EEG

Far from a normal variant, this is a highly abnormal tracing from a patient with Lennox Gastaut Syndrome.
Don't mistake the marked box for 14 and 6 positive spikes; even though they are indeed about 14 Hz, they are in the wrong area (14 & 6 spikes tend to be posterior) and come with too many other surrounding abnormalities including a highly disorganized background with multifocal phase reversing epileptiform discharges.

This page has periods of both bilateral and independent unilateral parasagittal arch-like, sharply contoured alpha activity maximal over the centroparietal region. This is consistent with mu rhythm, the idling activity of the sensorimotor cortex. Note that while some periods of mu are better formed than others, the activity never evolves or really changes in appearance.

While this activity might look kind of similar to mu, it is off in the wrong distribution--this is in the right temporal region--and has too broad a field, moving into the right parasagittal chain. Mu does not do either. You may also consider wickets, but this morphology does not have the arch-like activity of wickets, and is too prolonged; also, again, it has a disruptive field into the parasagittal chain that wickets typically don't. We'll look at this full seizure, focusing on its evolution in time and location, in the seizures section.

This tracing is marked by a mildly slow PDR of 6-7 Hz, along with a notably less organized AP gradient. Much of the usual architecture of a normal study is here though, including eye blinks and variability across the page. We see some excess theta admixed into the normal and expected alpha activity, but no major delta activity. The area marked by a purple arrow may appear to be frontal delta activity, but these are in fact epileptiform spike and slow waves (note the notch-like spike before each large slow wave), discussed in the epileptiform activity section.

This tracing shows persistent high amplitude delta slowing over the left hemisphere. Given the high amplitude particularly over the parasagittal chain, this may represent not just slowing but breach, which is seen after brain surgery and discussed more below. Note also the periods of mu rhythm--the resting alpha frequency of the sensorimotor cortex that is arch-like in morphology, over the right parasagittal chain; mu rhythm goes away with thoughts of or initiation of movement.

This tracing shows intermittent runs of approximately 3 Hz delta activity over the left temporal region, more prominent in the anterior than posterior temporal region. Because these intervals of 3Hz delta last at least 2 seconds, they complete the 6 cycle requirement for rhythmic delta activity. Note that the delta is very apparent in the lateral temporal leads below the traditional parasagittal and temporal chains--a reminder to always keep an eye on all of your available chains on EEG.

Recall that when assessing background you should look for a PDR, AP gradient, synchrony, symmetry, continuity, reactivity, and clear wave morphology. On this page, we have a continuous tracing that is synchronous and symmetric. However, there are only fragments of a PDR on both sides, and when present it is no faster than 5 Hz. Thus we have at least mild generalized slowing here. However, there is also no AP gradient--realistically, the page would look much the same if we flipped it upside down. Furthermore, we don't have much alpha activity, with the page mostly theta and delta activity that is relatively monotonous in appearance from beginning to end. Overall, then, this would be considered moderate generalized slowing.

This is a complicated tracing so let's look at one side at a time. First note that on the left side you have a clear PDR of 8-8.5 Hz, which is in the normal range. However, the AP gradient on the left is poor, with excess slow activity in the frontal regions. Note also that the F7 electrode is bad (note how Fp1-F7 and F7-T3 move in exact opposition to each other, a sign of a bad F7 electrode) and Fp1 and F3 are too close to each other (they've formed a salt bridge, seeing the same voltage as each other and thus cancelling each other out on bipolar, leading to a flat line; on an average montage, they'd have identical appearing lines).

Moving to the right side, we see a persistent / continuous, somewhat low amplitude delta activity in the frontal regions, and fragments of a PDR in the occipital leads. There is even less of an AP gradient and normal morphology than on the left side. As such, we say this tracing has persistent right hemispheric polymorphic delta slowing on a background of mild generalized slowing.

This tracing shows a burst of generalized high amplitude delta activity at about 3Hz, lasting several seconds and thus meeting the ACNS criteria for rhythmicity (aka requires 6 cycles). The delta activity is generalized but more prominent on the right side, particularly in the temporal chains. We would thus classify this as generalized intermittent rhythmic delta activity (GIRDA) with right-sided predominance. Recall that GIRDA is similar to FIRDA prognostically in that it suggests nonspecific and diffuse cerebral dysfunction.

Here we see a good PDR throughout of 10 Hz, but note while the right temporal region is alpha and beta activity, the left temporal region has a kind of "sloppier" appearance with admixed slower frequencies from theta and delta. In a normal awake adult, you should basically never see theta or delta activity. The slowing here is not rhythmic and seems to run across the entire page, so we'd classify it as focal left temporal persistent polymorphic theta to delta slowing.

