EEG Quantitative EEG (qEEG) and Trend Analysis — Questions and Answers
Question 1: A compressed spectral array (CSA) displays EEG information by showing:
- Raw EEG waveforms compressed to fit on a single page per hour
- Power spectral density plotted over time, with frequency on one axis and time on the other (Correct answer)
- Spike-and-wave discharge counts per epoch displayed as a histogram
- Amplitude envelope of the raw EEG plotted as a single line per channel
Correct answer: Power spectral density plotted over time, with frequency on one axis and time on the other
A CSA (also called a density spectral array or DSA depending on color coding) uses Fast Fourier Transform (FFT) to decompose the EEG into its component frequencies and plots the power at each frequency band over time. This allows hours of EEG to be reviewed at a glance for trends in frequency content, making it useful for detecting delayed cerebral ischemia, seizures, and drug effects.
Question 2: Amplitude-integrated EEG (aEEG) is most commonly used in which clinical setting?
- Adult outpatient epilepsy monitoring units for seizure localization
- Neonatal intensive care units for continuous cerebral monitoring of high-risk newborns (Correct answer)
- Intraoperative monitoring during spinal cord surgery
- Outpatient ambulatory studies in adults with suspected absence epilepsy
Correct answer: Neonatal intensive care units for continuous cerebral monitoring of high-risk newborns
aEEG was developed specifically for continuous cerebral function monitoring in neonates. It compresses and amplitude-integrates the EEG into a single trend channel, making it practical for bedside nurses and neonatologists to monitor brain activity, detect seizures, and assess background continuity in premature and term newborns with hypoxic-ischemic encephalopathy.
Question 3: On a qEEG trend panel, a sudden increase in the seizure probability (SP) line coincides with a visible change on the rhythmic run detector. The NEXT step for the EEG technologist should be:
- Immediately notify the physician that the patient is in status epilepticus
- Review the corresponding raw EEG segment to confirm whether the trend reflects a true electrographic seizure or artifact (Correct answer)
- Increase the high-frequency filter setting to reduce muscle artifact
- Decrease the epoch length to improve temporal resolution of the trend
Correct answer: Review the corresponding raw EEG segment to confirm whether the trend reflects a true electrographic seizure or artifact
qEEG trend algorithms are sensitive but not specific — they detect rhythmic and high-amplitude events but cannot distinguish between a true electrographic seizure and chewing artifact, electrode pop, or patient movement. The mandatory next step is always to review the raw EEG underlying the flagged epoch before any clinical action is taken.
Question 4: On a density spectral array (DSA) in a patient following subarachnoid hemorrhage, which finding would raise concern for delayed cerebral ischemia (DCI)?
- Stable alpha-frequency power bilaterally over 72 hours
- Progressive loss of alpha and sigma power with increase in delta power over the affected hemisphere (Correct answer)
- Intermittent increase in beta power bilaterally following sedation boluses
- Appearance of sleep spindles bilaterally during nighttime hours
Correct answer: Progressive loss of alpha and sigma power with increase in delta power over the affected hemisphere
DCI produces neuronal dysfunction that manifests on the DSA as a loss of faster frequencies (alpha 8–13 Hz, sigma 12–15 Hz) and an increase in slow delta power over the at-risk hemisphere. This qEEG signature can precede clinical deterioration by hours, making continuous trend monitoring valuable for early DCI detection in subarachnoid hemorrhage patients.
Question 5: Spectral edge frequency (SEF95) is defined as the frequency below which what percentage of the total EEG spectral power is contained?
- 50%
- 75%
- 90%
- 95% (Correct answer)
Correct answer: 95%
SEF95 is the frequency at or below which 95% of the total spectral power lies. It is a single-number summary of the 'upper edge' of EEG activity and is used in anesthesia monitoring and ICU trend analysis to track shifts in dominant frequencies over time. A falling SEF95 indicates slowing of brain activity.
Question 6: A patient's qEEG panel shows the asymmetry index trending toward the right hemisphere for 45 minutes. This means:
- Seizure activity is more likely occurring over the right hemisphere
- The right hemisphere has relatively more power (amplitude) than the left during this period (Correct answer)
- The left hemisphere's EEG is being contaminated by right-sided muscle artifact
- Electrode impedance is higher on the right side, inflating apparent amplitude
Correct answer: The right hemisphere has relatively more power (amplitude) than the left during this period
An asymmetry index that trends toward one hemisphere indicates that hemisphere has proportionally greater spectral power. Persistent rightward asymmetry over 45 minutes could reflect focal seizure activity, cortical spreading depression, or a structural lesion increasing amplitude on that side. It triggers review of the raw EEG to determine the cause, but by definition it means relatively higher power on the right.
EEG Quantitative EEG (qEEG) and Trend Analysis
A compressed spectral array (CSA) displays EEG information by showing: