Nonconvulsive seizures can be categorized electrographically as generalized or focal. Focal seizures are subdivided to include simple partial status epilepticus and complex partial status epilepticus. Seizures continue to be defined as a paroxysmal event with acute onset and offset that evolves in frequency and location. One of the challenges of making the diagnosis of NCSE is that the principles that define a seizure do not necessarily apply. The electrographic definition of nonconvulsive status epilepticus can be made in absence of electrographic evolution and often relies on the response to treatment. Given these features, the diagnosis of NCSE may not be straight forward. Current criteria state that the electrographic diagnosis of NCS or NCSE can be made if there are epileptiform discharges >3 Hz lasting for 10 s or longer, if there are epileptiform discharges <3 Hz with evolution in frequency or location, or if there are epileptiform discharges <3 Hz with an accompanying clinical or electrographic response to treatment (Fig. 1) [12].

The significance of periodic patterns that do not meet the criteria for nonconvulsive seizures remains controversial in both the adult and pediatric literature. Some patterns such as lateralized periodic discharges (LPDs) and specifically lateralized periodic discharges plus (LPDs with superimposed fast or rhythmic activity), as described by the new classification, may put patients at higher risk for seizures. The role of periodic patterns and their place on the ictal-interictal continuum remains a subject for debate. Much of the knowledge we have of periodic patterns is from the adult literature with regard to risk of seizure and prognosis [19], and there may be differences between periodic patterns identified in adults versus children [20]. For instance, while LPDs are frequently associated with acute lesions and altered mental status in adults, they can also be seen in children with chronic lesions in the absence of altered mental status [20]. Recent data on generalized periodic discharges (GPDs) in children suggest that they may carry a lower mortality rate compared to adults. EEG findings need to be considered in appropriate clinical context, history, and evolution, and it should be recognized that etiologies differ in pediatrics and adults [8]. For example, a child with known Lennox-Gastaut syndrome, who had slow spike-and-wave at baseline, may be approached differently than a child without EEG abnormalities at baseline. Due to uncertainty regarding their significance, there management also remains controversial. While some physicians would recommend treatment, few would treat aggressively. Moreover, treatment may be varied based on concomitant clinical symptoms and presentation.

The approach to seizures in critically ill children should take several factors into consideration. The most important factor is the prognosis based on the underlying etiology. Other factors to consider include the degree of encephalopathy, probability of seizures worsening preexisting acute injury, as well as systemic side effects of intravenous antiepileptic medications and the impact that they may have in already unstable patients [8, 10, 18]. Overall, NCS/NCSE in the critically ill, comatose patient should be treated aggressively given the impact of seizure burden on neurologic outcome, and in some instances mortality, independent of the underlying etiology [1, 4]. Among pediatric patients with encephalopathy, the degree of encephalopathy should also be taken into consideration when deciding on approach to treatment. The patient that is mildly obtunded or lethargic may not benefit from aggressive therapy. In encephalopathic patients with acute structural injury, seizures can impact the degree of injury and level of increased intracranial pressure. Adult studies suggest that patients presenting with traumatic brain injury are at risk of worsening increased intracranial pressure and worsening ratios of lactate to pyruvate [21].

Patients presenting with NCSE after CSE may or may not be treated aggressively, as NCSE patterns may be seen in transition for CSE to more normal brain patterns. However, NCSE after CSE has been shown to increase morbidity and mortality independent of etiology [22]. The uncertainty of the impact of NCS and NCSE should also be considered in the population that is paralyzed, sedated, and unable to show the outward manifestation of clinical seizures, including many patients in the intensive care unit and including patients presenting with convulsive status epilepticus that require intubation, patients after cardiac and neurosurgery and anesthesia, as well as those requiring extracorporeal membrane oxygenation.

Antiseizure medications also carry a degree of morbidity. Many medications have the potential to worsen preexisting systemic instability such as worsening cardiac function in a child status post cardiac arrest, worsening hypotension in a patient with sepsis, and decreasing respiratory drive in a patient with respiratory arrest. The underlying etiology and overall prognosis, in conjunction with background EEG findings, also need to be considered. In situations where the outcome is grim, such as severe hypoxic injury after cardiac arrest or near drowning, aggressive treatment may not be in the best interest of the patient, and risks of treatment versus benefits of seizure and NCS control need to be weighed against each other. To date, there is no clear evidence that treatment of NCS and NCSE improves outcome. However, there is a growing body of literature suggesting that even interictal epileptiform discharges may have an effect on cognition.

