A seizure is an abnormal, organized and evolving burst of cortical activity that interrupts the brain's usual function. Clinically, they can present as anything the brain can experience, including sensorimotor activity, emotion, autonomic changes and more.
Electrographically, like interictal activity, seizures (aka ictal activity) can be either generalized or focal but either way seizures must evolve temporally and/or spatially. Interictal activity can fluctuate and become very frequent, but if it never evolves into a broader area or its frequency doesn't become progressively faster or slower, it is not considered a seizure.
Especially in ICU patients, you may see patterns that are disturbing for seizures, such as fluctuating rhythmicity or periodicity, but if it does not consistently evolve (fluctuation is not evolution), it is not a seizure. Also remember the ten second rule: bursts of interictal activity that evolve but last less than 10 seconds in duration are not seizures (they could be classified as B(I)RDs--brief (ictal) rhythmic discharges--but that term is somewhat inconsistent in use throughout the EEG community).
Generalized seizures are marked by a nonlocalizable onset, often involving all the leads but sometimes having a predominance in the anterior or other regions. As with nearly any seizure type, the exact appearance varies widely, but must evolve in frequency (ex. discharges becoming faster until offset) or distribution (ex. spread from one region to another).
Generalized seizures are most often seen with idiopathic or symptomatic generalized epilepsy syndromes; subtypes of generalized seizures are discussed below and include generalized tonic clonic (GTCs, previously termed grand mal), absence, tonic, myoclonic, and atonic seizures. Any time you see a generalized seizure, look closely to ensure that it isn't actually a focal to bilateral tonic clonic seizure, in which the onset is localized but there is rapid propagation/generalization into both hemispheres.
On the example below, note that it evolves across time, starting with sudden onset generalized rhythmic theta activity for about a half second, then high amplitude spike wave activity at ~3Hz, which slows to about 2Hz and then slows further to 1.5Hz before offset towards the end of the page. It also lasts nearly 12 seconds, meeting the criteria for a seizure rather than a BIRD.
Because consciousness requires at least one cerebral hemisphere to be active along with the brainstem reticular activating system, and generalized seizures cause bilateral hemispheric dysfunction, all generalized seizures lead to loss of consciousness, even if only for a moment. This is particularly important for generalized subclinical seizures, in which there is no outward evidence of a seizure but a patient can have internal interruption of consciousness, leading to memory and attentional issues (note that subclinical seizures can be either focal or generalized).
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.
Focal seizures have a single area from which they originate. They can be categorized into focal aware, focal impaired aware, or focal to bilateral tonic clonic (previous terminology deemed these simple partial, complex partial, and secondarily generalized seizures, respectively).
In focal aware seizures the patient retains consciousness throughout the seizure; for example, they can continue talking while their left arm has seizure activity. Because of this retained consciousness, they can remember the full extent of the seizure from beginning to end. On the other hand, focal impaired aware / unaware seizures cause the patient to lose consciousness, and thus they do not recall the seizure afterward. Both types of focal seizures can involve motor activity or not.
If a focal seizure, be it focal aware or focal unaware, spreads to the contralateral hemisphere, it becomes a focal to bilateral tonic clonic seizure, and looks clinically like a generalized tonic clonic seizure at that point. It is important for such seizures to pay attention to the early clinical signs and electrographically trace them back to the beginning to find their focus, and not just assume they are generalized from the start.
In the example below (seen across two pages), note the onset of sharply contoured alpha/theta activity first in the left mid-temporal region with a field into the left central regions; this activity then proceeds into the next page where it becomes more frankly rhythmic, and then evolves further into 3Hz left temporal and left central spike and slow wave activity, which continues till the end of the page when it rather abruptly devolves back into the background, and the seizure ends. Note that this seizure never spreads into the right hemisphere.
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.
Tonic clonic seizures are the prototypical seizure, previously termed "grand mal." They can be either generalized tonic clonic (GTC) or focal evolving to bilateral tonic clonic. They are marked, as their name suggests, by an initial period of tonic motor activity that clinically appears as stiffening and on EEG appears as significant myogenic artifact; as the neurons use up their ATP stores with so much continuous activity, however, the tracing will evolve into intermittent bursts of spike/polyspike and slow waves, with each spike burst corresponding to the clonic jerks that you see clinically.
