07:56
Ultimately, during a seizure in
the ictal portion of a seizure,
we see spikes as well as
spike and wave activity.
08:03
And that corresponds to what's going
on electro-chemically in the brain.
08:07
There, those areas of repeated
paroxysmal depolarization shifts,
and then co-opting of
the surrounding neurons
to get involved in
that seizure activity.
08:16
After the seizure,
we see post-ictal depression
and that's a very calm,
flat, quiet brain,
there's very reduced amplitude,
low amplitude activity,
and really not a lot
of high frequency,
the brain typically
appears very slow,
with slow frequency waves
during that period of time.
08:33
And you can see correspondingly at the
bottom of the slide what's happening
within each individual cell.
08:38
Again, the surface EEG is measuring
what's going on in many cells
and intracellularly, you can see what's
happening at a neuron specific basis.