Electrodes are distributed over the brain to detect electrical changes associated with
Twenty electrodes are distributed symmetrically over the scalp.
The normal awake EEG shows 8- to 12-Hz, 50-muV sinusoidal alpha waves that wax and wane over the occipital and parietal lobes and > 12-Hz, 10- to 20-muV beta waves frontally, interspersed with 4- to 7-Hz, 20- to 100-muV theta waves.
The EEG is examined for asymmetries between the 2 hemispheres (suggesting a structural disorder), for excessive slowing (appearance of 1- to 4-Hz, 50- to 350-muV delta waves, as occurs in depressed consciousness, encephalopathy, and dementia), and for abnormal wave patterns.
Abnormal wave patterns may be nonspecific (eg, epileptiform sharp waves) or diagnostic (eg, 3-Hz spike and wave discharges for absence seizures, 1-Hz periodic sharp waves for Creutzfeldt-Jakob disease).
The EEG is particularly useful for appraising episodic altered consciousness of uncertain etiology.
If a seizure disorder is suspected and the routine EEG is normal, maneuvers that electrically activate the cortex (eg, hyperventilation, photic stimulation, sleep, sleep deprivation) can sometimes elicit evidence of a seizure disorder. Nasopharyngeal leads can sometimes detect a temporal lobe seizure focus when the EEG is otherwise uninformative. Continuous ambulatory monitoring of the EEG (with or without video monitoring) over 24 h can often determine whether fleeting memory lapses, subjective auras, or unusual episodic motor behavior is due to seizure activity.
If clinicians need to determine whether an episode is a seizure or a psychiatric disorder, a video camera may be used to monitor the patient while EEG is done in the hospital. This technique (called video EEG) is also used before surgery to see what type of seizure results from an abnormality in a particular epileptogenic focus.