Typically, after such intense neuron activity, the seizure is followed by a period of postical attenuation and/or slowing before an eventual return to the normal background. While seizures can be tonic clonic, they can also be solely tonic or solely clonic.
On the generalized tonic clonic seizure example below (the sensitivity of which is set at 20uV/mm to better show the high amplitude activity) note the progression from the tonic into clonic phases toward the end of the first page and across the second page, and how this seizure follows the rule of temporal evolution. Note also the period of postictal attenuation before the return of the background. While this particular example is generalized, if the onset was focal but there was evolution spatially into a generalized pattern, that would be termed focal to bilateral tonic clonic.
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.
Tonic seizures are clinically marked by tonic muscle stiffening, and are mostly generalized; they do not evolve into clonic activity. Their appearance on EEG can look somewhat like artifact to the untrained eye, classically with a predominance of low amplitude fast activity and an early overriding slow wave.
On the example below, note the initial sharp in the left temporal region ( a "herald spike," not always present despite so many being seen on this page), leading into subtle attenuation and left temporal fast activity that quickly evolves into a generalized slow wave with overriding very fast activity, then generalized tonic activity marked by myogenic artifact and fast activity.
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.
Myoclonus describes a brief, involuntary jerk. It is normal and commonly seen in individuals falling asleep (hypnagogic myoclonus). However, when present in the morning (hypnopompic myoclonus), coming in multiple simultaneous jerks, or in the setting of anoxic brain injury, it is abnormal.
On EEG, you can see myoclonic jerks--brief, sudden jerks (usually generalized) that may have only a second or so of epileptiform discharges associated with them, and which would be easy to pass up as just interictal activity if there is no documented clinical correlate; in the example below, for instance, the noted polyspikes are seen at various points in this patient's tracing, but in this instance there was a clinical jerk to correlate. You may also see longer runs of discharges with a series of jerks (often leading to significant myogenic artifact), more consistent with a formal myoclonic 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.
Absence seizures are a type of generalized seizures, but we'll discuss them separately because the syndrome of absence epilepsy has characteristic findings. Typical absence seizures are defined clinically by abrupt and brief cessation of awareness and activity, often with eye fluttering or blinking, with rapid return to normal after the seizure ends. Atypical absence seizures may come with other features such as motor activity or more prolonged postictal states.
Electrographically, typical absence seizures are characterized by 3 Hz generalized spike and waves or polyspike and waves; in other forms of generalized epilepsy, absence seizures can be at varying frequencies. Note that absence seizures are brief, have very sudden on and offsets electrographicaly (without any postictal slowing or attenuation), and often do not follow the "ten second rule" that other seizure types require.
This page shows anterior left temporal sharply contoured delta activity at about 1 Hz, but there is no evolution that we can see on this page so it doesn't fit criteria for seizure. This is likely an example of left temporal intermittent rhythmic delta activity (TIRDA), an epileptiform finding. The caveat to this is that we don't see the following page--if there is some evolution in frequency or distribution of this activity, seizure would return as a possibility.
Spasms are most commonly seen in the setting of infantile spasms, or West's Syndrome, and thus come along with the hypsarrhythmia background of high amplitude, very disorganized activity with multifocal spikes. However, spasms can also be present in other epilepsies such as Lennox Gastau.
Electrographically, spasms are an unusual seizure type, classically appearing as brief and diffuse, high amplitude slow waves the often have overriding fast activity. Because of the associated clinical correlate--usually a head drop with bilateral tonic arm raise and extension--there is often signifiant overlying movement and myogenic artifact.
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 example demonstrates nicely the two main phases of a tonic clonic seizure: the tonic phase, marked by diffuse and essentially constant myogenic artifact, and then the clonic phase, in which you get intermittent spike wave activity that corresponds to each clonic muscle contraction.
