Answer
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 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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Continuous slow, lateralized right hemisphere. While "spilling over" to the left frontal region, the polymorphic delta activity is clearly predominant over the right hemisphere. This type of slowing almost invariably is associated with a structural hemispheric lesion. This patient had a large right middle cerebral artery infarct.
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Continuous slow, lateralized left hemisphere. This polymorphic delta activity was continuous throughout the record. This patient had a left hemisphere neoplasm.
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Continuous slow, regional right temporal. This polymorphic delta activity is somewhat more focal than that shown in the first image above, with a maximum in the temporal chain. Little such activity is evident in the central chain, but enough to exclude a T4 electrode artifact. The slowing shows phase reversals at T4, indicating a maximum at that electrode.
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Continuous slow, regional left temporal. This polymorphic delta activity is somewhat more focal than that in the second image above, with a maximum in the temporal chain. The phase reversals at T3 indicate a maximum at that electrode.
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Intermittent slow, lateralized left hemisphere. This brief burst of delta activity is seen in the temporal and central areas. This is a much "weaker" finding than continuous slowing, and much less reliably associated with a structural lesion. This is indicative of mild dysfunction in that region.
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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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Periodic lateralized epileptiform discharges (PLEDS), regional left centrotemporal. The repetitive discharges occur with a periodicity of about 1 second. Polymorphic delta activity is seen over the left hemisphere, but the classification as PLEDS already implies severe focal dysfunction. In addition, it indicates an acute destructive process and very high (80%) risk of seizures.
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Periodic lateralized epileptiform discharges (PLEDS), lateralized right hemisphere. The repetitive discharges occur with a periodicity of 2-4 seconds. PLEDS are associated with severe focal dysfunction and with acute destructive processes and very high (80%) risk of seizures.
