Residency · Residency · Neurology

EEG Interpretation Fundamentals

Overview

Electroencephalography (EEG) records the summated postsynaptic potentials of cortical pyramidal neurons, providing a real-time window into brain electrical activity. EEG is essential for epilepsy diagnosis, classification, and monitoring, as well as for neuromonitoring in the intensive care unit. Systematic interpretation requires an understanding of normal patterns, benign variants that mimic pathology, true epileptiform discharges, and common artifacts. Continuous EEG monitoring has become increasingly standard in the neurological ICU for detecting nonconvulsive seizures and guiding management.

Technical Fundamentals

Signal Generation

The EEG primarily reflects synchronous excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) of cortical pyramidal cells oriented perpendicular to the cortical surface. The scalp EEG can only detect activity from at least 6 square centimeters of synchronously active cortex, meaning that small or deep generators may not produce a visible scalp signal. Deeper structures such as the hippocampus and thalamus are not directly recorded but exert important influences on cortical rhythms.

Montages

A bipolar montage displays each channel as the voltage difference between two adjacent electrodes. It is particularly useful for localizing sharp transients because a phase reversal indicates the point of maximum voltage of the discharge. A referential montage compares each electrode to a common reference (often the ears or an average reference) and is superior for assessing true amplitude and polarity. Electrodes are placed according to the 10-20 International System, with standard positions labeled Fp, F, C, P, O, and T. Odd numbers designate left hemisphere positions, even numbers designate the right hemisphere, and "z" denotes the midline.

Frequency Bands

Delta activity (below 4 Hz) dominates deep sleep (stage N3) and is seen diffusely in encephalopathy or focally over structural lesions. Theta activity (4-7 Hz) appears during drowsiness and early sleep stages; focal theta may indicate underlying pathology. Alpha activity (8-13 Hz) constitutes the posterior dominant rhythm (PDR) seen in relaxed wakefulness with eyes closed, and it attenuates with eye opening (alpha blocking). Beta activity (above 13 Hz) predominates frontally and is enhanced by benzodiazepines, barbiturates, and alertness. Gamma activity (above 30 Hz) is less relevant in routine clinical EEG.

Normal Adult EEG

Wakefulness

The posterior dominant rhythm (PDR) is an 8-13 Hz alpha rhythm that is maximal in the occipital regions and should be roughly symmetric (though amplitude may be slightly higher on the right). The PDR must attenuate with eye opening, demonstrating reactivity. A PDR frequency below 8 Hz in an adult suggests diffuse dysfunction or encephalopathy. Some normal adults display a low-voltage fast pattern with low-amplitude beta-dominant activity; this is a recognized normal variant. The mu rhythm is an 8-13 Hz arch-shaped rhythm over the central (rolandic) regions that attenuates with contralateral hand movement or even the thought of movement; it is a benign variant and should not be mistaken for epileptiform activity. Lambda waves are positive sharp transients over the occipital regions that occur during visual scanning and are similarly benign.

Drowsiness and Sleep

During stage N1 (drowsiness), the PDR attenuates and is replaced by diffuse theta activity, slow lateral eye movements appear, and vertex sharp waves emerge with maximum amplitude at Cz. Stage N2 (light sleep) is characterized by sleep spindles (12-14 Hz bursts with central predominance lasting 0.5-2 seconds) and K-complexes (high-amplitude biphasic waves with frontal predominance). Stage N3 (deep sleep) shows continuous high-amplitude delta activity exceeding 75 microvolts at frequencies above 2 Hz, occupying more than 20% of the recording epoch. REM sleep produces low-voltage, mixed-frequency activity resembling wakefulness, accompanied by rapid eye movements and muscle atonia on chin EMG. Positive occipital sharp transients of sleep (POSTS) are bilateral, surface-positive, triangular waveforms in the occipital regions during light sleep and represent a benign variant.

Activation Procedures

Hyperventilation involves 3-5 minutes of deep breathing and produces bilateral high-amplitude delta slowing that is more prominent in younger patients and during hypoglycemia. It may activate absence seizures by producing 3 Hz spike-wave, and the response should be symmetric. Photic stimulation uses a strobe light at varying frequencies; the normal response is a photic driving response where occipital activity follows the flash frequency. An abnormal photoparoxysmal response (generalized spike-wave triggered by photic stimulation) is associated with generalized epilepsy, especially JME. Sleep deprivation increases the yield of epileptiform discharges and is often requested before a routine EEG.