As discussed previously, the significance of many periodic patterns remains uncertain. Periodic patterns should be addressed within the context of the clinical presentation. Based on criteria for electrographic nonconvulsive status epilepticus, certain periodic patterns that respond to treatment are consistent with NCS/NCSE. Therefore, the acute onset and recognition of a periodic pattern in an encephalopathic patient may warrant a treatment trial, e.g., with benzodiazepines, to assess clinical change. In some instances, physicians suggest prophylaxis given the high probability of seizures in populations with certain periodic patterns, specifically LPDs and bilateral independent periodic discharges (BiPDs). Treatment with pharmacological coma and continuous infusions in patterns without clinical signs is usually not warranted.

The diagnosis of NCSE is not a diagnosis unique to the ICU. Patients frequently present for an outpatient appointment or to the emergency room. Patients presenting with lethargy or altered mental status, with intermittent or continuous impairment of cognition and cognitive function, may be at risk. These signs are often clear in developmentally normal children, but can be challenging in patients with underlying developmental delay due to a preexisting epileptic encephalopathy. The approach set forth by the Epilepsy Research Workshop focusing on the mental status examination may be helpful when assessing patients in the ambulatory clinic for NCSE where the diagnosis is not clear [7]. In these instances, the systematic assessment of several features may be helpful. Close attention needs to be paid to subtle motor phenomena such as brief behavioral arrest, subtle eye findings (e.g., brief nystagmoid movements or eye lid fluttering), and motor incoordination in the form of ataxia or negative myoclonus. Other subtle signs such as drooling and a delayed motor response may be apparent [7]. Other domains to assess include overall affect as it may relate to degree of eye contact, visual fixation, and social interactions, keeping in mind the need to have a strong understanding of baseline function. Cognition and memory decline may also be present. Children may present with regression or loss of speech. There may also be a change in school performance, overall behavior, and the ability to learn. Populations that are particularly at risk include patients with a preexisting diagnosis of epilepsy or epileptic encephalopathy and patients with genetic conditions that have the potential for comorbid epilepsy and in some instances NCS/NCSE [8]. Patients with juvenile myoclonic epilepsy are at risk for absence status epilepticus as well as subtle myoclonic status epilepticus; however, this is rare [8]. Studies have documented NCSE in up to 40 % of children diagnosed with SCN1a mutations that meet criteria for Dravet Syndrome [8]. In some cases, NCSE may be the initial presentation of epilepsy.

Despite deficits in some of these domains, the diagnosis of NCS/NCSE in the ambulatory setting is nearly impossible without the aid of EEG, especially in children with baseline cognitive dysfunction. While in some instances a routine study may be enough to make the diagnosis, prolonged monitoring is often necessary. In instances in which there is a concern for electrical status epilepticus of sleep (ESES), sleep should be assessed.

NCSE in the ambulatory setting can be classified as either generalized or focal. The most common generalized form is typical and atypical absence status epilepticus. Absence status epileptics (ASE) may be seen in up to 3 % of children with childhood absence epilepsy [8]. The symptoms may be subtle and some children continue to function cognitively with only minor deficits. The typical presentation is drowsiness and stupor; however, motor manifestations such as rhythmic blinking, clonic twitching, myoclonic jerks, or automatisms may be present. The ictal EEG pattern consists of 3–4 Hz generalized spike-and-wave discharges. The prognosis of children with typical absence status in the presence of an underlying idiopathic generalized epilepsy is good. Aggressive treatment is usually not necessary. ASE is often responsive to benzodiazepines as well as other medications such as valproic acid and ethosuximide. Atypical forms of absence status epilepticus are also seen in children with symptomatic etiologies or idiopathic/genetic epilepsy syndromes, such as Lennox-Gastaut syndrome. In these cases, the interictal EEG pattern shows 2–2.5 Hz slow spike-and-wave.