Electroencephalogram (EEG): Procedure and Preparation
What is an EEG?
An EEG is a non-invasive test that records the electrical activity of your brain using electrodes placed on your scalp, typically lasting 20-30 minutes, and requires minimal preparation beyond staying still during the recording. 1, 2
Standard Procedure Details
Equipment and Setup
- 19 electrodes are placed on your scalp according to the 10-20 International System for diagnostic purposes 2
- At bedside or in emergency settings, a minimum of 8 electrodes may be used 3
- Electrodes are attached with paste or gel that will need to be washed out of your hair afterward 4
During the Recording
- You will be asked to remain still with eyes open or closed as instructed 4
- The recording includes:
- You may be asked to perform simple tasks during the test 4
- Staying still is crucial for obtaining clear recordings 4
Duration
- Standard diagnostic EEG: 20-30 minutes 1, 2, 3
- Ambulatory EEG (at home): Average 32 hours (range 15-96 hours) 5
- Continuous monitoring in ICU: Variable, depending on clinical need 2
Patient Preparation
Before the Test
- No special fasting or medication changes are typically required (unless specifically instructed by your physician)
- Wash your hair the night before but avoid hair products (oils, gels, sprays) that may interfere with electrode placement
- Continue taking your regular medications unless told otherwise 3
What to Expect
- The test is completely painless and non-invasive 4
- You will not feel any electrical sensations 4
- The electrodes only record brain activity; they do not deliver any electrical current
After the Test
- You will need to wash your hair to remove electrode paste 4
- Results require analysis by a specialist before being reported to your physician 4
- You can typically resume all normal activities immediately
Clinical Context: When EEG is Ordered
Primary Indications
- First unprovoked seizure - EEG is recommended as part of standard neurodiagnostic evaluation 1, 2
- Known epilepsy - to characterize seizure type, frequency, or localization 5
- Altered consciousness or confusion - to detect nonconvulsive status epilepticus 1, 6, 3
- Unexplained neurological deficits - particularly in ICU patients 2
Timing Considerations
- Early EEG (within 16 hours of seizure) yields higher detection rates of epileptiform abnormalities (52.1% vs 20.2% after 16 hours) 2
- Within 48 hours of seizure event is statistically more likely to show epileptiform abnormalities 7
- For persistent altered consciousness after seizure, emergent EEG should be performed 1, 6
Diagnostic Yield
Expected Results
- Approximately 48% of routine EEGs are normal 7
- 8.9% demonstrate epileptiform abnormalities 7
- 72% of ambulatory EEGs provide clinically useful information when patients are carefully selected 5
Limitations
- A normal EEG does not exclude epilepsy or other brain disorders 2
- Routine EEG may miss approximately 50% of nonconvulsive seizures compared to prolonged monitoring 2
- EEG is nonspecific and may be influenced by metabolic disturbances (hyponatremia) or drugs 1
Special Populations
Pediatric Patients
- Use simple language for preschool children: "We're going to take special pictures of your brain" 4
- For school-age children, use analogies: "like taking a photograph of your brain's activity" 4
- Positive reinforcement and rewards may help with cooperation 4
ICU/Emergency Patients
- Continuous EEG monitoring is preferred over routine EEG for detecting nonconvulsive seizures 2
- Average response time for emergent EEG is approximately 3 hours 2
- Neurologic consultation before obtaining EEG is reasonable given specialized interpretation requirements 2