Overview of EEG

[dropcop]E[/dropcap]lectroencephalography (EEG) is the measurement of electrical activity produced by the brain, as recorded from electrodes placed on the scalp. EEG is used in both clinical and research settings.  In the clinical setting, it can be used as a diagnostic tool for sleep disorders, tumor, encephalopathy, epilepsy, coma, brain death, etc.  EEG is the basis for recording evoked potentials, and has several other important uses in IONM, which are described later.

Because brain activity produces electrical potentials, the scalp electrodes can record this activity, which is amplified and displayed on a computer screen.  EEG is continuously recorded throughout the stages of surgery that pose risk, but a snapshot of a computer screen during a recording might look like this:

EEG recording

Sample snapshot of an EEG recording. The wavy lines represent fluctuations in the summed electrical activity of many, many neurons in close proximity to the recording site.

Electrode positions are standardized and usually adhere to the international 10-20 system.  The “10” and “20” refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull (Figures A and B).

The international 10-20 system seen from (A) left and (B) above the head.

Each recording site is identified by a letter and number combination.  The letter tells you about the region of the brain or recording site. F=Frontal, Fp=Frontal Pole, C=Central, P=Parietal, O=Occipital, T=Temporal, A=Ear Lobe, Pg=Pharyngeal, z=Zero (midline). Odd numbers are on the left side of the head, and even numbers are on the right.  You may also sometimes see something like this Cp3, CP3 or C3′ which can either stand for C3 prime or Central-Parietal 3 to indicate that the electrode is positioned midway between the standard C3 and P3 recording sites (Figure C).  The number of recording sites used depends on the needs of the clinician.  Topographic specificity increases with the number of electrodes placed.

(C) Location and nomenclature of the intermediate 10% electrodes, as standardized by the American Electroencephalographic Society.

(C) Location and nomenclature of the intermediate 10% electrodes, as standardized by the American Electroencephalographic Society.

Whenever we describe an EEG recording, the activity is best described by including information from all of the following categories:

  • Location:  examples include Frontal, Generalized, Multifocal, etc.
  • Amplitude: examples include High Voltage, 50 µV sharp wave, etc.
  • Morphology: examples include sinusoidal, spike, etc.
  • Frequency: described as delta (0 to <4 Hz), theta (4 to <8 Hz), alpha (8-13 Hz) and beta (>13 Hz).
  • Rhythmicity: examples include rhythmic delta, irregular theta, etc.
  • Continuity: intermittent theta, continuous slowing, periodic sharp wave, etc.
  • Clinical State: asleep, awake, seizure, etc.

Different patterns of EEG in different areas of the brain are associated with arousal, inattention, drowsiness, different stages of sleep, presence of different types of drugs, and a whole host of clinical states. Generally speaking, the following elements are commonly observed when analyzing EEG in wakefulness and sleep:

  • Wakefulness:
    • Alpha rhythm is present over the parietal regions when eyes are closed (aka, posterior rhythm).
    • Beta activity is most dominant anteriorl and decreased posteriorly (aka, anteroposterior gradient).
  • Drowsiness:
    • The posterior rhythm slows, then disappears and is replaced by theta waves.
  • Sleep:
    • Appearance of vertex sharp waves, K-complexes, sleep spindles.
    • Delta waves become more dominant as you fall deeper into sleep.

By now you are probably wondering what is normal, and what is abnormal in EEG.  The answer to this question could fill a very large book and, to a certain extent, normality is something that is both context-specific and patient-specific.  The easiest way to answer that very big question is to say that the following elements are examples of what could be considered abnormal in any patient/context in which they are not expected:

  • Asymmetry (voltage, frequency, rhythm, etc.)
  • Slowing (focal, generalized, diffuse)
  • Presence of epileptiform activity
  • Continuous or intermittent slow waves.


Interpretation of EEG data takes many years of training and practice.  We haven’t even scratched the surface of this topic.  All of the information that’s I’ve presented thus far is meant  to give the reader a very basic idea of what EEG is, how it is recorded, what it looks like, and how it can be used to analyze brain activity.  Actually learning to read EEG must come from a combination of didactic training and clinical practice.