Here we see continuous polymorphic delta slowing involving essentially the whole left parasagittal chain, as well as more subtly the left frontotemporal region. Also of note here are several negative phase reversals over C3 with spike and slow wave morphology, consistent with epileptiform discharges.

This tracing shows diffuse theta activity with a slow PDR and relatively poor AP gradient, consistent with mild generalized slowing. however, there is also a polymorphic focal delta slowing over the right temporal region. Especially when you're starting to read EEGs, this sort of slowing-on-slowing can be hard to see.

Starting with the background here, we see a relatively poor AP gradient and slow, often poorly formed PDR. This is consistent with mild or even the beginnings of moderate generalized slowing. We also see, in the first half of the page, higher amplitude and somewhat rhythmic delta activity in the frontal regions that goes on for at least 6 cycles, consistent with FIRDA albeit not the cleanest example as such.

This tracing has a normal background with a PDR of about 12 Hz. The right temporal region, however, has higher and more sharply contoured morphology than does the left temporal or other regions, consistent with breach artifact as is seen after brain surgery due to the residual skull defects. Within the breach rhythm we see very nicely formed wickets, a benign finding in the temporal lobes that are marked by arch-like, phase reversing waves that come in runs. They should not be mistaken for epileptiform discharges, and are often more easily seen within breach.

This bipolar tracing shows a phase reversing spike and slow wave in the right anterior temporal region. Note the phase reversal (the point at which two tracings "point to one another") between Fp2-F8 and F8-T4, but also at F8-T2 and T2-T4. This puts the discharge between F8 and T2, in the anterior temporal region. Other notable findings on this tracing are a bad T3 electrode, seen as repetitive fast artifact over the T3 leads, and several low amplitude sleep spindles suggesting this is in stage II sleep.

This tracing shows a clear spike and wave discharge at Fp2-F8. However, despite this being a bipolar montage there is no phase reversal. This is a good example of the end of chain issue, where there is nothing anterior to Fp2 to which to compare its voltage. It is possible that the maximal discharge, where the phase reversal would be, is actually anterior to or deep to Fp2, but because there isn't an electrode there to measure it, we can't see the full extend of what would be the phase reversal on scalp EEG. Also note the good field in the right frontocentral region, with some reflection to the left frontal/frontotemporal region.

This tracing shows intermittent runs of approximately 3 Hz delta activity over the left temporal region, more prominent in the anterior than posterior temporal region. Because these intervals of 3Hz delta last at least 2 seconds, they complete the 6 cycle requirement for rhythmic delta activity. Note that the delta is very apparent in the lateral temporal leads below the traditional parasagittal and temporal chains--a reminder to always keep an eye on all of your available chains on EEG.

This is a complicated tracing. First note the state of the patient, who appears to be in slow wave sleep (relatively synchronized high amplitude delta activity diffusely, no eye blinks or other evidence of being awake). There are multiple epileptiform discharges on the page, including a run of right frontal paroxysmal fast activity, right frontal spike and waves (Fp1 max), right mid-temporal sharps (T6 max, although poorly formed), and left temporal spike and waves. Note the field of the right temporal sharps into the right parasagittal chain. This kind of tracing--disorganized with multifocal discharges--is typical for Lennox Gastaut Syndrome.

Here we see a three-phased generalized discharge with slight right hemispheric and bifrontal predominance, with a slight anterior to posterior lag in onset and in which each each phase is slightly longer than the preceding one. These are all hallmarks of triphasic waves, which are not epileptic in nature and are most often seen with toxic-metabolic encephalopathy.

Here we have multiple, very well formed phase reversing spike and slow wave discharges that point toward T4 and C4, with slightly better formed morphology over the temporal than central region. These would thus be right centrotemporal spikes. Note the field into the left central region, and also that in the right parasagittal chain, the frequent negative phase reversals can appear somewhat similar to electrode artifact, but you don't see evidence of such artifact elsewhere on the page.

First off, note that this tracing has mild generalized slowing, with a PDR not exceeding 7.5 Hz. There are multiple relatively high amplitude spike and slow waves in the bilateral occipital regions, fairly symmetric in amplitude, with a field extending anteriorly throughout the parasagittal, temporal and central chains.

This tracing shows left temporal lateralized periodic discharges (LPDs), also called periodic lateralized epileptiform discharges (PLEDs). This is an epileptogenic pattern commonly, but not exclusively, seen with HSV encephalitis. Recall that for both periodic and rhythmic patterns, a cycle of at least 6 is required by ACNS classification. Note the field of the discharges into the left central regions, and slightly into the posterior right central region, but the clear area of maximal amplitude is the left temporal region.

