Electroencephalography/Evoked Potential (EEG) Laboratory

• Thoru Yamada, MD (Director)
• Mark Granner, MD
• Jon M. Tippin, MD
• Malcolm Yeh, MD
• Mary Ann Werz, MD

Fellows:

• Robert Gilbert, MD

Services:

• Electroencephalography (EEG)
• Evoked Potential (EP)
• Intra-operative Neurophysiological monitoring
• Bedside EEG monitoring

Additional Information:

• The EEG: Electroencephalogram Test

I. Electroencephalography (EEG)

As a neurological diagnostic test, EEG has more than 60 years history and has evolved from analog EEG recording to the current digital recording, having advanced computerized data analysis system. EEG is recorded from the multiple electrodes placed over the scalp. Most common diagnostic utility of EEG is to establish diagnosis of seizure or seizure type, or spells (paroxysmal disorder) of unknown etiology. EEG test may be performed for many other neurological disorders such as brain tumor, stroke, encephalitis, encephalopathy, degenerative or demyelinating disease, etc.

EEG is recorded from multiple electrodes placed from the scalp. The recording is done while the patient is on the bed or in the reclining chair. The patient is asked to relax with eyes closed and encouraged to sleep during the recording. The EEG technologist performs the test and keeps eyes on the recording that appears on the video screen throughout the testing time. The preparation time for EEG record is 30 minutes and recording time is about 30 to 60 minutes. The patient may be sedated if indicated or needed to obtain sleep record.

II. Evoked Potentials (EPs)

The EP is the electrical response recorded from brain, spinal cord or peripheral nerve evoked by various external stimuli, such as visual, auditory or somatosensory stimulation. The recording electrodes are placed over the scalp, neck or spine surface, which vary depending on the type of stimulus modality to be tested.

1. Visual Evoked Potential (VEP)
   This test examines the integrity of visual pathway from retina to occipital cortex where visual input is perceived in the brain. During the test, the patient is asked to watch the video screen, which present moving checkerboard patterns. If the patient wears glasses, VEP should be tested with the best corrected glasses. Each eye is tested separately. The preparation for VEP takes about 20 minutes and recording time of 30 minutes. VEP may be tested for patient with suspected diagnosis of multiple sclerosis, with complaint of visual disturbance, or with suspected lesion involving visual pathway.
2. Brainstem Auditory Evoked Potential (BAEP)
   This test examines the integrity of auditory pathway through the brainstem. The sound enters ear canal and stimulates auditory nerve. The electrical impulse travels from auditory nerve through the brainstem to auditory cortex. During testing, the patient hears the repetitive click sound through the earphone. Each ear is tested separately. The preparation time for BAEP is 20 and recording time is about 30 minutes. BAEP may be tested for patients with hearing problem, dizziness or any lesion involving brainstem. BAEP may also be examined in patient s with suspected diagnosis of multiple sclerosis.
3. Somatosensory Evoked Potential (SEP)
   This test examines the sensory system from the peripheral nerve to the sensory cortex of brain. The weak electrical stimulations are applied to the peripheral nerve, for example median or ulnar nerve for upper extremity study and tibial nerve for lower extremity study. The patient feels tingling sensation with the electrical stimulation applied to the nerve but usually not painful. The preparation time for SEP is 30 minutes and recording time is 30 to 60 minutes. In some patients, both upper and lower extremity SEPs are tested. SEP may be tested in patients with numbness or weakness of arm or leg, or with suspected lesion in spinal cord or peripheral nerve. SEP is also common diagnostic test for multiple sclerosis.

III. Neurophysiological Monitoring during Surgery (Intra-operative Monitoring or - IOM)

Patient who undergoes brain, spinal cord or spine surgery may have potential risk for damaging the nervous system. In order to avoid neurological damage to peripheral nerve, spinal cord, brainstem or brain during surgery, various neurophysiological testing have been developed. In our laboratory, all operating rooms have Internet connection for transmitting EEG or EP data from operating room to review station in the EEG laboratories.

1. EEG Monitoring
   EEG monitoring is done most commonly during surgery of carotid artery, for example, carotid endarterectomy. EEG is a sensitive tool to reflect brain ischemia during cross clamping of the carotid artery. If EEG changes after cross clamping of the artery, shunt placement is required to restore the blood circulation to the brain and to avoid ischemic damage to the brain.
2. Brainstem Auditory Evoked Potential Monitoring (BAEP)
   BAEP monitoring is requested by surgeons for surgery of brainstem, for example, removal of acoustic tumor, vascular decompression of trigeminal nerve for trigeminal neuralgia or vascular decompression of facial nerves for facial spasms. Auditory nerve or brainstem is at risk during these surgeries. Monitoring BAEP helps to identify potential risk and to prevent permanent damage to auditory nerve or brainstem.
3. Somatosensory Evoked Potential Monitoring (SEP Monitoring)
   SEP monitoring is used for testing spinal cord function for surgeries of spine or spinal cord such as scoliosis, laminectomy, spine fusion or spinal cord tumor surgeries, etc. For cervical spine or cervical cord surgery, upper extremity SEP is usually tested by stimulation median or ulnar nerve. For thoracis/lumbar spine or cord surgery, posterior tibial nerve is stimulated. Changes of SEP during surgery will provide warning signs to surgeons before permanent spinal cord damage occurs.
4. Motor Evoked Potential Monitoring (MEP monitoring)

Indication of MEP monitoring is similar to those for SEP monitoring. Although SEP monitoring is usually sufficient to protect spinal cord damage, there have been cases in which SEP remained unchanged during surgery, but the patient ended up with motor deficit. This is because SEP deals only with sensory system but not motor system. Ideal monitoring for spinal cord function is to combine both SEP and MEP monitoring. MEP is performed by electrically stimulating brain via electrodes placed over the scalp (same electrodes with EEG recording). High stimulus intensity current is painful if applied during awake but the patient will not perceive any pain during anesthesia. Responses are recorded from spinal cord or muscles. MEP monitoring may be avoided in patients who has history of epilepsy or potential seizure risk secondary to various brain diseases or patient who has implanted metallic device in the brain.

IV. Continuous Bedside EEG Monitoring

Utilizing the characteristic of EEG, which reflects sensitivity and dynamically the level of consciousness, continuous EEG recording at patient’s bedside provides information about progress and prognosis of brain disease process in acutely ill comatose patients. Continuous bedside EEG monitoring is also useful for patients having intermittent seizures, which may not always be clinically visible. Review of long term EEG recording can be effectively achieved by computerized EEG data analysis.