Focal slowing in the EEG suggests an underlying abnormality but is of nonspecific etiology. It may reflect structural (i.e., tumor or infarct) or functional (i.e., postictal or migraine) abnormalities. There exist two spectra of severity, one pertaining to frequency, with slower rhythms representing more severe lesions, and one pertaining to persistence, with continuous slowing a more significant abnormality than intermittent slowing. An interhemispheric frequency difference of less than 1 Hz is not considered significant.
Continuous slowing in the EEG, whether focal or generalized, tends to take the appearance of either rhythmic monomorphic or arrhythmic polymorphic waveforms. These patterns often have differing significance. Continuous focal arrhythmic polymorphic slowing (Fig. 1) usually suggests some type of structural lesion in the underlying subcortical white matter. Abscesses, ischemic strokes, tumors, contusions, and so on, all may produce this pattern. The mechanism for this form of slowing may reflect disordered intracortical connectivity. Even transient functional disturbances, such as migraine and the postictal state, can be responsible. This illustrates the value of follow-up EEGs looking for evolution or resolution of any focal slowing that may be present. Rarely, focal slowing (and even focal seizures) can suggest toxic-metabolic disturbances, especially in hypoglycemic and hyperglycemic states. Continuous focal rhythmic monomorphic slowing, by contrast, is more commonly associated with underlying gray matter lesions. Note that focal cortical lesions are more likely to produce focal voltage attenuation or epileptiform abnormalities rather than focal slowing. Focal rhythmic monomorphic slow activity can also be intermittent. This is a less common pattern. The manner of appearance should be considered. Recurrent bursts of paroxysmal focal slowing may raise
From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ
a suspicion for an underlying epileptogenic focus. A seizure focus is also suggested when the focal slowing shows exceptional rhythmicity or frequency evolution, that is, rhythmic slow waves that speed up and slow down. The location of focal intermittent rhythmic slowing may be significant. For example, if observed in the temporal regions, that is, temporal intermittent rhythmic delta, an underlying epileptogenic focus is more likely.
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