electroencephalography

Electroencephalography (EEG) is a well established diagnostic tool with a long history outside the operating room primarily in diagnosing and treating seizure disorders (epilepsy). EEG measures at the level of the scalp the summated spontaneous electrical signals generated in the brain by excitatory and inhibitory post synaptic potentials.

Uses:

EEG has been adopted for intraoperative use over the past 2 decades. It is employed to measure depth of coma or anesthetic effect and to look for new focal areas cerebral dysfunction or seizure activity. These functions are useful in surgical  procedures involving barbiturate coma, deliberate hypothermia or hypotension (e.g. hypothermic circulatory arrest), surgery for seizures and vascular procedures such as carotid endarterectomies (CEA),  intracranial aneurysm treatment and arterio-venous malformation treatment.  

Technique:

Although intraoperative EEG uses the same recording principles and international 10-20 system for electrode placement,  it differs from diagnostic EEG in that:

• In the operating room the patient is either awake and anxious with muscle artifact (pre-anesthetic) or under general anesthesia, compared to awake and relaxed or sleeping in the diagnostic setting
• There is considerably more extraneous noise in the operating room than in the diagnostic laboratory complicating interpretation
• Fewer channels of recording are often available in the operating room due to equipment limitations

  Current use of intraoperative EEG may include several real time quantitative or algorithmic analysis of the EEG waveforms including time-domain analysis and frequency domain analysis. These allow the EEG to be displayed in a non-traditional graphical style such as compressed spectral array (CSA)(a sequential plotting of spectral power in a stack) and density spectral array (DSA) (a similar stacked color coded display of spectral power). Spectral edge displays show the frequency below which a certain percentage (usually 95-97%) of EEG spectral power lies. A downward shift in the spectral edge suggests a move to predominantly lower frequencies.

Upside

• well established and internationally used recording technique
• offers relatively full cortical coverage depending upon number of electrodes used

Downside

• sensitive to anesthetic effect, which may attenuate predictive value in vascular cases (e.g. CEA)
• requires a different EEG reading experience than for typical diagnostic EEG
• insensitive to most subcortical surgical events

Last Word on EEG

EEG is a well supported monitoring modality that is extremely useful for monitoring vascular and epilepsy surgeries and for gauging anesthetic and hypothermic/hypotensive effect

R. O'Brien MD

Selected References:

Blume, W, Ferguson, G, McNeill D. Significance of EEG changes at carotid endarterectomy. 1986;Stroke 17:891

Redekop G, Ferguson G. Correlation or contralateral stenosis and intraoperatrive electromencephalographic change with risk of stroke during carotid endarterectomy. Neurosurgery 1992;30:191

Sundi T, Sharbrough F, Anderson R, Michenfelder J. Cerebral blood flow measurements and electroencephalograms during carotid endarterectomy. Electroencephalogr Clin Neurophysiol 1973;34-61