EEG basic definition

Definitions and Terms

▪ Frequency: The measurement used to categorize several ranges of electrical activity found in the normal and abnormal brain:
- —Delta: High-amplitude, low-frequency waves seen in sleep
- —Theta: Low-frequency waves seen in younger children and meditation in adults—may be indicative of pathological activity, such as toxic encephalopathy
- —Alpha: Higher frequency range seen in relaxation and certain forms of coma
- —Beta: High frequencies seen during active thinking
▪ Epileptiform discharges: Rhythmic discharges seen focally or diffusely in patients with epilepsy.
▪ Diffuse slowing: Combination of delta and theta frequencies seen in many pathological states—when seen with preserved reactivity to external stimuli (ie, pain, sound); this pattern may have a better prognosis than when responsiveness is absent.
▪ Intermittent rhythmic delta activity: Pathological pattern that can be seen with metabolic, toxic, hypoxic, or other diffuse intracranial diseases.
▪ Burst suppression: High-voltage bursts of activity alternating with background suppression—seen in deep coma and pharmacologically induced coma.
▪ Bispectral index (BIS): A limited form of EEG analysis in which brain electrical activity is indexed to a dimensionless number between 0 and 100, wherein a value of 0 represents the absence of EEG activity and 100 is typical for an awake person—values between 40 and 60 are found in well-anesthetized patients.

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