5.1 Normal EEG Patterns
Key Takeaways
- The posterior dominant rhythm (PDR) in a normal awake adult is 8.5-13 Hz alpha, attenuates with eye opening, and should be symmetric within 1 Hz between hemispheres.
- Sleep architecture markers in order of N2 onset are vertex sharp waves, sleep spindles (12-14 Hz), and K-complexes; POSTS appear in N1-N2 over the occipital region.
- Mu rhythm is a 7-11 Hz centrally located arciform rhythm that reacts to contralateral movement, not to eye opening, distinguishing it from alpha.
- Benign variants such as wicket spikes, RMTD, BETS/SREDA, and 14-and-6 positive bursts mimic epileptiform activity but have no clinical significance.
- The PDR matures with age: roughly 4 Hz at 4 months, 6 Hz at 12 months, 8 Hz by age 3, and reaches the adult 9-10 Hz range by about 8 years.
Why Normal Patterns Matter for the R.EEG.T. Exam
The single most important skill tested under the Performing the Study domain (75% of the exam) is knowing what normal looks like. You cannot reliably flag an abnormality until you have internalized the normal background for the patient's age and state of consciousness. ABRET questions frequently present a described pattern and ask whether it is normal, a benign variant, or pathologic. A technologist who calls a wicket spike "epileptiform" or a vertex wave "a sharp transient" creates a record that can mislead the interpreting physician.
This section covers four areas: the awake background, the drowsy/sleep transition and its hallmark elements, the benign normal variants that mimic epileptiform activity, and how all of these change with age.
The Awake Background: Posterior Dominant Rhythm
The posterior dominant rhythm (PDR) is the foundation of every adult EEG read. In a relaxed, awake adult with eyes closed it is an alpha rhythm of 8.5-13 Hz, maximal over the occipital and parietal regions, and it attenuates (blocks) with eye opening and returns with eye closure.
| PDR Property | Normal Adult Finding | Why It Matters |
|---|---|---|
| Frequency | 8.5-13 Hz (>=8.5 Hz awake) | <8 Hz awake suggests encephalopathy |
| Reactivity | Attenuates with eye opening | Loss of reactivity is abnormal |
| Symmetry | Within ~1 Hz / ~50% amplitude side to side | Persistent asymmetry suggests a focal lesion |
| Distribution | Maximal posteriorly | Anterior "alpha" may be alpha coma, not normal |
A normal feature often mistaken for pathology is alpha squeak (a brief frequency rise of the PDR right after eye closure) and the normal slight amplitude asymmetry favoring the right occipital region (the left is commonly a few microvolts lower).
Beta, Theta, and Delta in the Normal Record
Low-amplitude beta activity (>13 Hz) over the frontocentral regions is normal and is markedly increased by benzodiazepines and barbiturates — a medication effect the technologist should note in the recording. Scattered low-amplitude theta (4 to <8 Hz) is normal in the awake young adult, especially frontocentrally and during drowsiness. Sustained delta (<4 Hz) is not normal in the awake adult but is the dominant frequency of deep sleep and of the normal young infant.
The Drowsy-to-Sleep Transition
As the patient becomes drowsy, the PDR slows and fragments, slow lateral eye movements appear, and theta increases. Recognizing this sequence prevents the technologist from mislabeling normal drowsy slowing as focal pathology.
Sleep Architecture and Its Hallmark Elements
| Sleep Stage | Hallmark EEG Features |
|---|---|
| N1 (light) | PDR dropout, theta, slow rolling eye movements, vertex sharp waves, POSTS |
| N2 | Sleep spindles (12-14 Hz) and K-complexes on a theta background |
| N3 (slow-wave) | High-amplitude (>75 uV) delta occupying >=20% of the epoch |
| REM | Low-voltage mixed frequency, sawtooth waves, rapid eye movements, muscle atonia |
Key normal transients and their correct identification:
- Vertex sharp waves (V waves): Sharply contoured negative transients maximal at the Cz electrode in N1-N2. They are symmetric and do not indicate epilepsy, though they can be sharply contoured in children.
