How is amplitude identified in the visual analysis of neonatal electroencephalogram (EEG)?

Updated: Aug 20, 2019
  • Author: Samuel Koszer, MD; Chief Editor: Selim R Benbadis, MD  more...
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Amplitude: The spectrum of abnormalities of background varies from the ominous pattern of electrocerebral inactivity to the more benign finding of low-amplitude activity during discontinuous sleep. Note that caput succedaneum, scalp edema, and subdural effusions or hematomas may affect the apparent amplitude of the EEG.

  • The most extreme abnormality in amplitude consists of electrocerebral inactivity (isoelectric/inactive) (see image below). This is a record in which no cerebral electrical activity is present. Before concluding that this pattern is present, the electroencephalographer must be certain the test was performed satisfactorily and for a long enough time to include potential sleep cycling. Increased sensitivities, long time constants, and long interelectrode distances are used. Artifacts are identified and eliminated, when possible. The technician always performs various stimulations (auditory and nociceptive) to establish lack of reactivity. This EEG pattern carries a grave prognosis in both PT and FT infants if not due to postictal state, hypothermia, acute hypoxia, or drug intoxication. Most infants with persistent electrocerebral inactivity either die or have severe neurological sequelae. See the image below.

    Artifacts and a low-voltage record. A full-term in Artifacts and a low-voltage record. A full-term infant aged 4 days with anoxia at birth and seizures. No spontaneous respirations occur. The record is low voltage with no activity of cerebral origin greater than 2 mV. All other activity can be accounted for by physiologic or nonphysiologic artifacts. ECG artifacts are diffuse. Pulse artifact is time locked to the ECG. An IV artifact occurs in multiple electrodes every 1.75 seconds and is most prominent in the right electrooculogram (REOG). The RESP channel demonstrates mechanical ventilation artifact.
  • A second abnormality of amplitude occurs when the low-voltage undifferentiated pattern consists of background activity between 5-15 µV in all states. Distinct state changes typically do not occur. This background pattern is easier to recognize in PT infants because discontinuity and high-voltage bursts are normal, and a decrease or absence of delta brushes, which should be quite prominent at this age, is common. The low-voltage undifferentiated pattern is associated with poor outcomes, particularly when the pattern persists beyond the first weeks of life. Low-voltage records can be observed in a variety of neonatal encephalopathies, including hypoxia-ischemia, congenital hydrocephalus, and severe intracranial hemorrhages such as large subdural hematomas. They also may occur in various toxic or metabolic disturbances. A low-voltage record shortly after a hypoxic or ischemic event is less concerning than one obtained days after the event.

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