How we view it:

There are many different ways to view EEG, and two are quite common to surgery.  The first way is called unprocessed, or raw EEG. In this view, the X-axis is either paper speed (mm/sec) or time (sec/division), and the Y-axis is voltage.  Raw EEG gives nearly immediate feedback and requires no averaging, so it has has excellent temporal resolution. The problem is that trends over time are difficult to evaluate on a computer screen, particularly as they relate to changes in frequency. The second way to view EEG is called processed, or spectral EEG. It is called processed because the computer analyzes the waveform and determines how much of the waveform falls into certain frequency divisions.  In this view, the X-axis is frequency and the Y-axis is time.  The major benefit of the spectral view is that it shows you how EEG frequencies change over time.

Raw EEG (above) and Spectral EEG (below).

Raw EEG (above) and Spectral EEG (below).

Why we use it:

1.  Monitoring Cerebral Blood Flow (CBF).

Sometimes surgery carries risk for cerebral ischemia, which can result in permanent loss of brain function due to anoxia-related cell death.  Multi-channnel scalp EEG is particularly sensitive to reduced blood flow. The brain can tolerate minor reductions in cerebral blood flow fairly well; however, when CBF falls below a functional threshold, changes in EEG activity are observed. First, you may see a decrease in EEG amplitude and/or EEG slowing. Continued reduction in CBF may lead to flattening of EEG activity.

2. Identify Seizure Locus

Seizure activity is extremely rare during surgery anesthetic agents (like Propofol) which have some anticonvulsant properties. Also, if the patient has epilepsy or is at risk for seizure, anticonvulsant medications are usually administered prior to surgery. Sometimes, the purpose of the surgery is to remove cortical tissue that is generating the epileptic activity. In these cases, seizure activity is facilitated and commonly recorded by electrodes placed on the surface of the cortex (grid arrays) or deep within cortical or subcortical structures (depth electrodes).

3. Evaluate depth of hypnosis and interpret changes in evoked potentials.

When high doses of anesthetic drugs are administered, cortical brain activity can be dramatically reduced and EEG activity reflects this change. As a consequence of reduced cortical activity, evoked potentials (SSEPs, VEPs, MEPs) can change. There are so many reasons why evoked potentials can change. Is the patient having a stroke? Is there a positioning issue? Is the change just related to the heavy hand of the CRNA? There are lots of ways to answer these questions, and one common approach is for the neurophysiologist to record just 2 EEG channels (one over each hemisphere) to evaluate generalized cortical excitability over time. This simply helps to correlate changes in evoked potentials.  It cannot tell you whether the patient is experiencing awareness, pain, etc.

Benefits of EEG in Surgery: 

  • Noninvasive
  • Useful to evaluate gross function of specific regions of cortex.
  • High temporal resolution (milliseconds)
  • High sensitivity and specificity for adequacy of cerebral blood flow.
  • Can identify locus of seizure activity.
  • Tells you about generalized cortical excitability.
  • Can be informative to anesthesia regarding depth of hypnosis (not awareness), but this is controversial.
  • Can guide neuroprotective burst suppression.
  • Helps interpret changes in evoked potentials.

Limitations of EEG in Surgery: 

  • Limited spatial resolution.
  • Lengthy set-up, depending on the number and type of electrodes.
  • Insensitive to subcortical changes in blood flow.
  • Cannot monitor surgical site because electrodes must be moved to accommodate incision and retraction.
  • Not a measure of motor or sensory function.
  • Inadequate for depth of anesthesia (controversial as noted above).
  • Cannot tell if patient is experiencing awareness.
  • Cannot tell if patient is experiencing pain.
  • Saturated by electrocautery

Surgical Procedures for which scalp EEG is indicated (>8 channels):

Intracranial Tumor Surgery:

  • Endoscopic, transnasal, transsphenoidal approach for tumor resection.

Intracranial Vascular Surgery:

  • Aneurysm clipping.
  • Arteriovenous malformation (AVM) repair.
  • Aneurysm coiling (interventional radiology; IR)

Peripheral Vascular Surgery:

  • Carotid endarterectomy (CEA).
  • Open heart total/partial bypass.
  • Aorta (ascending, descending, arch aneurysm/repair).

Surgical Procedures for which scalp EEG is indicated (<8 channels):

As noted above, EEG is frequently used to correlate changes in evoked potentials with anesthetic depth. Therefore, 2-4 channel EEG can be used in any surgical procedure during which evoked potentials are recorded.


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