Abnormal Neonatal Background

Abnormal Neonatal Background

John Giang, MD

Why it matters

The background is the strongest EEG marker of encephalopathy severity. Always read it against postmenstrual age (PMA), state, medications, and hypothermia, all which ACNS says belong in the report. ACNS rates the background on eight domains: continuity, symmetry, synchrony, voltage, variability, reactivity, dysmaturity, and sleep–wake cycling. Findings like discontinuity or asynchrony can be normal at 30 weeks and abnormal at term.

Key points

• Continuity:
• A background is continuous if no stretch of attenuation below 25 µV lasts 2 s or longer.
• An interburst interval (IBI) is activity attenuated below 25–50 µV for at least 2 s.
• Excessive discontinuity: interburst intervals (IBIs) too long or too low in voltage for PMA, but the bursts still contain some normal graphoelements and the record retains some variability or reactivity.
  ACNS upper limits for IBI length:
  • under 30 weeks: 35 s
  • 30–33 weeks: 20 s
  • 34–36 weeks: 10 s
  • 37–40 weeks: 6 s
• Burst suppression is a major abnormality:
  • Bursts are invariant and abnormally composed, with no normal graphoelements.
  • IBIs are below 5 µV.
  • The record does not vary on its own and does not react to stimulation.
• Low voltage:
  • Borderline low voltage means persistent voltages of 10 to under 25 µV with some normal features.
  • Abnormally low voltage (low-voltage suppressed) means a baseline under 10 µV that is invariant and unreactive. It suggests severe diffuse cortical injury.
  • An infant with persistently low voltage that continues beyond 43 weeks PMA is abnormal.
• Electrocerebral inactivity (ECI): no cerebral activity of 2 µV or more at a sensitivity of 2 µV/mm. You can only call it when the recording meets brain-death recording standards.
• Asymmetry or focal attenuation: a persistent voltage difference greater than 2:1 between homologous regions, or a clear difference in frequencies or graphoelements. If only amplitude differs, check electrode placement, scalp edema, and subdural collections. If frequency and graphoelements also differ, think stroke or a structural lesion.
• Asynchrony: burst onsets more than 1.5 s apart between hemispheres. Some asynchrony is expected at 30–37 weeks, but little should remain by 38 weeks. Excess asynchrony suggests diffuse encephalopathy or callosal agenesis.
• Unreactivity is normal before about 30 weeks. After that it points to thalamocortical disruption.

Common pitfalls

• Calling tracé alternant "burst suppression." Tracé alternant has normal graphoelements in the bursts, IBIs above 25 µV, and it reacts to stimulation.
• Mistaking scalp edema or a misplaced electrode for focal attenuation.
• Judging continuity without adjusting for PMA or medication effects.