The Posterior Dominant Rhythm (PDR) is a crucial feature in an EEG, primarily observed in the occipital region when the patient is at rest with their eyes closed. It represents the brain's resting frequency, typically falling between 8.5 to 12 Hz in adults, within the alpha range (7-13 Hz). This rhythm is an important indicator of normal brain activity and should be symmetric in both frequency and amplitude.

Key Aspects of PDR:

  • Symmetry: The PDR should be symmetric in frequency and amplitude between the left and right occipital regions. A difference greater than 50% in amplitude or 1 Hz in frequency between the sides is considered abnormal.
  • Left Side Attenuation: It is normal for the left side PDR to be slightly attenuated due to factors like a thicker skull, though it should still remain within the normal range.
  • Alpha Squeak: A transient, quickening of the PDR immediately after eye closure is called alpha squeak. This transient acceleration can make the PDR seem faster than it is. Care should be taken to avoid basing measurements on areas showing alpha squeak.
  • Drowsiness Effect: If the patient is drowsy or entering stage I sleep, the PDR may appear slower than usual, even though it’s still within normal limits.
  • Age Variations: In children, the PDR differs based on age. Younger children may have a slower rhythm, while older children and adults show a PDR between 8.5-12 Hz.
  • No PDR in Some People: Up to 5% of normal individuals might not exhibit any discernible PDR.

Measurement:

To measure the PDR:

  1. Wait for the patient to close their eyes.
  2. Count the number of waves per second (frequency) in the occipital region.
  3. Multiple observations may be needed, as the PDR can fluctuate.
  4. Ensure the patient is awake and relaxed to avoid errors due to drowsiness or alpha squeak.

The PDR serves as a marker of brain health and is one of the first things to check during an EEG. An abnormal PDR may indicate underlying issues such as generalized slowing, which is an important sign for clinicians to investigate further.