A reading order
Before any waveform: age, level of consciousness, medication, skull defects, and whether the patient is awake, drowsy or asleep. Most reading errors start with skipping this step. Then describe the background before hunting for abnormalities.
Posterior dominant rhythm
In relaxed wakefulness with the eyes closed, an 8–13 Hz rhythm appears over the occipital regions and attenuates with eye opening. 3
These are the usual milestones for the maturing posterior rhythm. 11 In an awake adult a posterior rhythm below 8 Hz is abnormal at any age. Mild amplitude asymmetry, higher on the right, is common.
The organisation of a normal background
- Anterior–posterior gradient: lower-amplitude faster activity in front, higher-amplitude alpha behind.
- Symmetry between homologous regions.
- Reactivity to eye opening and stimulation.
- Variability with state: a record that never changes is not normal.
Beta, mu and lambda
Beta (above 13 Hz) is low in amplitude and frontocentral; abundant beta suggests sedatives. Mu is an arciform central rhythm that blocks with contralateral hand movement, not with eye opening. Lambda waves are sharp, positive occipital transients seen while the eyes scan a patterned scene; they vanish when the patient stares at a blank surface. 3,11
Drowsiness
The posterior rhythm slows and drops out, slow lateral eye movements appear, theta emerges, and in children bursts of high-amplitude rhythmic slowing (hypnagogic hypersynchrony) are normal. Many benign variants live in drowsiness, which is where over-reading happens.
Sleep
| Stage | Defining features |
|---|---|
| N1 | Alpha replaced by low-amplitude mixed frequency; vertex sharp waves; slow eye movements |
| N2 | K-complexes and sleep spindles (11–16 Hz, at least 0.5 s) |
| N3 | Slow waves of 0.5–2 Hz, above 75 µV frontally, in at least 20% of the epoch |
| REM | Low-amplitude mixed frequency, rapid eye movements, low chin tone, sawtooth waves |
Definitions follow the AASM scoring manual. 10 POSTS (positive occipital sharp transients of sleep) are normal, bilateral, positive occipital transients in N1 and N2.
Sleep activates interictal epileptiform discharges, and a record with drowsiness and sleep is more informative than a purely awake one. That is the reason for sleep-deprived recordings.