The most basic and probably the most common use of EEG monitoring is to determine whether a patient with paroxysmal clinical disturbances is having epileptic seizures. Monitoring can help both in identification of interictal epileptiform discharges and in recording clinical seizures or seizure-like events. It can help identify whether an individual episode in question is epileptic or nonepileptic in origin.
Logically, it is probably impossible to exclude an epileptic basis for a given event. Some seizures are not well recorded at the surface EEG leads. Nevertheless, maintenance of a normal EEG background (even amid extensive movement and other artifact) suggests strongly that the event is not epileptic. Even if epileptiform discharges are not seen during some true epileptic seizures, there is usually a substantial alteration in the EEG with seizures such as rhythmic slowing, background suppression, or some other change.
Atlas of Ambuiatoiy EEG
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Some paroxysmal clinical episodes are associated with clear electrographic seizures, helping to make the diagnosis. The absence of epileptiform EEG abnormalities at the time of paroxysmal behavioral disturbances increases the likelihood that the events are not epileptic, but rather manifestations of other illnesses. Two other groups of illnesses produce symptoms similar to those of seizures. There are events of other physiologic bases such as cardiac arrhythmias causing syncope, episodes caused by cerebrovascular disease, movement disorders, and unusual manifestations of sleep disorders. There are also events of psychiatric origin. Taken together, all of these events are generally referred to as "nonepileptic seizures." All of these can produce bizarre and unusual movements and behavior with some resemblance to epileptic seizures.
For some events, review of a video recording alone is persuasive for a psychogenic origin of spells. Nevertheless, even very experienced epileptologists will see bizarre behavior that appears to be psychogenic but, by EEG, actually has an epileptic basis.
The events of psychiatric origin are often referred to as psychogenic nonepileptic seizures (PNES), or pseudo-seizures or nonepileptic events. (Although used often, the word seizure in this context is likely to promote confusion.) EEG monitoring, especially with simultaneous video recording, is particularly valuable in the evaluation of possible PNES. Although not always successful, it is usually the best way to determine the nature of these events.
Epileptic seizures and PNES differ in several respects. [4-6] PNES tend to be of longer duration and may peak in frequency early in a monitoring session, whereas epileptic seizures tend to occur later, as medication is reduced. Medication withdrawal does not appear to influence the occurrence of nonepileptic events. Whereas PNES tend to occur in the daytime and not during sleep, epileptic seizures are common in sleep. The presence of observers and family members may prompt the occurrence of nonepileptic events and facilitate their recording. Many other clinical features help to distinguish epileptic from psychogenic events. [5, 6] An accurate diagnosis of PNES can be just as helpful as a diagnosis of epilepsy, at least for avoiding anticonvulsant toxicity.
Ambulatory EEG has been very effective at separating seizures from PNES in children, especially when the events in question are stereotyped and reliably discerned by observers such as parents. In these cases, video recording is often unnecessary.  Still, events need to occur with a reasonable frequency (such as three times per week) for monitoring to be successful.
As with any other test the value and, in this case, specificity of monitoring depend on the appropriateness of the events being monitored.  Monitoring to confirm a clinical suspicion of epilepsy or analyze clinically diagnosed epilepsy has a far greater yield than monitoring for vague or poorly characterized symptoms. In almost all cases, however, the yield is far greater than with routine EEG recordings. Still, events can be missed, and if the events in question do not occur during the session, monitoring may yield no useful results.
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