How Long Does an EEG Take?
A standard diagnostic EEG recording typically takes 20-30 minutes of actual recording time, though the entire appointment may be longer to allow for electrode placement and removal. 1, 2, 3
Standard Diagnostic EEG Duration
- The recommended recording duration is 20-30 minutes to capture variations in vigilance levels and different brain states 1, 2
- A minimum of 20 minutes of artifact-free EEG recording is required by current neurophysiology standards 3, 4
- The recording should include both eyes-closed and eyes-open periods when possible 1
- Standard setup uses 19 electrodes placed according to the 10-20 International System for diagnostic purposes 1, 2
Extended Recording Options
For patients where routine EEG fails to detect abnormalities, longer monitoring durations significantly improve diagnostic yield:
- Extending routine EEG to 40 minutes increases abnormality detection by an additional 11% compared to 20-minute recordings, identifying findings that would otherwise require more expensive long-term monitoring 4
- Two-thirds of epileptiform discharges are detected within the first 30 minutes of video-EEG monitoring 5
- Outpatient video-EEG monitoring (3 hours) detects epileptiform discharges in 54.1% of patients versus only 16.4% with routine 20-minute EEG 5
Ambulatory and Inpatient Monitoring
When capturing actual seizure events is the goal, substantially longer recording periods are necessary:
- For ambulatory home monitoring: 48 hours is optimal for children (capturing 98% of first events), while 72 hours is optimal for adults and geriatric patients (capturing over 97% of first events) 6
- In inpatient video-EEG monitoring, 81% of epileptic seizures occur within the first 24 hours, with an additional 10% in the second 24 hours 7
- 48 hours of inpatient monitoring is generally adequate for detecting target clinical events, with a second peak of seizure occurrence during hours 33-36 5
Bedside/ICU Monitoring
- For critically ill patients, bedside EEG may use a minimum of 8 electrodes rather than the full 19-electrode montage 3
- Duration and need for repeat examinations depend on the specific clinical indication 3
- For monitoring purposes (rather than diagnosis), as few as 4 electrodes may be sufficient 1
Important Caveats
- The actual appointment time will be longer than recording time due to electrode placement (which can take 15-30 minutes) and removal, plus time for hair washing afterward to remove electrode paste 8
- Patients with generalized epilepsy show epileptiform discharges earlier (mean 22.1 minutes) compared to those with focal epilepsy (mean 33.9 minutes) 5
- Approximately 43% of epileptiform abnormalities are only detected during sleep, making longer recordings that capture sleep more diagnostically valuable 4