- K-complexes: A high-amplitude diphasic slow wave (initial sharp negativity followed by a slower positivity) maximal frontocentrally, often followed by a spindle. They define N2 and can be evoked by a sudden sound.
- Sleep spindles: 12-14 Hz sinusoidal bursts, frontocentral, lasting >=0.5 s. They appear by ~2-3 months of age and should be roughly symmetric by ~2 years.
- POSTS (Positive Occipital Sharp Transients of Sleep): Triangular, surface-positive, occipital transients in light sleep; benign and frequently mistaken for occipital sharp waves.
Benign Normal Variants That Mimic Epileptiform Activity
These are the highest-yield "trick" patterns on the exam because they look pathologic but are clinically insignificant. Document them accurately; never describe them as epileptiform.
| Variant | Description | Why It Is Mistaken | Significance |
|---|---|---|---|
| Mu rhythm | 7-11 Hz arciform rhythm, central (C3/C4) | Looks like alpha or focal slowing | Normal; blocks with contralateral movement, not eye opening |
| Lambda waves | Surface-positive occipital sharp transients in the alert, visually scanning patient | Look like occipital spikes | Normal response to visual exploration |
| Wicket spikes | 6-11 Hz arciform temporal spikes, often in trains | Resemble temporal sharp waves | Benign; no slow-wave aftercoming, no disruption of background |
| RMTD | Rhythmic Mid-Temporal Theta of Drowsiness; 5-7 Hz notched temporal trains | Resemble temporal seizure activity | Benign; does not evolve in frequency or spread |
| BETS / SREDA | Benign Epileptiform Transients of Sleep (small sharp spikes); Subclinical Rhythmic Electrographic Discharge of Adults | Mimic epileptiform spikes / electrographic seizure | Benign; no clinical correlation |
| 14-and-6 positive bursts | Arciform positive bursts at 14 Hz and/or 6 Hz, posterotemporal, in drowsiness/sleep | Look like epileptiform bursts | Benign, mostly in adolescents |
The defining rule for distinguishing a benign variant from a true epileptiform discharge: a true epileptiform discharge disrupts the background and is typically followed by a slow wave, whereas benign variants do not perturb the ongoing background and lack an after-coming slow wave.
Age-Related Normal Patterns
The normal EEG is age-dependent, and the technologist must not apply adult norms to an infant record. Maturation of the posterior dominant rhythm is the most testable progression.
| Age | Expected Posterior Dominant Rhythm |
|---|---|
| 3-4 months | ~3.5-4 Hz reactive posterior rhythm appears |
| 6 months | ~5 Hz |
| 12 months | ~6 Hz |
| 3 years | ~8 Hz (reaches adult lower limit) |
| 8 years and older | ~9-10 Hz (adult range) |
Other normal age-related findings the technologist should expect rather than flag:
- Neonatal patterns: discontinuous tracé alternant in quiet sleep of the term newborn and tracé discontinu in the premature infant are normal for age.
- Hypnagogic hypersynchrony: high-amplitude rhythmic 3-5 Hz activity at drowsiness onset in children ages ~3 months to 12 years — normal, not a spike-wave burst.
- Posterior slow waves of youth: delta intermixed with the PDR posteriorly in children and adolescents; suppresses with eye opening; benign.
- Elderly: mild temporal theta and slight PDR slowing toward the lower alpha range can be a normal variant of aging if reactive and not focal.
Clinical Application
When you describe a record, anchor every statement to age and state: "awake, eyes closed, symmetric 10 Hz PDR that attenuates with eye opening; drowsiness with vertex waves and POSTS; benign wicket spikes in the left temporal region without background disruption." That phrasing demonstrates the exact pattern-recognition discipline ABRET tests.
An awake adult with eyes closed shows an 11 Hz rhythm over C3 and C4 that does not attenuate with eye opening but disappears when the patient clenches the opposite fist. What is this rhythm?
Which set of transients correctly characterizes stage N2 sleep?
A 12-month-old infant has a reactive posterior rhythm of approximately 6 Hz. How should the technologist regard this finding?
Which feature best distinguishes a benign variant such as wicket spikes from a true epileptiform discharge?