When an EEG is reviewed, the frace can be considered at several levels. At the highest level, the overall organisation of the trace can be assessed. The organisation of the EEG describes the big picture and includes assessment of symmetry, the anteroposterior gradient and continuity. More on all of these later. Next, rhythms can be identified. A rhythm is a specific pattern of electrographic activity with a characteristic frequency, location, morphology and reactivity. Finally, the frequencies and waveforms of the trace can be reviewed.
The posterior dominant rhythm (PDR) is also known as the alpha rhythm. It is important to stress the difference between the alpha rhythm and alpha frequency.
| Feature | Description |
|---|---|
| Frequency | Alpha frequency range |
| Location | Most prominant posteriorly |
| Morphology | Monomorphic. Asymmetry of up to 50% may occur in healthy people. |
| Reactivity | Attenuates with eye opening. A transient increase in frequency and amplitude occurs following eye opening known as alpha squeak. The rhythm also attenuates with drowsiness, so called alpha dropout |
Features: - 14 or 6Hz - Posterior quadrant predominance - More common in drowsiness - Bilateral and synchronous - Positive - 1-2s runs
Arch like Can appear sharp Alpha range Most common in the temporal chains
Can be confused with Mu rhythm