What are focal EEG waveform abnormalities of the posterior dominant rhythm (PDR)?

Updated: Oct 09, 2019
  • Author: Alexis D Boro, MD; Chief Editor: Selim R Benbadis, MD  more...
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Differences in the amplitude of the PDR alone are rarely significant. Asymmetries in the extent of the field of the PDR also occur in healthy patients. Similarly, asymmetries in the amplitude of the response to photic stimulation alone are not reliable indicators of pathology. Asymmetries in frequency and regulation of the PDR are much more suggestive of a focal lesion. While the causative lesion may be at some distance from the occipital pole, the lesion is usually ipsilateral to the abnormal PDR.

This EEG abnormality can occasionally be falsely lateralizing; this can occur in patients in whom the dominant generator of the PDR involves the medial surface of the cerebral hemisphere and projects contralaterally. Rarely, epileptogenic lesions react to photic stimulation by producing large-amplitude driving responses or spikes ipsilateral to the lesion. Bancaud phenomenon is an uncommon finding in which there is unilateral failure of the PDR to attenuate with eye opening. This is typically associated with ipsilateral posterior subcortical lesions. See the images below.

Abnormal asymmetric photic driving in a 4-year-old Abnormal asymmetric photic driving in a 4-year-old boy with seizures. While lesser degrees of amplitude asymmetry in photic driving are not reliable indicators of pathology, in this case the amplitude difference is at least 5-fold. The photic driving response is usually attenuated on the abnormal side. Here, the preponderance of the slowing over the right posterior head regions with the eyes open, along with the probable spikes in the same area, suggest that the affected side may be the right. In addition, the abnormality occurs in the presence of scanning eye movements (note the horizontal eye movements just before, during, and after photic stimulation), which would often attenuate the driving response.
Patient is a 4-year-old boy with seizures. Photic Patient is a 4-year-old boy with seizures. Photic stimulation at a higher frequency provokes spikes with a widespread field with the principal phase-reversal at 02.

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