The commonly used standard defines low-voltage activity as the persistent absence of any cerebrally generated waves greater than 20 μV (Niedermeyer, 1999). The value of 20 μV is arbitrary but it effectively eliminates spontaneous variations in amplitude that are expected to occur in EEGs, and thereby sets a definition for abnormality. Since an accurate measurement of amplitude is especially important when assessing for abnormal low voltage, channels with long interelectrode distances should be used. This decreases the likelihood of the appearance of low voltage due to broad, isoelectric fields. Broad, isoelectric fields result in neighboring electrodes recording similar activity, which produces a low-voltage output after differential amplification. Longer interelectrode distances maximize the likelihood that a channel’s two recording electrodes are not recording the same activity. For this reason, common reference montages with distant reference electrodes are preferred when determining amplitude. With especially broad rhythms, a contralateral reference may be necessary to assure that the amplitude measurement is accurate. Even when not especially broad, cerebrally generated low-voltage activity usually extends beyond a focal region, and it may be hemispheric, bilateral, or generalized. Assessment of generalized low-voltage activity requires that the technologist is diligent in checking each electrode and the overall recording because a reference outside the low-voltage field will not be available. Low-voltage activity sometimes contain high-frequency activity that is not apparent at usual sensitivity settings. Such activity often is clinically relevant despite its amplitude, and careful scrutiny at a high-gain setting is warranted. Identification of high-frequency activity is sometimes easier after increasing the low-frequency filter. This stabilizes an undulating baseline, which commonly accompanies low-voltage activity, and benefits the visualization of the other rhythms.

Electrocerebral inactivity (ECI) is an extreme example of a low-voltage EEG in that the amplitude is sufficiently low that it lacks any visible cerebrally generated activity. In some clinical circumstances, low-voltage EEG may be a precursor to ECI (Bauer, 1999). The absence of cerebral activity generally is defined as the absence of potentials greater than 2 μV when reviewed at a sensitivity of 2 μV/mm. This definition has been based on the historic limits of EEG resolution, but it remains clinically useful because of widespread experience over many years and the difficulty in obtaining reliable recordings of even lower voltage activity in the clinical environment of the patients with low-voltage EEG, which is often an intensive-care unit. In addition to the sensitivity criterion, accurate determination of ECI requires that the EEG be recorded and reviewed according to specific criteria (Silverman et al., 1970), (American Electroencephalographic Society, 1994). In total, the criteria are:

Low-voltage EEGs may occur in any context; thus, they have no specific accompanying waves. ECI commonly is accompanied by artifact due to electrical and mechanical medical devices that often are at the bedside because of the patient’s medical condition. This becomes particularly evident at the high sensitivity used during the review. The high sensitivity also results in more pronounced cardiac artifact.

Persistent and generalized low-voltage activity on an EEG may be a normal variant and has increasing prevalence with advancing age. It is rare in childhood, increases in adulthood, and reaches a maximum prevalence of about 10% by middle adulthood (Fisch, 1999; Niedermeyer, 1999

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