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VNS Surgery

If you (or your loved one) have had a vagus nerve stimulator implanted, what happened after it?

no more seizures
5% (11 votes)
helped control seizures a lot
34% (76 votes)
helped a little bit
35% (78 votes)
didn’t help at all
17% (37 votes)
worse off
10% (22 votes)
Total votes: 224

View results
View past poll results

How to Read an EEG

How are the electrodes arranged?

The electrodes are placed on the scalp by an EEG technician according to internationally agreed-upon criteria. In order to locate the exact electrode position, this system uses four anatomical landmarks from which measurements can be made. The nasion is the indentation between the forehead and the nose, the inion is a ridge that can be felt at the midline of the back of the skull, over the occipital area, and the preauricular points are defined as the indentations just above the cartilage that covers the external ear openings. The electrode locations and distances between the electrodes are then defined as 10% or 20% of these anatomical distances. Occasionally, additional electrodes (sphenoidal and suboccipital, for instance) are placed to enhance the likelihood of detecting and determining the location of EEG waves that may be too small or too deep in the brain to be detected by routinely placed electrodes.

What do the letters and numbers mean?

The names of the electrode sites use alphabetical abbreviations that identify the lobe or area of the brain to which each electrode refers:
F = frontal
Fp = frontopolar
T = temporal
C = central
P = parietal
O = occipital
A = auricular (ear electrode).

The localization of the brain waves within the brain regions or lobes is further narrowed by adding electrodes, which are given numbers such as T3, T4, P3, P4. Even numbers identify electrode positions on the right side of the head, and odd numbers refer to the left side. The label "z" points to electrode sites in the midline of the head. For example, Cz refers to the midline central region of the head.

Often an EKG (electrocardiogram), a record of the heartbeat, also appears on the EEG printout.

How are the electrodes related to each other?

The EEG records brain waves from different amplifiers using various combinations of electrodes called montages. In bipolar montages, consecutive pairs of electrodes are linked by connecting the electrode input 2 of one channel to input 1 of the subsequent channel, so that adjacent channels have one electrode in common. The bipolar chains of electrodes may be connected going from front to back (longitudinal) or from left to right (transverse).

Another type of montage is the referential montage. In this type, various electrodes are connected to input 1 of each amplifier and a reference electrode is connected to input 2 of each amplifier. Ideally, inactive electrodes (ones that are uninvolved in the electrical field being studied) are chosen as references.

What does the doctor learn from looking at the EEG?

Being able to locate the origin of electrical activity ("localization") is critical to being able to interpret the EEG tracings meaningfully. Localization of normal or abnormal brain waves in bipolar montages is usually accomplished by identifying "phase reversal," a deflection of the two channels within a chain pointing to opposite directions. (See sample.) In a referential montage, all channels may show deflections in the same direction. If the electrical activity at the active electrodes is positive when compared to the activity at the reference electrode, the deflection will be downward. Electrodes where the electrical activity is the same as at the reference electrode will not show any deflection. In general, the electrode with the largest upward deflection represents the maximum negative activity in a referential montage.

Topic Editor: Steven C. Schachter, M.D.
Last Reviewed: 12/15/06


This content is user-generated. Content is not monitored nor consistently reviewed by the epilepsy.com Editorial Board. Epilepsy.com therefore cannot guarantee the accuracy of any content edited with the Wiki sections. While epilepsy.com, the Epilepsy Therapy Project, and its partners encourage visitor interaction and publishing within these sections, users should use caution when exploring content, especially as it pertains to health concerns. No content on epilepsy.com is intended to replace the care of a doctor. We encourage you to contact your own health care provider for individual medical advice. We cannot provide second opinions or make specific recommendations regarding therapy, nor does this Wiki content constitute a recommendation for any diagnosis or treatment options.

How are the electrodes arranged?

The electrodes are placed on the scalp by an EEG technician according to internationally agreed-upon criteria. In order to locate the exact electrode position, this system uses four anatomical landmarks from which measurements can be made. The nasion is the indentation between the forehead and the nose, the inion is a ridge that can be felt at the midline of the back of the skull, over the occipital area, and the preauricular points are defined as the indentations just above the cartilage that covers the external ear openings. The electrode locations and distances between the electrodes are then defined as 10% or 20% of these anatomical distances. Occasionally, additional electrodes (sphenoidal and suboccipital, for instance) are placed to enhance the likelihood of detecting and determining the location of EEG waves that may be too small or too deep in the brain to be detected by routinely placed electrodes.

What do the letters and numbers mean?

The names of the electrode sites use alphabetical abbreviations that identify the lobe or area of the brain to which each electrode refers:
F = frontal
Fp = frontopolar
T = temporal
C = central
P = parietal
O = occipital
A = auricular (ear electrode).

The localization of the brain waves within the brain regions or lobes is further narrowed by adding electrodes, which are given numbers such as T3, T4, P3, P4. Even numbers identify electrode positions on the right side of the head, and odd numbers refer to the left side. The label "z" points to electrode sites in the midline of the head. For example, Cz refers to the midline central region of the head.

Often an EKG (electrocardiogram), a record of the heartbeat, also appears on the EEG printout.

How are the electrodes related to each other?

The EEG records brain waves from different amplifiers using various combinations of electrodes called montages. In bipolar montages, consecutive pairs of electrodes are linked by connecting the electrode input 2 of one channel to input 1 of the subsequent channel, so that adjacent channels have one electrode in common. The bipolar chains of electrodes may be connected going from front to back (longitudinal) or from left to right (transverse).

Another type of montage is the referential montage. In this type, various electrodes are connected to input 1 of each amplifier and a reference electrode is connected to input 2 of each amplifier. Ideally, inactive electrodes (ones that are uninvolved in the electrical field being studied) are chosen as references.

What does the doctor learn from looking at the EEG?

Being able to locate the origin of electrical activity ("localization") is critical to being able to interpret the EEG tracings meaningfully. Localization of normal or abnormal brain waves in bipolar montages is usually accomplished by identifying "phase reversal," a deflection of the two channels within a chain pointing to opposite directions. (See sample.) In a referential montage, all channels may show deflections in the same direction. If the electrical activity at the active electrodes is positive when compared to the activity at the reference electrode, the deflection will be downward. Electrodes where the electrical activity is the same as at the reference electrode will not show any deflection. In general, the electrode with the largest upward deflection represents the maximum negative activity in a referential montage.




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