EEG and Evoked Potentials
Definitions
Electroencephalography (EEG): recording spontaneous brain electrical activity.
Evoked potentials: electrical responses to specific sensory stimuli.
Electroencephalography
Normal Adult EEG
Awake Resting State
Alpha rhythm: 8–12 cycles per second (Hz), over parieto-occipital regions.
Beta rhythm, 13–25 Hz, over frontal regions.
Sleep Stages
Stage 1: alpha replaced by slower rhythm; vertex waves (high-voltage sharp waves at top of head) present.
Stage 2: sleep spindles (symmetric 12 to 14 Hz sinusoidal waves).
Stages 3, 4: high-voltage, widely distributed, slow wave activity.
Rapid eye movement (REM) stage: low-voltage mixed frequency activity together with rapid eye movements and generalized atonia; begins about 90 minutes after sleep onset.
REM in daytime EEG suggests sleep deprivation, withdrawal from REM-suppressant drugs, alcohol withdrawal, narcolepsy (see Chapter 145).
Common EEG Abnormalities
Diffuse background slowing: the most common EEG abnormality. Nonspecific. Associated with diffuse
encephalopathies of diverse causes, multiple structural abnormalities.
Focal slowing: localized parenchymal dysfunction; lesion seen on CT, MRI in 70% of cases.
Triphasic waves: generalized synchronous waves in brief runs; toxic metabolic encephalopathies (hepatic encephalopathy in 50%).
Epileptiform discharges: brief discharges of characteristic shape (spike, sharp, slow wave, or combinations of these); occur intermittently; strongly associated with seizure disorders; type of discharge may suggest specific epileptic syndrome.
Periodic lateralizing epileptiform discharges (PLEDs): seen with acute destructive cerebral lesion; focal epileptiform discharges recurring at 1–2 Hz over focally slow or low-voltage background. Acute cerebral infarct (35%); other mass lesions (26%); cerebral infection, anoxia, other causes (39%).
Generalized periodic sharp waves: recur at 0.5–1 Hz on attenuated background. Most common after cerebral anoxia; also seen in CJD.
Clinical Utility of EEG
Epilepsy
EEG essential in diagnosis and classification of epilepsy (Table 14.1; see also Chapter 141).
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