Encyclopedia of Health and Medicine

E
electroencephalogram (EEG) A diagnostic procedure
that records the electrical activity of the
BRAIN. The neurologist uses EEG to assess the
brain’s function. Electrodes attached to the scalp
detect the brain’s electrical impulses and carry the
signals to the EEG machine. An amplifier converts
the impulses into patterns that the machine
records either in analog form (in which styluses
create tracings on a slowly moving roll of paper)
or digital form (in which a computer creates an
electronic record).
Preparation, Procedure, and Recovery
EEG generally requires no preparation or recovery
and does not cause discomfort. To conduct the
EEG, the technologist first measures the scalp to
determine the sites for placing the electrodes. The
sites represent a standard pattern, the most common
of which is called the 10/20 system in reference
to the relationships among the sites. A key of
letters and numbers denote the lobe and the electrode’s
position. The technologist then attaches
electrodes to locations on the scalp. Small dots of
glue hold the electrodes in place; the glue may be
difficult to remove when the EEG is over. During
the EEG the person lies on a table in a quiet, darkened
room while the technologist allows the EEG
machine to record the electrical impulses the electrodes
pick up and conduct to the machine. The
technologist may use flashing or steady light to
stimulate areas of the brain. A typical diagnostic
EEG of the brain may take 15 to 90 minutes to
complete.
The different regions of the brain generate
characteristic patterns of electrical activity, measured
in Hertz (Hz). EEG typically captures five
types of electrical activity or brain waves:
Reasons for Doing This Test
Neurologic conditions, from BRAIN HEMORRHAGE to
BRAIN TUMOR to SEIZURE DISORDERS, cause predictable
and detectable deviations from the normal electrical
patterns. EEG also shows the level of electrical
activity in the brain of a person who is UNCON-
SCIOUS, in a COMA, or in a PERSISTENT VEGETATIVE
STATE. The neurologist must interpret the EEG
findings in context with the person’s age, PERSONAL
HEALTH HISTORY, medications, and other clinical
findings to arrive at a diagnosis.
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• Alpha waves are 8 to 13 Hz, originate from the
forward lobes, normally are present only when
the eyes are closed, and form a moderateamplitude
symmetrical pattern.
• Beta waves are 2 to 13 Hz, originate from the
back lobes, normally are present during wakefulness,
and form a low-amplitude symmetrical
pattern.
• Delta waves are 0 to 4 Hz, normally are present
only during deep sleep, and form a high-amplitude
symmetrical pattern.
• Theta waves are 4 to 8 Hz, normally are present
during the transition from wakefulness to
sleep, and form a moderate-amplitude erratic
pattern.
• A pattern of spikes and waves is always abnormal,
features erratic amplitude and cycle, and
typically indicates a seizure disorder.
Risks and Complications
There are no risks or complications from EEG.
There is no discomfort from attaching the electrodes
or during the recording process. Sometimes
removing the electrodes pulls the HAIR, and the