Epileptiform Discharges

Interictal Epileptiform Discharges (IEDs)

Spikes are sharp transients with a duration of 20-70 milliseconds that stand out from the background and are followed by an aftergoing slow wave. Sharp waves have similar morphology but a longer duration of 70-200 milliseconds. IEDs support a diagnosis of epilepsy but are not pathognomonic, as they are found in approximately 0.5% of normal adults. Conversely, the absence of IEDs does not exclude epilepsy; the first routine EEG shows IEDs in only about 50% of epilepsy patients, though sensitivity increases with repeat studies and sleep deprivation.

Localization

Focal spikes and sharp waves localize to the epileptogenic zone. Temporal discharges are most common in adults, while centrotemporal discharges characterize SeLECTS in children. Generalized spike-and-wave (GSW) is bilateral, synchronous, and symmetric, associated with generalized epilepsies. Phase reversal on a bipolar montage indicates the electrode of maximum negativity and is the key technique for localizing a discharge.

Generalized Spike-and-Wave Patterns

The 3 Hz GSW pattern is characteristic of childhood absence epilepsy. A 4-6 Hz polyspike-and-wave pattern is seen in JME. Slow GSW below 2.5 Hz characterizes Lennox-Gastaut syndrome. Fast GSW above 3 Hz may be seen in various generalized epilepsies.

Benign EEG Variants (Not Epileptiform)

Wicket spikes are arc-shaped, 6-11 Hz discharges in the temporal regions of adults over 30; they lack an aftergoing slow wave and are benign. Small sharp spikes (also called benign epileptiform transients of sleep, or BETS) are brief, low-amplitude temporal transients seen in drowsiness. The 14-and-6 Hz positive bursts are positive sharp waves at 14 or 6 Hz in the posterior temporal regions during drowsiness. Rhythmic mid-temporal theta of drowsiness (RMTD, formerly called "psychomotor variant") consists of 5-7 Hz notched theta in the temporal regions during drowsiness. Subclinical rhythmic electrographic discharges of adults (SREDA) produce sudden onset bilateral theta that mimics a seizure but has no clinical correlate. Breach rhythm refers to increased amplitude and sharp-appearing activity recorded over a skull defect (such as a craniotomy site) and results from altered bone conduction rather than epileptiform pathology.

Common Artifacts

Eye movement artifact produces frontal-predominant slow deflections; eye blinks generate deflections at Fp1 and Fp2, while lateral eye movements produce out-of-phase deflections at F7 and F8. Muscle artifact is high-frequency (above 20 Hz), irregular, often temporal, and can obscure underlying activity. ECG artifact appears as regular deflections time-locked to the QRS complex and is best identified by simultaneously recording an ECG channel. Electrode pop produces abrupt, high-amplitude transients restricted to a single electrode and is distinguished from spikes by its morphology and isolation. Sixty-hertz (or 50 Hz) electrical interference appears as uniform sinusoidal activity affecting multiple channels. Sweat artifact produces very slow, undulating baseline drift caused by changes in skin impedance.

Continuous EEG (cEEG) Monitoring in the ICU

Indications

cEEG monitoring is indicated for unexplained altered mental status in critically ill patients, detection of nonconvulsive seizures (NCS) and nonconvulsive status epilepticus (NCSE), monitoring after convulsive status epilepticus to detect ongoing subclinical seizures, evaluation following acute brain injury (traumatic brain injury, SAH, intracerebral hemorrhage, cardiac arrest), monitoring burst suppression during therapeutic barbiturate coma, and ischemia detection in patients at risk for delayed cerebral ischemia after SAH.

