EEG in Status Epilepticus

Status epilepticus (SE) is a life-threatening, neurologic emergency that the International League Against Epilepsy (ILAE) defines as “seizure that persists for a sufficient length of time or is repeated frequently enough that recovery between attacks does not occur." ^[1] Due to early research that showed irreversible neuronal damage after about 30 minutes of continuous epileptic activity, ^[2] the conventional definition of status epilepticus as continuous clinical seizure activity lasting greater than 30 minutes or 2 or more repetitive seizures without recovery of the baseline level of consciousness between attacks was adopted. Although this definition initially referred to clinically obvious or generalized convulsive status epilepticus (GCSE), the advent of continuous electroencephalographic (EEG) monitoring has facilitated the recognition of subtle convulsive and nonconvulsive (NCSE) forms of status epilepticus as well. ^[3]

However, studies have suggested that this definition might need revision. Work by Jenssen et al ^[4] and Shinnar et al ^[5] demonstrated that seizures that do not spontaneously resolve within 5-10 minutes are unlikely to terminate without intervention. Additionally, Treiman et al showed that the duration of generalized convulsive status epilepticus before treatment was an important determinant of treatment success, whereas later studies have shown that delayed treatment in nonconvulsive status epilepticus is associated with increased mortality and refractoriness to treatment. ^{[6, 7, 8]} Based on these findings, many authors have suggested a more aggressive definition of status epilepticus that is based on a duration of 5 rather than 30 minutes. ^[9]

In Europe, the annual incidence of generalized convulsive status epilepticus is estimated to range from 3.6 to 6.6 per 100,000, whereas nonconvulsive status epilepticus is estimated from 2.6 to 7.8 per 100,000. ^{[10, 11, 12]} In the United States, a prospective study that included all forms of status epilepticus (GCSE, subtle convulsive status epilepticus, and NCSE) cited a markedly higher incidence rate of 41 per 100,000. ^[13] Mortality rates for status epilepticus range from 3% to 33%, and such variability is heavily dependent on etiology, age, and clinical seizure form. ^{[10, 11, 12, 13, 14, 15, 16]}

Any seizure type can develop into status epilepticus, and hence, there are as many types of status epilepticus as there are types of seizures. ^[17] Because the type of status epilepticus can often inform on the etiology, prognosis, and treatment of the seizures, distinguishing between these various forms of status epilepticus is crucial. Such classification is based on both the clinical semiology and the findings on the EEG. With the increased recognition of subtle convulsive and nonconvulsive status epilepticus over the past 2 decades, ^{[7, 13, 18, 19, 20]} as well as the controversies regarding periodic patterns that lie on the ictal-interictal continuum, EEG in particular is indispensable in identifying and characterizing the various forms of status epilepticus. ^{[21, 22]}

This review presents a concise categorization of the various types of status epilepticus with attention to their particular hallmarks on EEG. It also discusses the more ambiguous periodic patterns (generalized periodic discharges [GPDs]; periodic lateralized epileptiform discharges [PLEDs]; PLEDs with transitional rhythmic discharges [PLEDs Plus]; bilateral, independent PLEDs [BIPLEDs]; and stimulus-induced rhythmic, periodic, or ictal discharges [SIRPIDs]), which have been the focus of debate regarding their potential ictal nature and clinical relevance. ^[23] Consideration of these more controversial patterns serves to underscore some of the limitations and uncertainties involved with EEG interpretation in status epilepticus. ^[24]

For more information, see Status Epilepticus and Pediatric Status Epilepticus.

When characterizing status epilepticus (SE), the first consideration is a clinical one, which categorizes this condition as either convulsive or nonconvulsive, depending on whether the patient presents with or without motor manifestations. ^[25] Both convulsive status epilepticus (CSE) and nonconvulsive status epilepticus (NCSE) can be further subdivided into partial (also called focal) or generalized subtypes, determined by what portion of cerebral cortex is involved at seizure onset. Although clinical semiology may help, often the electroencephalogram (EEG), ideally obtained at the start of ictal activity, is required to make this distinction. This distinction is critical, as it informs on the etiology and prognosis of the seizures.

Seizures or status epilepticus that begin with generalized ictal activity are usually sequelae of idiopathic epilepsy, thought to be the result of a genetic predisposition toward seizures (examples are childhood absence epilepsy [CAE], juvenile myoclonic epilepsy [JME], and epilepsy with grand mal seizures on awakening). Seizures that originate from a specific cortical focus are usually sequelae of focal brain dysfunction or injury. When the underlying cause of such focal dysfunction is apparent (ie, stroke, tumor, cortical malformation) the seizure is considered symptomatic of this lesion. If no associated focal lesion is identified despite the footprint of a focal onset, either clinically or on EEG, the etiology is termed cryptogenic.

Generalized seizures (or generalized status epilepticus) by definition involve the entire cortex and therefore result in an alteration of consciousness. Partial seizures (or partial status epilepticus) only involve specific portions of the cortex and may or may not result in altered consciousness. This leads to further classification of partial status epilepticus as either simple partial status epilepticus, identified by preserved consciousness, or complex partial status epilepticus, distinguished by the finding of altered consciousness. Based on these categorizations, we will now briefly describe and provide examples of the most common EEG patterns in status epilepticus.

This section will discuss primary generalized tonic-clonic status epilepticus, generalized myoclonic status epilepticus, generalized clonic status epilepticus, generalized tonic status epilepticus, and generalized atonic status epilepticus.

Primary generalized tonic-clonic status epilepticus

Primary generalized tonic-clonic status epilepticus (also called grand mal status epilepticus or primary generalized convulsive status epilepticus [PGCSE]) is thought to be less common than secondarily generalized convulsive status epilepticus (SGCSE), but the 2 conditions look clinically identical.

PGCSE is seen in patients with a history of primary generalized epilepsy, often in the setting of intentional or unintentional changes in their antiepileptic medications. Clinically, patients present with loss of consciousness and a repetitive, stereotyped sequence of bilateral stiffening of the face, trunk, and extremities (tonic phase), alternating with repetitive, symmetric clonic contractions in the face, trunk, and extremities (clonic phase).

Typically, the tonic phase starts with a brief period of flexion in which the eyes are open and rotate upward and the neck, trunk, and extremities are in some degree of flexion. Classically, the arms are elevated, adducted, and externally rotated, and the legs are less notably flexed at hips and knees, adducted, and externally rotated. The mouth may be seen to be held in a rigid, half-open position.

The tonic extension phase follows this and consists of more prolonged neck and back extension, with the arms and legs also moving into extension. At this point, forced closure of the mouth may produce oral trauma. The arms may remain flexed to some degree at the elbows or may show complete extension, with some degree of internal rotation at the shoulders and pronation at the elbows. The patient’s wrists may be extended with fists clenched, or the wrists may be flexed with fingers extended. The legs are usually extended, adducted, and externally rotated with feet and big toes extended as well. During the tonic phase, the diaphragm also contracts, which may elicit a characteristic ictal cry.

This is then followed by the clonic phase in which the generalized tonic muscle contraction is replaced by synchronous, rhythmic jerking movements of the face, trunk, and limbs. The tongue may also be bitten during this period, and rhythmic ictal vocalizations may be heard. As the clonic phase progresses, the rhythmic contractions are seen to occur with decreasing force, amplitude, and frequency. Of note, contraction of the urinary sphincter prevents urinary incontinence, which occurs only after the end of the clonic phase. Fecal incontinence and ejaculation are rare phenomenon.

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 generalized myoclonic form of status epilepticus can be seen in a variety of patient populations, including the following:

• Patients with idiopathic generalized epilepsy: Juvenile myoclonic epilepsy (JME)

• Patients with symptomatic or cryptogenic generalized epilepsy syndromes: Myoclonic epilepsy of infancy (Dravet syndrome), myoclonic-astatic epilepsy of Doose, and Lennox-Gastaut syndrome

• Patients with progressive myoclonic epilepsy syndromes: Lafora body disease, neuronal ceroid-lipofuscinosis, Unverricht-Lundborg disease, mitochondrial encephalopathy with ragged-red fibers (MERRF)

• Patients who have suffered an acute anoxic brain injury

The clinical history, time course of the seizures, and mental status of the patient are the key features that help to distinguish among these various forms of myoclonic status epilepticus.

Withdrawal of antiepileptic medications, sleep deprivation, or treatment with narrow-spectrum antiepileptic drugs such as phenytoin or carbamazepine can rarely precipitate myoclonic status epilepticus in patients with juvenile myoclonic epilepsy. Clinically, these patients present with large amplitude, shocklike jerks of the trunk and extremities that occur at irregular intervals. Typically the jerks are synchronous and symmetric. Similar to the typical myoclonic jerks experienced by these patients, myoclonic status epilepticus often presents in the morning. Importantly, these patients have preserved awareness during these events and can often give a history compatible with the diagnosis of juvenile myoclonic epilepsy, if this was not previously known. The EEG shows irregular bursts of generalized, bisynchronouspolyspikesandpolyspike-waves at a frequency of 2-5 Hz that precede and are time-locked with the observed myoclonic jerks.

