generalized seizures

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).

focal seizures

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.
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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.

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tonic clonic seizures

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.
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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.

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tonic seizures

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.

myoclonic seizures

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.

Myoclonic Jerk

Myoclonic Seizure

Absence seizures

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.

spasms

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.
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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.
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