EEG is a high yield topic for resident and board exams, so take the time to review this topic carefully! This chapter will teach you to read EEG: from normal findings to epileptiform discharges to associated clinical syndromes. You will also learn commonly tested EEG artifacts. Complete with real EEG tracing examples. Test your skills at the end with a practice quiz! Of note, medical students are not expected to know how to read EEG for exam purposes.

Authors: Brian Hanrahan MD, Steven Gangloff MD

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Table of Contents

EEG Basics

  • EEG measures the potential difference between two electrodes on the scalp.
  • The electrical fields that generate EEG signals are the result of inhibitory and excitatory postsynaptic potentials (IPSPs and EPSPs) on the apical dendrites of cortical neurons. Pyramidal neurons contribute to the plurality of the signal (Figure 1).
    • The EEG does not measure single action potentials. It measures a summation of postsynaptic potentials.
  • The region of the brain with the lowest threshold to electrical stimulation is the hippocampus.
  • It takes a minimum area of 6 cm² of cortical activation to produce recordable epileptic discharges on standard scalp EEG.

Figure 1: Postsynaptic Potential Summation

EEG review
The result of the summation of postsynaptic potentials is the overall change in the membrane potential. At point A, several different excitatory postsynaptic potentials (EPSPs) add up to a large depolarization. At point B, a mix of excitatory and inhibitory postsynaptic potentials result in a different result for the membrane potential. Licensed under a Creative Commons Attribution 4.0 International License. Download the image for free here.

EEG Patterns Throughout Development

Preterm EEG

  • <32 weeks: Tracé discontinu is seen, which is a discontinuous background pattern that resembles burst suppression. There are bursts of mixed frequency activity lasting <15 seconds followed by long periods of low voltage generalized background suppression.
    • There is no difference in the EEG background with state changes.
  • >32 weeks: Trace alternant replaces trace discontinu; High-voltage bursts are separated by low-amplitude voltage waves. Other findings include delta brushes, frontal sharp transients, and theta bursts.

Term and pediatric EEG

  • Term (40 weeks): Frontal sharp waves can be seen during sleep.
    • All pre-term findings resolve by the 42nd-44th week.
  • 44 weeks: Sleep spindles form. They can be asynchronous until 2 years of age.
  • 46 weeks: Vertex transients can be seen.
  • 1 year: Theta frequency posterior dominant rhythm develops.
  • 8-10 years old: Alpha rhythms of 9 Hz to 10 Hz become the predominant frequency.

Preterm and pediatric EEG findings

Background Rhythms

  • Delta activity (<4 Hz): When generalized it is Indicative of generalized cerebral dysfunction/encephalopathy produced by processes involving subcortical white matter. If focal, it can represent focal cerebral lesions such as a tumor, cerebral infarction, or cerebral abscess.
  • Theta activity (4-8 Hz): Can be seen normally in drowsiness or pathologically in patients with increased intracranial pressures or mild encephalopathy.

EEG review theta of drowsiness

  • Alpha activity (8-12 Hz)/normal background: At rest, a quiet awake individual has posterior (occipital) dominant background alpha activity.
    • Alpha activity can be suppressed with eye-opening.
Review EEG normal awake EEG
Normal Awake (alpha range waves) EEG
  • Beta activity (13-30 Hz): Can be seen normally in adults, but more often seen secondary to medications like benzodiazepines, barbiturates, drug toxicity, or anesthesia.
    • Increases in drowsiness and light sleep.
EEG review Beta range activity in the frontal head
Beta-range activity in the frontal (left>right) head regions


 

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Table of Contents

Table of Contents