Seizures: Evaluation and Functionality of the EEG

by Roy Strowd, MD

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    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.

    About the Lecture

    The lecture Seizures: Evaluation and Functionality of the EEG by Roy Strowd, MD is from the course Seizures and Epilepsy.

    Included Quiz Questions

    1. Depth electrodes offer the best resolution to identify a seizure focus deep within the brain.
    2. Cortical electrode signals are weaker and less accurate than surface electrode signals.
    3. Depth electrodes are the most commonly used EEG electrodes.
    4. Cortical electrodes are placed underneath the skin but outside the skull.
    5. Surface electrodes offer the most accurate identification of the seizure focus.
    1. Odd numbers are used to signify the left side of the brain.
    2. The first four lines signify the right parasagittal chain.
    3. The “Z” chain represents lateral temporal lobes.
    4. By convention, the posterior leads are listed first.
    5. All electrodes show equal activity when a seizure begins.
    1. Beta waves are seen in healthy individuals.
    2. Alpha waves are seen when an individual is attentive.
    3. Theta waves are normal in an adult who is awake.
    4. Delta waves have a frequency of < 4-7 Hz.
    5. Muscle activity will not alter the waveform.
    1. A seizure will appear as a synchronized pattern within a centralized area.
    2. The amplitude of waves during normal brain activity is high.
    3. The end of a seizure is determined based on the gradual cessation of electrical activity.
    4. Neurons fire at random times and locations during a seizure.
    5. Electrical activity remains high during the postictal phase.

    Author of lecture Seizures: Evaluation and Functionality of the EEG

     Roy Strowd, MD

    Roy Strowd, MD

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