As their names imply, myoclonic status epilepticus is commonly seen in patients with severe myoclonic epilepsy of infancy (Dravet syndrome) and myoclonic-astatic epilepsy of Doose, and it is also described, although less frequently, in patients with Lennox-Gastaut syndrome. In all of these symptomatic generalized epilepsies, myoclonic status epilepticus is associated with a depressed level of consciousness, although this can be difficult to discern in these young patients who have significant cognitive impairments at baseline. The myoclonic jerks in these patients are often small amplitude, asymmetric, and asynchronous, and they affect the face and distal extremities. The myoclonic status epilepticus can also be quite prolonged in these patients, lasting from days to many weeks. The EEG often shows multifocal spikes, which may or may not correlate with the clinical jerks, and a diffusely slow and disorganized background.

In patients with one of the progressive myoclonic epilepsy syndromes, the clinical myoclonus is often also small amplitude, multifocal, asymmetric, and asynchronous. Notably, particularly early in the disease course, the myoclonus may exhibit a reflex component whereby it is exacerbated by action or stimulation. However, unlike myoclonic status epilepticus associated with the symptomatic generalized epilepsies, mental status is not depressed as a result of the myoclonic status epilepticus. Again, this may be difficult to tease out due to the concomitant cognitive decline related to the underlying pathophysiology of these diseases. The EEG in these cases typically shows mild to moderate background slowing, with multifocal or generalized spikes and polyspike-waves that variably correlate with the clinical jerks.

The most common cause of myoclonic status epilepticus is anoxic brain injury, usually following cardiopulmonary arrest. In this clinical scenario, comatose patients develop continuous myoclonus within hours to days following the anoxic injury and these movements persist for up to 1-5 days. The myoclonus can be rhythmic or irregular and characteristically consists of bilaterally synchronous jerks of the face, trunk, and limbs. Repetitive blinking, eye opening, upward eye rolling, and mouth twitching are often prominent features.

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.

Traditionally, the presence of acute myoclonic status epilepticus has portended a dismal prognosis. ^{[26, 27]} However, this conclusion will need to be revisited in patients treated with therapeutic hypothermia after cardiopulmonary arrest.

In evaluating patients in post–anoxic coma with myoclonic status epilepticus, it is additionally important to distinguish between acute post–anoxic myoclonic status epilepticus and chronic post–anoxic reflex myoclonus (Lance-Adams syndrome). In contrast to acute post–anoxic myoclonic status epilepticus, chronic post–anoxic status epilepticus usually presents days to weeks after the inciting anoxic event and persists for months to years. Also, in contrast to the acute form, chronic post–anoxic myoclonic status epilepticus classically presents in patients who have recovered their mental status and consists of action-induced, focal myoclonic jerks. ^[28]

Generalized clonic status epilepticus

Classically, clonic status epilepticus distinguished from myoclonic status epilepticus in that the seizures consist of rhythmic, repetitive muscle movements as opposed to the single or irregularly recurrent movements seen with myoclonic seizures. The International League Against Epilepsy (ILAE) has proposed that when clonic seizures occur in isolation, their mechanisms may be different from those of the clonic phase seen in generalized tonic-clonic seizures. ^[29] In isolated clonic seizures, the repetitive clonic movements are thought to be the result of rhythmic excitatory discharges, whereas in generalized tonic-clonic seizures, the clonic movements are thought to result from the interruption of tetanic contraction caused by the activation of seizure-suppressing mechanisms. ^[29]

Clonic status epilepticus occurs predominantly in children and may be seen with febrile seizures or in patients with Lennox-Gastaut syndrome. ^[30] Clinically, consciousness is impaired and these patients will have synchronous rhythmic jerking movements of the face, trunk and limbs. The EEG typically shows generalized, synchronous spikes or spike wave complexes time-locked with the clinical movements. ^[31]

Generalized tonic status epilepticus

Tonic status epilepticus is most commonly seen in patients with Lennox-Gastaut syndrome, one of the symptomatic generalized epilepsies. The individual tonic seizures are usually brief, typically lasting 5-20 seconds, and range from being so subtle as to often go unnoticed by caregivers, to dramatic clinical events similar to the tonic phase described in the section on generalized tonic-clonic status epilepticus. The more subtle forms may involve only upward deviation of the eyes or brief but sustained contractions of the neck, facial, or masticatory musculature (termed axial tonic seizures). Usually, these consist of flexion of the head at the neck, opening of the mouth, and grimacing of the facial muscles. When the abdominal and diaphragmatic muscles are involved, the patient may produce an ictal cry or may have brief periods of apnea.

