Electroencephalography

Brain Wave Activity. Brain activity consists of waveforms that vary in polarity, shape, and frequency, and usually range in voltage from 20 to 60 microvolts. Scalp EEG activity shows oscillations at a variety of frequencies, representing synchronized activity over a network of neurons. EEG waveforms are labeled according to their frequency, measured in cycles per second or Hertz (Hz). Alpha activity ranges between 8 and 13 Hz. The alpha rhythm is predominantly over the posterior head region and is the characteristic background frequency of the normal awake person. It occurs when the eyes are closed and attenuates when the eyes are open. Beta activity is low amplitude, fast activity with a frequency of 13 to 30 Hz and is usually present over the anterior head regions. Theta activity ranges from 4 to 7 Hz, and delta activity occurs at a frequency of less than 4 Hz. There is a developmental maturation of the EEG. For example, in the newborn infant, the EEG does not show continuous mixed-waveform activity, as would be expected in an adult. Instead, an infant has continuous amorphous delta activity. The other waveform frequencies progressively emerge as the infant’s brain develops. EEG patterns change during different stages of sleep and contribute to the definition of sleep stages. The EEG patterns are very different for rapid eye movement (REM) stages compared with non-REM sleep. For stage II non-REM sleep, the EEG shows spindle activity (10-14 Hz sinusoidal activity) and vertex sharp waves. During stage III to IV, non-REM sleep high-voltage delta activity predominates. The EEG during REM sleep resembles the EEG during wakefulness, with a low-amplitude background consisting of a mix of frequencies.

The main types of abnormalities that may be seen in the EEG are slowing of background frequencies, epileptiform activity, and suppression of activity. Slowing of background activities can either be diffuse or focal. Diffuse slowing suggests widespread brain dysfunction, which can be caused by a variety of insults, such as global brain injury, toxins, inflammation, or degenerative processes. Focal slowing is often indicative of a structural lesion, such as a tumor or a stroke. Epileptiform activity indicates the patient is at risk for seizures. Suppression of activity can be either focal or diffuse, and it indicates a severe derangement of brain function.

Indications for EEG. The main indications for obtaining an EEG are to assess for seizure disorders, intracranial disease processes, coma, and brain death. The most common reason for an EEG is to characterize a seizure disorder. The EEG is useful in defining epilepsy syndromes and for localization of a seizure focus. Because seizures occur infrequently, EEG activity is usually measured between seizures to determine whether characteristic waveforms, such as sharp waves or spikes, are present. These waveforms signify a predisposition to epilepsy. Capturing a seizure on EEG usually requires long-term monitoring with video. For some disease processes, the EEG shows specific diagnostic patterns, such as generalized periodic sharp waves in Creutzfeldt-Jakob disease. The EEG is also very useful in the evaluation of comatose patients. There may be distinctive patterns that can confirm that diagnosis of an underlying condition, such as triphasic waves in hepatic coma, spike discharges in nonconvulsive status epilepticus, and excessive beta activity associated with a benzodiazepine or barbiturate drug overdose. Finally, the EEG can be used to confirm brain death in patients in whom the EEG activity has ceased and the clinical criteria for brain death are present.

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