This page shows an average tracing, so recall that you aren't looking for a phase reversal here, just for the point of maximal amplitude for a sharp, spike, or spike wave discharge. As such, F3 has a clear spike and slow wave as marked, with a more subtle field from that spike throughout the rest of the electrodes. Average montages are useful to determine the point of maximal amplitude for a discharge if a phase reversal is somehow unclear.

This was kind of a trick question (sorry?), as these discharges are, actually, not localizable. They are generalized discharges, albeit with some anterior/frontal predominance, which is very common in generalized epilepsies. Even though the discharges are much higher amplitude frontally, you can see the field extend with clear (much lower amplitude) spikes all the way back in the occipital regions. The discharges have a spike or polyspike prior to each slow wave, and those spikes are fairly (though not completely) symmetric in morphology and amplitude.

This awake tracing (note the eye blink around the middle of the page, and frontal muscle artifact in the second half of the page), shows a clear phase reversing sharp maximal at T4, with a field into the right central region.

This is not a seizure, as it does not evolve either temporally or spatially. It starts with 3 Hz spike wave activity that is maximal temporally with a field into the parasagittal chain, and it ends that same exact way. It also appears to be slightly less than the ten second requirement for seizure, although there may be some subtle lead-in in the first few seconds of the page so that is debatable. The lack of evolution, however, is clear and thus this is just an interictal run.

This seizure begins with a herald slow wave in the right frontotemporal region, which leads into subtle rhythmic delta of the right temporal region that evolves in amplitude and morphology, and within a few seconds is quite prominent with a field / spread into the right frontocentral region. Shortly thereafter, there is some reflection into the left frontotemporal region.

This is a subclinical seizure that may at first appear generalized in onset, but if you look closely you can see a left centrotemporal spike (note the clear positive spike temporally and prominent phase reversal centrally) immediately before onset of generalized fast activity leading into sharply contoured 6-7 Hz activity that progresses to very high amplitude generalized, frontally predominant spike wave activity at about 1 Hz, which on the second page slows into a rather abrupt offset.

This seizure begins with a generalized albeit somewhat fragmented generalized spike and wave discharge (note that the discharge is seen in all the chains with similar morphology in them all), with subsequent slowing for several seconds before onset of generalized attenuation with overriding fast activity, the hallmark appearance of a tonic seizure.

This seizure starts with a second of rhythmic spikes best seen over the left frontocentral regions, but with simultaneous albeit less well formed spikes over the left anterior temporal region. With the frontal regions of both the parasagittal and temporal chains being involved, we'd call this a left anterior quadrant onset seizure. Note that this seizure continues to evolve via higher amplitude spikes waves that increase in frequency for several seconds, then slow prior to offset; this seizure remains focal to the left hemisphere, but there's associated muscle artifact over the right hemisphere.

This tracing shows about seven seconds of bifrontal rhythmic activity. Note, however, that while there might appear to be evolution here, the change is very localized to the frontal leads, and their morphology is not consistent with typical epileptiform spike wave activity. This is actually a pretty classic example of eye flutter. Also remember that seizures require a least ten seconds of evolution if there is no clinical correlate, so even if this were epileptifrom in nature, it still wouldn't be categorized as a seizure unless there was a clinical correlate.

This seizure is not localizable, as it is generalized. This particular seizure was subclinical, with no outward clinical signs, but electrographically is marked by a sudden onset of generalized spike and slow waves (with frontal predominance) that continue at about 2Hz for 12 seconds, before slowing suddenly with an abrupt offset, and no clear postictal changes that we can see in the last second of the page.

Here we see prominent left posterior quadrant spike and slow wave discharges, at about 2 Hz and with a prominent field into the right posterior region. However, they only last for 7.5 seconds and are non evolving, and thus do not fit criteria for a seizure. Recall that technically seizures require evolution, and should be longer than 10 seconds. There are, of course, exceptions to this rule; myoclonic jerks, absence seizures, and tonic seizures, for example, are often less than ten seconds long but are still considered seizures as long as there is a consistent clinical correlate (seen on video) to electrographic epileptiform activity.

Here we see a tonic clonic seizure that might at first look rather generalized in onset, but note the half a second of sharply contoured fast activity over the left hemisphere (in red boxes) prior to the onset of similar activity in the right hemisphere (in purple boxes). So while this seizure has very rapid generalization, it does arise from the left hemisphere.
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Note also that in terms of evolution, we see that fast spike activity on the left, then right side, then diffuse myogenic artifact from which arises several seconds of diffuse slowing and attenuation, followed by generalized polyspike and waves with overriding myogenic artifact, which slow from 1-2Hz to <1Hz prior to offset.