Global tonic seizures occur when both axial and appendicular musculature are involved, and these characteristically consist of the axial features discussed above plus abduction of the arms at the shoulders and flexion at the elbows with lower extremities, which are either in flexion at the hip, knees, and ankles or in extension at these joints. Intermediate forms of tonic seizures also exist in which, in addition to axial involvement, only the proximal muscles of the upper extremities are affected (termed axorhizomelic tonic seizures).

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. 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

Whereas tonic seizures are characterized by the sudden onset of sustained, increased muscle tone, atonic seizures consist of the sudden loss of muscle tone. This loss of tone is usually very brief, lasting 1-2 seconds, and may affect the neck, jaw, trunk, or limb musculature. Clinically, the manifestations of these seizures range from subtle head drops to often injurious falls.

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.

In this section, secondary generalized tonic-clonic status epilepticus, simple partial status epilepticus, and complex partial status epilepticus are reviewed.

Secondary generalized tonic-clonic status epilepticus

Secondary generalized tonic-clonic status epilepticus is more common than primary generalized tonic-clonic status epilepticus and occurs when focal seizure activity spreads to involve all of the brain. If observed either clinically or electrographically early in its course, localizing or lateralizing signs may be present that verify the focal onset of the seizures. However, more commonly, initial clinical descriptions, much less initial electroencephalographic (EEG) recordings, are not available, and the clinician is left to look for other clues to help distinguish between the 2 diagnostic possibilities.

Although historical features such as history and semiology of past seizures, age of seizure onset, and family history of epilepsy can often help, EEG can also provide critical diagnostic information. In the cases in which the seizures are controlled and the status epilepticus is broken, interictal findings such as generalized interictal epileptiform discharges (IEDs), focal interictal slowing, or focal interictal epileptiform discharges can often help cinch the diagnosis. However, when generalized convulsive status epilepticus is ongoing, making the distinction between primary generalized convulsive (PGCSE) and secondary generalized convulsive status epilepticus (SGCSE) based on EEG alone is often more challenging.

Treiman and colleagues ^{[6, 18, 32]} have described a sequence of clinical and EEG changes seen in prolonged SGCSE that have not been reported with PGCSE. According to their observations, patients in SGCSE initially have generalized tonic-clonic convulsions that correlate with discrete generalized seizure patterns on EEG (termed the overt status epilepticus phase). This is followed by a stage of electroclinical dissociation (termed subtle status epilepticus). During this phase, the patient may have only subtle twitching of the face or limbs or slight eye jerking movements, despite ongoing generalized ictal activity on EEG. Often, coma may be the only clinical manifestation of the ictal discharges seen on EEG during prolonged SGCSE.

Whether the presence of the clinical-electrographic sequence described by Treiman and colleagues is useful in differentiating SGCSE from PGCSE is debatable, but the observation of electroclinical dissociation is well established and is significant, as it serves to emphasize the importance of ongoing EEG monitoring for patients who present in generalized convulsive status epilepticus (GCSE), even after their clinical convulsions have subsided. This point was reinforced by De Lorenzo et al, who found that 48% of 164 patients who presented with GCSE continued to have electrographic seizures after all convulsive activity had ceased, and 14% of these patients met criteria for nonconvulsive status epilepticus. Additionally, mortality was seen to be significantly higher in those with nonconvulsive status epilepticus (51%) or ictal patterns (32%) relative to those without ictal activity (13%). ^[33]

Simple partial status epilepticus

Simple partial status epilepticus is caused by the continuous, abnormally excessive and hypersynchronous firing of a focal, circumscribed population of cortical neurons. By definition, consciousness is not impaired, and the symptoms experienced by the patient will vary depending on what area of the brain is involved in the seizure. When the seizure involves motor pathways, the clinical manifestations are obvious to observers and may consist of either excitatory phenomena (such as clonus, tonic, or dystonic posturing) or inhibitory phenomena, whereby negative motor symptoms such as asterixis may be seen. When the seizures consist of focal, continuous, rhythmic clonic movements of the face, eyes, tongue, trunk, arms, or legs, it is termed epilepsia partialis continua (EPC). When motor signs are not present, the manifestations of simple partial seizures can be subtle or imperceptible to the outside observer, who must then rely on the patient’s subjective reports.

