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Electroencephalogram (EEG) : Definition, Procedure, Measure and Indication

Thursday, August 30th 2012. | Other Examination

Electroencephalogram Definition

Electroencephalography is the examination which records the spontaneous electrical activity of neurons in the brain. The plot thus obtained is called electroencephalogram.

Electroencephalography (EEG) is a test that allows the recording of spontaneous electrical activity of the brain through the skull, through electrodes placed on the scalp in a standardized way (or scalp) and interconnected by montages variables.

The electrodes (the number of 10 to 20), made of an alloy of silver, are distributed over the scalp symmetrically. They may be maintained by a helmet rubber, or bonded with a paste. The electrodes are connected to a recording apparatus which measures the electric potential detected by each electrode and compares the electrodes pairs.

The unit draws 5-10 plotted below each other. Each track is composed of successive waves characterized by their shape (peak or wave) frequency, the effect of stimulation. It is the study of the frequency that is used to analyze the type of waves that predominate waves (alpha, beta, delta and theta).


Electroencephalography (EEG) (img thanks to mc.vanderbilt.edu)

How is it?

The EEG is a simple and safe.

The examination takes place in a medical office or hospital. The person may be simultaneously filmed.

The standard EEG awake lasts about 20 minutes during which are recorded successively three to four fixtures. It is a painless test. A plot of rest is obtained in quiet, eyes closed.One tests the reactivity of brain rhythms when opening and closing the eyes.

Two tests are systematically associated activation:

  • Hyperpnea March to May is minutes to breathe slowly and ample,
  • A photic stimulation frequency gradually increasing from 1 to 40 Hz applied immediately after the closing of the eyes.

Infants and children up to age 4 or 5, it is desirable to record routine EEG sleep. In the subject unconscious reactivity is studied by auditory stimuli or sensory pain.

Special techniques registration may be required in certain circumstances: EEG sleep in adults, Polygraphy (simultaneous recording of several parameters such as heart rate and breathing, blood pressure, eye movements) EEG activation drug, EEG coupled with a video recording of the patient’s behavior, recording or remote telemetry, ambulatory often called “Holter EEG,” Signal Processing with EEG-mapping.

For surgical treatment of epilepsy focused rebel severe medical treatment, electrodes can be placed directly in contact with the cerebral cortex (electrocorticography) or within the brain itself (stereo-EEG).

As measured by the EEG?

The EEG records the brain activity of the brain.

The EEG records brain waves or “EEG activities” which are characterized by their frequency, amplitude, stability, morphology, topography, their reactivity. They are classified according to their frequency in

Delta wave (frequency below 3.5 Hz,

  • Theta waves (frequency of between 4 and 7.5 Hz,
  • Alpha waves (frequency between 8 and 13 Hz and
  • Beta waves (frequency greater than 13 Hz

The EEG can record activities pathological (abnormal), or paroxysmal non permanent or intermittent, diffuse across the scalp or localized to one region only, or not rhythmic, periodic or pseudo periodic. We also study their magnitude, and their reactivity to different stimuli (sensory, hyperpnoea that is to say with the respiratory movements amplified …).

Taken together, these data still face the statement of symptoms and clinical examination of the patient, can formulate diagnostic hypotheses topographical (site of injury) and etiological (cause symptoms).

Which indications?

The EEG is a valuable aid in the diagnosis of many diseases of the central nervous system.

It remains an irreplaceable examination for the diagnosis and treatment of epilepsy. It provides important information in other central nervous system pathologies (trauma, tumor, vascular, infectious, and metabolic), epilepsy, sleep disorders, encephalitis, meningitis, confusion disorders, dementia

A number of its former markings have disappeared with the advent of modern medical imaging of the nervous system (or CT scan, magnetic resonance imaging or MRI).

- Epilepsy

In epilepsy, the EEG are different depending on the recording takes place during a crisis (ictal EEG) or outside attacks (EEG inter criticism). The diagnosis of epilepsy is primarily a diagnostic examination, EEG may help confirm the diagnosis and topographic rarely etiological (causal) household favorable to the development of epilepsy.

- Other neurological diseases

The EEG can be used at the balance sheet of some head injuries, stroke, phlebitis brain, encephalitis ….

It is also used in the ICU to confirm the “brain dead” patients who are still breathing with the resuscitation.

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