How is an EEG finding of amplitude asymmetry interpreted?

Updated: Aug 06, 2019
  • Author: Selim R Benbadis, MD; Chief Editor: Helmi L Lutsep, MD  more...
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Destructive lesions clearly can attenuate the amplitude of normal rhythms. However, normal rhythms are never perfectly symmetric in amplitude, therefore which asymmetries to consider significant is not always clear. (Some have proposed a greater than 50% side-to-side difference as abnormal.)

A good rule of thumb is that, with very few exceptions, significant focal asymmetries are associated with slowing. The authors recommend that any amplitude asymmetry associated with slowing of frequency be considered significant.

Amplitude asymmetry or suppression of normal rhythms is somewhat more likely to be seen in structural abnormalities that increase the distance or interfere with the conduction of the electrical signal between the cortex and the recording scalp electrodes. Examples include subdural collections (eg, hematoma, empyema), epidural collections (eg, hematoma, abscess), subgaleal collections, and calcifications such as those seen in Sturge-Weber syndrome.

Amplitude asymmetry also may be more common than slowing in subdural hematomas. However, caution must be exercised before considering isolated nonepileptiform focal findings other than slowing as abnormal. In general, as with other types of focal EEG abnormalities such as slowing, amplitude asymmetry is nonspecific as to etiology.

Although asymmetry in amplitude is usually indicative of dysfunction on the side of depressed amplitude, one notable exception to this rule is the so-called breach rhythm (see image below). This is caused by a skull defect, which attenuates the high-frequency filter function of the intact skull. As a result, faster frequencies (eg, alpha, spindles, beta) are of higher amplitude on the side of the defect. Since morphology often is sharply contoured, determining the epileptogenicity of these discharges can be extremely difficult, and in this situation erring on the conservative side, by not interpreting them as epileptiform, is clearly preferable. Because of a cancellation effect between frontopolar (Fp1/Fp2) and frontal (F3/F4), eye movements often are not increased on the side of a skull defect and may indeed be of lesser amplitude on that side.

Asymmetry, increased beta, regional right frontoce Asymmetry, increased beta, regional right frontocentral. The beta activity is increased in amplitude in the right frontocentral region. This is a "breach rhythm" and is caused most often by a skull defect (in this case a burr hole).

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