The range of symptoms produced by simple partial status epilepticus are as varied as the brain regions that can be affected; some of the more common manifestations include somatosensory symptoms (typically focal paresthesias), special sensory symptoms (flashes of light or colors in one hemi-field; ringing or buzzing sounds in the ears; noxious tastes or odors; or more complex visual, auditory, gustatory, or olfactory illusions or hallucinations), autonomic symptoms (sweating, salivation, pallor, flushing, piloerection, pupillary dilatation, palpitations, tachy-/bradycardia, hyper-/hypotension), or psychic symptoms (anger, fear, ecstasy, déjà vu, jamais vu).

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, ^{[34, 35]} 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

Complex partial status epilepticus is also caused by abnormal, excessive synchronous firing of a localized group of neurons. As such, similar clinical signs and symptoms as those described for simple partial status epilepticus may be present in complex partial status epilepticus and vary depending on the location of the seizure focus. The critical additional feature that defines and distinguishes complex partial status epilepticus is the presence of impaired consciousness, which is generally defined as the loss of awareness of and/or inability to respond in a purposeful and appropriate manner to external stimuli.

Practically, awareness is assessed by the determining whether or not the patient was aware of the events that occurred during the ictal period. Responsiveness is assessed by testing the patient’s ability to follow commands and interact in a meaningful way with an outside observer during the ictal event.

Of note, care should be taken to distinguish between complex partial seizures and simple partial aphasic seizures, in which the patient is aware of events going on around him or her and can interact appropriately but cannot produce speech. Due to the presence of impaired consciousness with complex partial seizures, the patient is often unable to report any subjective symptoms caused by the ongoing seizures. Thus, if motor symptoms are not present, the seizure may be undetectable without the use of EEG monitoring and may be best classified as nonconvulsive status epilepticus.

Because a significant amount of cortex (ie, >6 cm²) ^[34] 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.

Both typical and atypical absence status epilepticus are discussed in this section.

Typical absence status epilepticus

Typical absence status epilepticus (petit mal status epilepticus) occurs in about one third of patients with typical absence seizures. ^[36] As such, it is a common occurrence in patients who suffer from the idiopathic generalized epilepsy syndromes such as childhood absence epilepsy (CAE), juvenile absence epilepsy (JAE), and juvenile myoclonic epilepsy (JME). Clinically, the seizures may divide into simple and complex subtypes.

In simple absence seizures, the hallmark manifestation is the abrupt onset and offset of impaired consciousness. Classically, this is noted as staring and behavioral arrest. The seizures characteristically last 2-30 seconds, during which time the patient is unaware of and unresponsive to the surrounding environment. The patient has neither a warning (aura) that the seizure is about to occur nor postictal confusion or lethargy and is therefore usually unaware of the seizure. At the cessation of the seizure, the patient often returns to the previous activity as if nothing had happened.

Complex typical absence seizures are more common than the simple type and are characterized by the addition of clonic, tonic, myoclonic, or automatic motor activity. Commonly, these consist of clonic eyelid jerking or oral-motor and manual automatisms. However, myoclonic jerks of the axial and appendicular muscles may be seen as may head drops or tonic muscle contractions. The seizures can be precipitated by prolonged hyperventilation (3-5 min) in about 90% of untreated patients and photic stimulation may provoke them as well.

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. ^[37]

Atypical absence status epilepticus

Atypical absence status epilepticus is usually seen in patients with symptomatic generalized or cryptogenic epilepsies such as the Lennox-Gastaut syndrome, myoclonic-astatic epilepsy of Doose, and epileptic encephalopathy with continuous spike and waves during sleep. These patients usually have some degree of baseline cognitive impairment and suffer from other seizure types such as myoclonic, tonic, and atonic seizures in addition to the atypical absence seizures. The seizures manifest with some degree of impaired consciousness, which may be difficult to detect due to an already abnormal baseline cognition.

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.

Simple partial status epilepticus and complex partial status epilepticus were previously discussed (see Convulsive SE with Partial Onset). As noted, complex partial status epilepticus without motor manifestations (nonconvulsive status epilepticus) presents a particular problem for clinicians, as without electroencephalographic (EEG) monitoring a definitive diagnosis is impossible. ^[38]

Certain patterns on the EEG are unequivocally ictal; however, other patterns remain controversial and pose a particular dilemma regarding the need for treatment. ^[39] In an attempt to define and standardize criteria for what constitutes an electrographic or nonconvulsive seizure, Chong and Hirsh modified the earlier work of Young et al ^[7] and proposed primary and secondary criteria the described below. ^[40]

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 ^[40] :

• 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. ^{[7, 40]}

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.