Here we see high frequency photic stimulation (the red lines on the bottom of the screen) that sets off a rhythmic delta activity in the left temporal lobe, leading into rhythmic spikes in the left temporal lobe that eventually spread and generalize into tonic clonic activity.
Tonic seizures are classically marked by a higher amplitude, generalized wave, then diffuse attenuation with overriding fast activity amongst widespread myogenic artifact. They do not have to follow the 10 second rule, and can be very brief.
Here we see a tonic seizure with a few preceding generalized discharges; in considering when the seizure begins, it likely begins with those discharges, because there is a change in the background (diffuse slowing) from the first discharge until onset of the tonic phase.
Typical absence seizures are classically seen with absence epilepsy, and are marked by 3 Hz generalized spike waves or polyspike waves, with abrupt onset / offset and no postictal state. Absence seizures can be a part of many idiopathic generalized epilepsies, and don't have to have the classic 3 Hz spike wave description.
Sharply contoured alpha activity begins in the left temporal region, which spreads into the parasagittal chain and slows in frequency to about 2-3 Hz while increasing in amplitude and evolving in morphology to frank spike and slow waves. Note that the left mid-temporal beta activity seen prior to seizure onset on the first page is also seen throughout this patient's tracing not associated with seizures, but low amplitude beta activity like that can sometimes be the seizure onset itself.
From sleep there's arousal artifact, then emergence of right anterior to mid-temporal 2-3Hz spike and slow wave activity, which increases in frequency to 7 Hz with spread throughout the right parasagittal, then the left temporal and parasagittal chains, with continued increase in amplitude of the spike waves into generalized tonic clonic activity.
Sharply contoured alpha activity begins over the left posterior quadrant, more temporally, that evolves to rhythmic theta spikes with reflection to the right posterior temporal region; after brief obscuration by myogenic artifact, rhythmic 4-5 Hz spike wave activity arises in the left posterior quadrant and spreads to the right hemisphere; the left sided activity increases in amplitude and frequency to 6 Hz, spreads through the left > right hemisphere, with generalized 3 Hz spike wave activity prior to offset.
Out of sleep, a low amplitude and rather broad left hemispheric discharge, followed immediately by onset of left sided, poorly formed spikes that evolve quickly into higher amplitude, well formed left discharges, more prominent posteriorly, which slow slightly before an abrupt offset. The only clinical sign of this seizure was a subsequent arousal, and note that this seizure does not follow the 10 second rule.
This seizure is marked by very sudden onset, generalized 3 Hz spike and wave discharges with a strong frontal predominance; within a second or so, these discharges become more well formed and slow to 2Hz, and remain as such for the remainder of the seizure, which has a very abrupt offset without clear postictal changes (at least not in the second or so we can see on this page).
Myoclonic seizures consist of a series of myoclonic jerks. They can be brief, as this one is, and are most commonly seen in the setting of Juvenile Myoclonic Epilepsy (JME), Lennox Gastaut Syndrome (LGS), or a progressive myoclonic epilepsy. They're commonly generalized in onset, but can be lateralized; this one, for example, has a slight left lead in but is generally bisynchronous in onset.
Here we have a left occipital seizure, seen first in AP bipolar (across two screens), then again in a circumferential montage. Note how the AP bipolar montage shows us a left sided predominance, but the circumferential montage clearly shows a phase reversal over O1, verifying that this is indeed a left occipital onset seizure, and the spiky activity seen in the right occipital region on the first AP bipolar page is just a field from the left side.
Spasms are a highly concerning seizure type, classically seen in infants amongst a background of hypsarrhythmia (very high voltage, chaotic background with multifocal spikes). Electrographically, they are very brief and diffuse, high amplitude slow waves, often with overriding fast activity. To give a sense of how high amplitude spasms can be, the example above was taken at a rather low sensitivity of 20uV/mm.
In this tracing, captured at a low sensitivity of 25uV/mm, we see a background more like those expected with infantile spasms (aka West Syndrome): high amplitude, disorganized background with multifocal spikes. The seizure is brief, and again shows a diffuse slow wave with overriding fast activity. Clinically, spasms are often sudden and brief flexion/extension movements of the arms and/or legs, often with a brief head drop.