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Issues of Concern

Even normal EEG waveforms can be considered potentially abnormal, depending upon various factors. For example, alpha waves are seen over the posterior head regions in a normal awake person and considered as the posterior background rhythm. However, in certain comatose states, there can be diffuse alpha activity (alpha comma) and may be considered pathognomonic. Delta waves can be seen in drowsiness and also in very young children; however, the appearance of focal delta activity can be abnormal (see below). Beta activity is present in the frontal regions of the brain and can spread posteriorly in early sleep. Focal beta activity sometimes seen in structural lesions and also in various epilepsies (generalized fast activity/GFA). Medications like sedatives (phenobarbital, benzodiazepines) commonly cause diffuse beta activity.

Triphasic waves: Triphasic waves were initially described in 1950 by Foley, and in 1955 Bickford and Butt gave it the name. Triphasic waves were first believed to be pathognomic of hepatic encephalopathy. However, these are nonspecific and can be seen in any metabolic encephalopathy. They are high amplitude sharp waves, with the duration of each phase longer than the next. They are sharply contoured with three phases. The first phase is always negative, hence the name triphasic waves. Triphasic waves are seen diffusely with bifrontal predominance and are synchronous. They are not seen in an awake state. They are seen in patients with altered levels of consciousness. It is hypothesized that they occur due to structural or metabolic abnormalities at the thalamocortical levels due to the changes in the thalamocortical relays.[1][2][3]

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Interictal Epileptiform Discharges (IED)

Interictal epileptiform discharge is an abnormal synchronous electrical discharge generated by a group of neurons in the region of the epileptic focus.[4] They represent the epileptic focus in patients with seizures. They have a low sensitivity in routine 30 minute EEG recording, and the yield increases with repeat EEG and prolonged EEG recordings. The presence of IED in a routine EEG in children with a new-onset seizure is 18% to 56%, while in adults, it is 12% to 50%.[5] Though uncommon, they can occur in healthy persons without a history of seizures.[6] IEDs can be subdivided into spikes or sharps.

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The following patterns of interictal epileptiform discharges may be seen:

Non-epileptiform Abnormalities

Other Diffuse or Focal Abnormal Patterns in EEG

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