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electroencephalogram (EEG) A diagnostic pro-
cedure 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).

Reasons for Doing This Test
Neurologic conditions, from BRAIN HEMORRHAGEto
BRAIN TUMORto SEIZURE DISORDERS, cause predictable
and detectable deviations from the normal electri-
cal 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.

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 com-
mon of which is called the 10/20 system in refer-
ence to the relationships among the sites. A key of
letters and numbers denote the lobe and the elec-
trode’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, dark-

ened room while the technologist allows the EEG

machine to record the electrical impulses the elec-

trodes 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, meas-

ured in Hertz (Hz). EEG typically captures five

types of electrical activity or brain waves:

• Alpha waves are 8 to 13 Hz, originate from the
  forward lobes, normally are present only when
  the eyes are closed, and form a moderate-
  amplitude symmetrical pattern.


• Beta waves are 2 to 13 Hz, originate from the
  back lobes, normally are present during wake-
  fulness, 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-ampli-
  tude 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 abnor-
  mal, 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 elec-

trodes or during the recording process. Sometimes

removing the electrodes pulls the HAIR, and the