Generalized background slowing indicates diffuse cerebral dysfunction, which, similar to focal slowing, is also not specific as to cause. See Figure 54 for an example of focal temporal regional slowing, which also shows a “breach rhythm,” with focally elevated background amplitude as a result of a skull defect and previous surgery in this area. The various causes are too numerous to be comprehensive, but common examples include transient or permanent ischemia resulting from stroke, brain hemorrhage, tumors, traumatic injury, malformations of cortical development, nonstructural focal cerebral dysfunction corresponding to a focal epileptic focus, focal involvement of the cortex by neurodegeneration, arteriovenous malformations, and focal brain infection caused by bacterial cerebritis or viral encephalitis. Focal brain lesions of a variety of causes to cortex, underlying white matter, or both may induce focal slowing. When intermittent, focal slowing may indicate unveiling of subtle focal cerebral dysfunction owing to the effects of a sedating or hypnotic medication, although usually medication-induced slowing is generalized in nature. A variety of etiologies for focal cerebral dysfunction may be seen. Slowing may be intermittent or persistent, with more persistent or consistently slower activity generally indicating more severe underlying focal cerebral dysfunction. Focal slow wave activity on the EEG is indicative of focal cerebral pathology of the underlying brain region.
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