Common ICU EEG Patterns

Generalized periodic discharges (GPDs) are repetitive sharp waves or spikes at regular intervals seen in metabolic encephalopathy, anoxic brain injury, Creutzfeldt-Jakob disease, and NCSE. They lie on the "ictal-interictal continuum." Lateralized periodic discharges (LPDs, formerly PLEDs) are periodic discharges confined to one hemisphere, associated with acute structural lesions such as stroke, abscess, or encephalitis, and carry a high seizure risk. Generalized rhythmic delta activity (GRDA) is frontally predominant rhythmic delta that often reflects encephalopathy and may or may not be ictal. Lateralized rhythmic delta activity (LRDA) is focal rhythmic delta that raises concern for seizure risk. Burst suppression shows alternating high-amplitude bursts and periods of suppression below 10 microvolts, seen in deep coma, anesthetic overdose, severe anoxic brain injury, or therapeutic coma. Electrocerebral inactivity shows no identifiable cerebral activity above 2 microvolts and is consistent with brain death as an ancillary test, though confounders such as hypothermia and drug intoxication must be excluded.

<image>A comprehensive EEG reference panel showing labeled representative EEG tracings. Row 1 (Normal patterns): posterior dominant rhythm (alpha), mu rhythm, sleep spindles and K-complexes, vertex sharp waves, and POSTS. Row 2 (Epileptiform discharges): focal temporal sharp wave with aftergoing slow wave and phase reversal on bipolar montage, 3 Hz generalized spike-and-wave (absence), 4-6 Hz polyspike-and-wave (JME), and focal seizure with rhythmic temporal theta evolution. Row 3 (Benign variants): wicket spikes, small sharp spikes, 14-and-6 positive bursts, and RMTD. Row 4 (Artifacts): eye blink, lateral eye movement, muscle artifact, ECG artifact, and electrode pop. Each tracing is annotated with montage type, amplitude scale, and time scale.</image>

<image>An infographic of the 10-20 International Electrode Placement System shown on a schematic head (superior and lateral views). Each electrode position is labeled (Fp1, Fp2, F3, F4, F7, F8, C3, C4, T3/T7, T4/T8, P3, P4, T5/P7, T6/P8, O1, O2, Fz, Cz, Pz). The anatomical correlate (underlying cortical region) is indicated for each position. A bipolar montage chain is illustrated with arrows showing how phase reversal localizes the maximum of a discharge.</image>

<image>An ICU EEG pattern atlas showing representative examples of: (1) lateralized periodic discharges (LPDs) with clinical context (acute stroke), (2) generalized periodic discharges (GPDs) with clinical context (anoxic brain injury), (3) nonconvulsive status epilepticus showing continuous rhythmic epileptiform activity, (4) burst suppression pattern, and (5) electrocerebral inactivity. Each pattern is annotated with the clinical significance, associated conditions, and management implications. A sidebar describes the ictal-interictal continuum concept with a gradient bar from clearly interictal to clearly ictal.</image>

Clinical Pearls

A normal routine EEG does not exclude epilepsy. The sensitivity of a single EEG is only about 50%, and repeat studies with sleep deprivation significantly increase the yield. Phase reversal on a bipolar montage localizes the maximum of an epileptiform discharge and is the most important localizing technique in EEG interpretation. Benign variants such as wicket spikes, BETS, RMTD, and SREDA are commonly misread as epileptiform discharges, leading to overdiagnosis of epilepsy. Breach rhythm over a craniotomy site mimics epileptiform activity, so the surgical history must always be noted. In the ICU, generalized periodic discharges lie on the ictal-interictal continuum and may warrant a trial of intravenous benzodiazepine to determine whether clinical or electrographic improvement occurs. Hyperventilation is the single best activation procedure for childhood absence epilepsy and should always be performed in children being evaluated for staring spells. The mu rhythm is not epileptiform; it attenuates with contralateral movement and represents normal sensorimotor cortex activity.

References

• Tatum WO. Handbook of EEG Interpretation. 3rd ed. Demos Medical; 2021.
• Hirsch LJ, et al. American Clinical Neurophysiology Society's standardized critical care EEG terminology. J Clin Neurophysiol. 2021;38(1):1-29.
• Noachtar S, Remi J. The role of EEG in epilepsy: a critical review. Epilepsy Behav. 2009;15(1):22-33.
• Benbadis SR, Tatum WO. Overinterpretation of EEGs and misdiagnosis of epilepsy. J Clin Neurophysiol. 2003;20(1):42-44.
• Herman ST, et al. Consensus statement on continuous EEG in critically ill adults and children. J Clin Neurophysiol. 2015;32(2):87-95.

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