Abnormal Electroencephalogram
General characteristics of the abnormal EEG include asymmetry
with respect to frequency or amplitude, or both, recorded from corresponding electrodes
on each hemisphere, and patterns of amplitude and frequency that are not predictable
or expected in the normal recording. These abnormal patterns reflect anatomic or
metabolic alterations in the underlying brain. Regional asymmetry can be seen with
tumors, epilepsy, and cerebral ischemia or infarction. Epilepsy may be recognized
by high-voltage spike and slow waves, whereas cerebral ischemia manifests with electroencephalographic
slowing with preservation of voltage (mild or moderate) or loss of voltage (severe).
Factors affecting the entire brain may produce symmetric abnormalities of the signal.
Identifying pathologic abnormal patterns in the global electroencephalographic signal
is very important, although sometimes quite difficult, in the clinical situation.
Many of the normal global pattern changes produced by anesthetic
Figure 38-3
Characteristic sleep spindles in normal sleep are shown
in the center. (Courtesy of the Mayo Foundation, Rochester, MN.)
drugs are similar to pathologic patterns produced by ischemia or hypoxemia. Control
of the anesthetic technique is important when the EEG is being used for clinical
monitoring of the nervous system.
