EEG Basics and IOM

EEG Basics and IOM

EEG Basics and IOM

The EEG is generated from the difference in voltage between any two electrodes which are secured on the patient’s scalp according to the International 10-20 System.  The EEG is recorded from different pairs of these electrodes termed derivations and groups of these derivations are referred to as the montage., e.g. double banana montage

Examples of Bipolar (A) and Monopolar (B) montages in reference to recording site.

Bipolar (A) and Monopolar (B) Montages

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Examples of alternatives for EEG Location Recording

EEG Location Recording

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EEG patterns in a normal and awake patient:

During eye opening and eye closing, there will be the appearance of alpha rhythm in the occipital area and disappearance of the alpha activity in the occipital area, respectively.

EEG patterns - eyes open and eyes closed

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During mental activity, the EEG will show low amplitude, high-frequency beta rhythm

The lower frequencies such as delta or theta are not seen in the normal awake patient

EEG During Mental Activity

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There are four distinct EEG wave frequencies:

  1. Delta (0 – 4 Hz) Delta and Theta occur in pathological states except for sleep.
  2. Theta (4 – 8 Hz)
  3. Alpha (8 – 13 Hz) Relaxed and eyes closed
  4. Beta (13 – 30 Hz ) Awake and alert

EEG Wave Frequencies

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Some Intraoperative uses of EEG:

  1. CEA
  2. C-P bypass
  3. Burst suppression
  4. Depth of anesthesia

Basic analysis of EEG (Frequency, Amplitude, and Symmetry):

EEG Basic Anaylsis

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Refers to the number of times a specific band or type of wave occurs in 1 second time interval and is referred to in Hertz (Hz).

The frequency of the EEG waves seen ranges according to the established four specific bands:  delta, theta, alpha and beta with a Hertz range from 0 – 30 Hz.


Refers to the height of the EEG waves and is measured in microvolts (uv)

For example, a low amplitude EEG may be in the range of 20 uv and a high amplitude EEG would be greater than 50 uv.

Lower amplitude, lower frequencies may indicate episodes of ischemia for example in CEA surgery during clamping.


Refers to the appearance of the EEG waveforms; similarity or symmetry in the two cerebral hemispheres.

Various anesthetic agents will affect the hemispheric symmetry.

Clamping during  CEA surgery can affect the symmetry between the two hemispheres.

The EEG signal is very low voltage, to begin with, and it is also contaminated with various noise artifacts. To reduce this noise artifact and allow the EEG signal to pass through intraoperative monitoring machines currently, have a differential amplifier.  The differential amplifier has two inputs designated as positive (+) and negative(–) and this will cancel signals that are common to both inputs (Common Mode Rejection-  CMR) and will only amplify the difference in the voltage between the two inputs.

Frequency Analysis of the raw EEG

The EEG waveforms are actually a summation of waveforms of different frequencies and to readily break down the EEG into all of its frequencies (Fast Fourier Transform)  is used and then converts these frequencies into a digital number which will reflect the predominant frequency spectrum for each of the samples in a specific period of time.  This frequency is then displayed in the Compressed Spectral Array (CSA)  on the IOM machine as the most common frequency or the Spectral Edge Frequency (SEF).

Other topics to follow:

  1. EEG and anesthesia
  2. EEG and cerebral ischemia
  3. Various types of ischemia and EEG changes