Normal and Abnormal EEG Patterns
Domain 4 - Normal and Abnormal EEG Patterns
The exam asks you to recognize normal rhythms and variants, stage sleep, and identify epileptiform and periodic patterns without overcalling.
Core rules
- Normal adult rhythm. Posterior dominant rhythm of 8–13 Hz (alpha), maximal at O1/O2, attenuating with eye opening; beta above 13 Hz, often frontal; mu at C3/C4, arciform and blocked by movement.
- Sleep. N1: loss of alpha, vertex sharp waves, slow eye movements. N2: sleep spindles (11–16 Hz, typically 12–14) and K-complexes. N3: delta below 2 Hz and above 75 µV. REM: low-voltage mixed frequency with sawtooth waves.
- Normal variants. 14 and 6 Hz positive bursts, wicket spikes, breach rhythm, lambda waves, POSTS in children, and rhythmic midtemporal theta of drowsiness are not epileptiform.
- Epileptiform activity. Spikes (<70 ms), sharp waves (70–200 ms), spike-and-wave, polyspike, and hypsarrhythmia. Use ACNS terminology; describe location, polarity, frequency, and reactivity.
- Periodic patterns. PLEDs (lateralized, 1–2 Hz), triphasic waves (often hepatic), FIRDA, burst suppression, and generalized periodic discharges each have distinct associations.
Worked example
A 9-year-old has staring spells with eyelid flutter. The EEG shows 3 Hz generalized spike-and-wave lasting 3–10 seconds, activated by hyperventilation, with normal background between bursts. This is the classic absence pattern. Reasoning: the frequency, abrupt onset and offset, normal background, and clinical correlate fit absence epilepsy; the same discharges at 1–2 Hz would suggest a different syndrome.
Traps
Calling breach rhythm or wicket spikes epileptiform; overreading drowsiness as slowing; missing photoparoxysmal responses; confusing eye movement with frontal delta; failing to state frequency and location.