What are the normal types and abnormalities of EEG (electroencephalogram) waves?

EEG Waves: Normal Types and Abnormalities

Normal EEG consists of five primary frequency bands—delta, theta, alpha, beta, and gamma—each reflecting distinct states of brain arousal and cognitive processing, while abnormalities include pathological slowing, epileptiform discharges, and loss of normal reactivity patterns.

Normal EEG Wave Types

Delta Waves (1-4 Hz)

• Delta activity is the slowest frequency band and represents deep sleep in healthy adults 1, 2
• During normal waking states, delta should be minimal or absent; prominent delta during wakefulness indicates brain dysfunction or metabolic encephalopathy 3, 2
• Delta waves have the highest amplitude of all frequency bands 1

Theta Waves (4-7 Hz)

• Theta activity normally has small amplitude during waking states and increases during drowsiness 2, 4
• Theta (particularly around 7 Hz) is the most dominant oscillation type across lateral central, temporal, and frontal cortical areas during rest 4
• Increased theta during full wakefulness may indicate pathology or metabolic disturbance 2
• Rhythmic temporal theta activity of drowsiness is a normal variant 5

Alpha Waves (8-12 Hz)

• Alpha rhythms are the dominant posterior oscillations during resting state with eyes closed, most prominent in occipital leads 6, 4
• Normal alpha frequency is 8-12 Hz, with dominant activity around 10 Hz reflecting background synchronization of neural networks regulating arousal and consciousness 1
• Alpha shows characteristic amplitude reduction when transitioning from eyes-closed to eyes-open conditions due to visual-cortical activation 6
• Low-frequency alpha (8-10 Hz) reflects low levels of general brain arousal and attention 6, 2
• High-frequency alpha (10-13 Hz) may reflect low levels of perceptual, somatomotor, and memory processes 6, 2
• Alpha is largely limited to parietal and occipital regions in normal individuals 4

Beta Waves (12-30 Hz)

• Beta activity (13-30 Hz) is prominent peri-Rolandically, over middle frontal gyrus, and pars opercularis 4
• Low-frequency beta (12-20 Hz) during rest reflects low levels of somatomotor and memory processes 2
• Beta power density positively correlates with preserved cognitive function 2
• Normal beta activity indicates intact thalamocortical connectivity 2

Gamma Waves (30-100 Hz)

• Gamma activity (30-70 Hz and higher) is associated with active sensorimotor and cognitive processing 2
• Gamma reflects cholinergic inputs and thalamocortical projections during active mental states 2
• Gamma activity is variable across individuals and less reliably characterized than lower frequency bands 4

Major EEG Abnormalities

Pathological Slowing

• Generalized slowing with increased delta (< 4 Hz) and theta (4-7 Hz) activity during wakefulness indicates diffuse brain dysfunction 1, 3
• Progressive slowing of EEG activity occurs in metabolic encephalopathies, ranging from mild slowing to generalized delta waves 3
• In Alzheimer's disease, pathological slowing manifests as reduced alpha amplitude with increased delta and theta power, reflecting thalamocortical "disconnection mode" 1
• Focal slowing indicates regional brain dysfunction or structural lesion 7

Epileptiform Abnormalities

• Epileptiform discharges include spikes, sharp waves, and spike-wave complexes that indicate increased seizure risk 8, 7
• Epileptiform patterns can be focal (indicating localized epileptogenic zone) or generalized (suggesting genetic/idiopathic epilepsy) 8
• Periodic discharges may indicate specific pathologies: lateralized periodic discharges suggest HSV-1 encephalitis, while generalized periodic discharges suggest Creutzfeldt-Jakob disease 9
• Triphasic waves are characteristic of metabolic encephalopathy, particularly hepatic encephalopathy, though not specific 3, 9

Loss of Normal Reactivity

• Absence of alpha rhythm reactivity to eye opening indicates severe cortical dysfunction 6
• Continuous or nearly continuous background without reactivity predicts poor neurological outcome in post-cardiac arrest patients 1
• Unilateral decrease in alpha reactivity is an unusual but significant abnormal pattern 5

Abnormal Background Patterns

• Suppressed or discontinuous background indicates severe brain injury 1
• Burst-suppression pattern (alternating periods of high-voltage activity and suppression) suggests severe encephalopathy or anesthetic effect 1
• Reversed anteroposterior gradient (faster frequencies posteriorly than anteriorly) is abnormal 1
• Extreme delta brushes are characteristic of anti-NMDA receptor encephalitis 9

Clinical Context for Interpretation

Timing of EEG Recording

• Standard laboratory EEG should last at least 20 minutes with 19 active electrodes 7
• Artifact-free resting state data of 20-60 seconds is sufficient for reliable quantitative analysis 6
• EEG performed as soon as possible after a seizure is essential for epilepsy diagnosis 7

Critical Pitfalls to Avoid

• Regional specificity matters: Frontal beta excess has different implications (behavioral dysregulation) than posterior beta reduction (cognitive decline) 2
• Normal variants can mimic pathology: wicket spikes, breach rhythm, and rhythmic temporal theta of drowsiness are benign patterns 5
• Age modulates findings: Beta abnormalities in Alzheimer's disease are more pronounced in younger patients (≤65 years) 2
• Never interpret a single frequency band in isolation—comprehensive assessment requires evaluating all frequency bands together 2

Prognostic Value

• Continuous or nearly continuous normal-voltage EEG background within 12-48 hours post-cardiac arrest predicts favorable neurological outcome with 82-100% specificity 1
• Severity of EEG changes correlates with clinical status and prognosis in metabolic encephalopathies 3
• Test-retest reliability of resting state EEG is high (correlation coefficients 0.8-0.9 at 4 weeks), making it suitable for longitudinal monitoring 6