Of note, up to 5% of normal people have no PDR at all. It is helpful to check at more than one period, as the PDR can fluctuate mildly and you want to give a patient their best possible PDR. To determine the PDR, wait till the eyes are closed and then count the number of waves per second in the occipital region. Of note, it is normal for the PDR on the left to be slightly attenuated compared to the right, thought to reflect a thicker skull on the left side in most people. The PDR should be symmetric in both frequency and amplitude if there is a more than 50% difference in amplitude or a more than 1 Hz difference in frequency between sides, this is abnormal. It is a vital part of a normal EEG and among the first things you should look for the PDR used to be called the alpha rhythm because the normal PDR (8.5-12 Hz ) is in the alpha range (7-13 Hz).
The posterior dominant rhythm (PDR) is the resting frequency of the occipital region when eyes are closed and the patient is resting quietly.
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