What is the significance of asymmetries of faster activities on focal EEG?

Updated: Oct 09, 2019
  • Author: Alexis D Boro, MD; Chief Editor: Selim R Benbadis, MD  more...
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Widespread asymmetries in the amplitude of background activities, when not accompanied by other abnormalities, should be interpreted with caution because they can occur in healthy patients. Relatively focal asymmetries are more often associated with significant pathology. Focal abnormalities in beta activities are often most easily appreciated in patients taking benzodiazepines or other medications that increase the amount of beta in the background.

The most common cause of focal enhancement of beta activities is the breach rhythm. This occurs in the setting of a skull defect, most commonly due to a craniotomy or burr hole. The amplitudes of other frequency components of the background are increased as well, but to lesser extent. A common pitfall in EEG interpretation is to classify the sharply contoured background evident in the region of a skull defect as epileptiform. While the enhancement of beta activities is maximal near the margins of the skull defect, breach rhythms are usually widespread and are often maximal over the central regions, where the mu rhythm can be greatly accentuated.

Focal enhancement of beta activities in the absence of a skull defect is encountered rarely. While this can be seen with focal cortical dysplasias, [3] tumors, stroke, and vascular malformations, focal attenuation of beta activities is more common in all of these conditions. Focal attenuation of faster activities is a hallmark of a lesion involving the cerebral cortex. Attenuation of faster activities can also be caused by an intervening fluid collection in the subgaleal, epidural, or subdural spaces. Extra-axial fluid collections alone do not cause abnormal slow waves; their presence is indicative of parenchyma involvement.

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