Electrographically, the seizures characteristically begin with a flattening of the normal background rhythms, followed by generalized low voltage fast activity or polyspikes that increase in amplitude and decrease in frequency until these patterns become obscured by muscle and movement artifact. As the seizure clinically moves into the clonic phase, the EEG characteristically shows a checkerboard type pattern of muscle artifact corresponding to the rhythmic jerking movements observed clinically. During breaks between seizures, the EEG shows diffuse suppression of cerebral activity.
Generalized myoclonic status epilepticus
The EEG in myoclonic status epilepticus may show generalized, bisynchronous polyspikes, spikes, or sharp waves preceding and time-locked with the clinical myoclonus, superimposed on a diffusely slow and suppressed background. A burst-suppression pattern may also be seen. Of note, due to the accompanying muscle activity associated with the myoclonic movements, discerning true epileptiform activity from muscle artifact can often be challenging. In this case, the use of a short-acting paralytic agent may aid in determining if the myoclonus is cortically generated or whether it originates lower down the neuro-axis (ie, brainstem, spinal cord, or peripheral site). Ultimately, this determination may be difficult without simultaneous EEG and electromyography (EMG) with jerk-locked back-averaging techniques.
Generalized clonic status epilepticus
The EEG typically shows generalized, synchronous spikes or spike wave complexes time-locked with the clinical movements.
Generalized tonic status epilepticus
Tonic seizures have a propensity to cluster and are more common during non–rapid eye movement (REM) sleep. The electrographic appearance of tonic seizures consists of moderate to high amplitude, frontally predominant, generalized 10-25 Hz spikes, sometimes termed generalized paroxysmal fast activity (GPFA). A second ictal tonic EEG pattern consists of an abrupt, generalized attenuation or flattening of the background EEG activity (to < 5-10 µV) that can be the sole manifestation of the ictal activity or can precede the development of the 10-25 Hz generalized spikes.
Generalized atonic status epilepticus
Atonic seizures are common in patients with Lennox-Gastaut syndrome and are a prominent feature in patients with myoclonic-astatic epilepsy of Doose. The ictal EEG during atonic seizures typically shows either generalized polyspike-and-wave or generalized slow-spike-and-wave (SWS) activity, followed by diffuse, high-amplitude, generalized slow waves maximal over the central head regions.
Simple partial status epilepticus
The ictal EEG in simple partial status epilepticus may show any of a range of patterns from focal spikes, polyspikes, spike and waves, suppression, or focal rhythmic discharges of any frequency to a completely normal background without evidence of ictal activity. Because approximately 6 cm2 of synchronously firing cortex must be involved for EEG to detect ictal activity, it should not be surprising that many focal seizures will be beyond the resolution of scalp EEG, because the ictal focus is too small, too distant, or unfavorably oriented in relation to the electrodes (ie, originating deep in a sulcus) to be detected by scalp recording.
In such instances when the focal seizures cannot be detected by scalp recording, functional imaging modalities such as cerebral positron emission tomography (PET)—which measures cerebral glucose metabolism—or single photon emission computed tomography (SPECT) scanning—which measures regional cerebral blood flow—may be helpful in confirming the diagnosis. If such studies are performed during ongoing clinical signs or symptoms, increased local glucose metabolism or regional cerebral blood flow would verify the suspected seizure focus.
Complex partial status epilepticus
Because a significant amount of cortex (ie, >6 cm2) is typically involved to produce impaired consciousness, complex partial seizures will have an ictal correlate on EEG. The ictal footprint is variable and may consist of focal spikes, polyspikes, spike waves, suppression, or focal rhythmic discharges of any frequency.
NONCONVULSIVE PATTERNS
Typical absence status epilepticus
The classic EEG finding in typical absence status epilepticus is generalized 3 Hz spike-and-wave activity (range 2.5-4 Hz). However, generalized polyspike-and-wave may also be seen. The intradischarge frequency is classically constant but may vary over the course of the seizure.
Atypical absence status epilepticus
In contrast to typical absence seizures, in atypical absences, the onsets and offsets are clinically less abrupt and distinct and the seizures are longer in duration (lasting up to minutes). Additionally, changes in tone are more prominent than in typical absence seizures. The ictal EEG shows slow (< 2.5 Hz) generalized spike-and-wave complexes that may be more irregular and asymmetric than what is classically seen in typical absence status epilepticus.
Nonconvulsive SE with Partial Onset
Primary criteria
An electrographic or nonconvulsive seizure may be demonstrated by any electrographic pattern lasting at least 10 seconds and satisfying any 1 of the following 3 primary criteria:
*Repetitive generalized or focal spikes, sharp-waves, spike-and-wave, or sharp-and-slow wave complexes at a frequency of 3 or more per second.
*Repetitive generalized or focal spikes, sharp-waves, spike-and-wave, or sharp-and-slow wave complexes at a frequency of 3 or less per second AND one of the secondary criteria below.
*Sequential rhythmic, periodic, or quasi-periodic waves at 1 or more per second and unequivocal evolution in: (1) frequency (increasing or decreasing by at least 1/sec), (2) morphology, or (3) location. Of note, evolution in amplitude alone is not sufficient to meet the criteria for evolution. Additionally, change in sharpness of the waveform without other change in morphology is also not adequate to qualify as evolution of morphology.
Secondary criteria
An electrographic or nonconvulsive seizure may be additionally demonstrated by significant improvement in the patient’s clinical state or the appearance of previously-absent normal EEG patterns (such as a posterior dominant rhythm or sleep transients) temporally coupled to the acute administration of a rapidly-acting antiepileptic drug such as a benzodiazepine. Of note, resolution of the suspected ictal pattern without clinical improvement or the appearance of previously absent normal EEG patterns would not satisfy the secondary criteria.
When rhythmic, periodic, or quasi-periodic electrographic patterns fail to fulfill these criteria in an obtunded or comatose patient who lacks other clinical signs of seizure activity, the diagnosis of nonconvulsive status epilepticus becomes more difficult and controversial. Patterns such as lateralized periodic discharges (LPDs, formerly termed PLEDs); bilateral, independent periodic discharges (BIPDs, formerly termed BIPLEDs); generalized periodic discharges (GPDs, formerly termed GPEDs); and stimulus-induced rhythmic, periodic, or ictal discharges (SIRPIDs) represent ambiguous but potentially ictal patterns whose clinical significance and management remain controversial topics.
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