How to interpret the EEG

How to interpret the EEG

Utility of the Eeg 1

The electroencephalogram (EEG) is a very useful test that adds significant value in the diagnosis and treatment of seizures and/or seizure like events.


  • •Scalp EEG:
    • 1.Evaluation of episodes of loss of awareness or consciousness, especially if repeated and stereotypical
    • 2.For assessment of nonconvulsive seizures or status epilepticus in critically ill patients in the ICU
  • •In surgical planning as a treatment option for medically refractory epilepsy—these studies include scalp EEGs and intracranial EEGs

Contraindications (Relative)

  • •Open wound of the scalp would limit appropriate placement of the electrodes
  • •Noncooperative patient


  • •EEG machine—includes the computer, amplifier, and display monitor (the hardware) and an appropriate software system for analysis
  • •Scalp electrodes with a conductive medium
  • •Intracranial EEG requires placement of electrodes in the OR by the neurosurgeon
  • •EEG technologists—trained and certified to perform the test according to standards established by the American Board of Electroencephalographic and Evoked Potentials Technologists (ABRET) 2
  • •Internet availability with remote accessibility, especially in extended studies for intermittent reviewing
  • •Guidelines for minimum technical requirements are provided by the American Clinical Neurophysiology Society (ACNS) 3

Interpretation of Results

  • •General neurologists with adequate training and experience in interpretation of EEGs can provide interpretation of most outpatient EEGs.
  • •Neurophysiologists or epileptologists are required (higher level of training) for interpretation in complex situations or for highly specific requirements (e.g., evaluation in the ICU, evaluation for epilepsy surgery).



  • •Non-ICU EEG:
    • 1.Studies may be diagnostic and definitive when the result supports the clinical diagnosis.
    • 2.When inconclusive or not supportive of the clinical diagnosis, additional testing is required, and the patient requires evaluation at a level III or level IV NAEC National Association of Epilepsy Center (NAEC) 3 for final diagnosis.
  • •ICU EEG: Patients with abnormal EEG typically require a higher level of care, typically in association with a neuro-intensivist and epileptologist or neurophysiologist.

Special considerations

Types of EEGs

  • •Routine or standard EEG studies are usually done in the outpatient setting and last up to 30 minutes of recording.
  • •Extended EEGs can be done in the outpatient and inpatient setting and are typically done to record a typical “event” of the patient.
  • •Ambulatory EEGs: These are usually 24 to 72 hr in duration and may be done with concomitant video recording to record events in the natural habitat of the patient.
  • •Long-term EEG monitoring is typically done for patients admitted to the hospital with episodes of altered mentation due to various etiologies as well as for critically ill patients in the ICU. The duration depends on the diagnosis and response to appropriate treatment.
  • •EEG monitoring in the epilepsy monitoring unit (EMU): These are elective inpatient admissions for diagnosis and to quantify the episodes. Medications may be withdrawn or tapered during the admission rendering the patient at risk of seizures and/or status epilepticus and therefore warrant close monitoring. There are usually technicians or other designated individuals continuously monitoring the patient during this evaluation.
  • •Intracranial EEG monitoring: During evaluation for epilepsy surgery with electrodes directly implanted in the brain. Requires a very high level of monitoring in the hospital, done in the ICU or specialized EMU.


  • •There are established guidelines 4 for EEG electrode placement; by convention, the international “10-20” approach is used for scalp EEGs.
  • •The reader (neurologist) typically arranges the electrode tracings in specific arrangements, referred to as montages before interpretation.
  • •Medications, especially sedation, can change EEG patterns and need to be considered while interpreting EEGs.

Basic Concepts

Scalp EEG

  • •Non-ICU EEG:
    • 1.Standard EEGs are 30 minutes in duration and ideally incorporate awake and either drowsy or sleep states.
    • 2.Activation measures such as photic stimulation or hyperventilation are sometimes used to provoke typical events and help to confirm the diagnosis.
  • •ICU EEG:
    • 1.Several studies 5 have shown the utility and indications of continuous EEG monitoring in critically ill patients.
    • 2.The primary goal is to identify critical patterns that can be corrected and preventfurther brain injury. Some patterns of particular concern include lateralized epileptiform periodic discharges that may suggest underlying destructive lesions, generalized periodic discharges that signify toxic or metabolic underlying etiology, and burst suppression pattern with epileptiform bursts that suggest a severe encephalopathy with underlying potential to develop seizures.
    • 3.Interpretation is based on the ACNS guidelines. 6 , 7

Intracranial EEG

  • •Done as part of evaluation for surgical management of medically refractory epilepsy. 8
  • •Done in a level IV NAEC 3 with a comprehensive epilepsy team available to offer all therapeutic options for epilepsy, including epilepsy surgery.
  • •Options include superficial (grids) brain electrodes, depth (typically with stereotactic techniques) electrodes, and other specialized methods to record seizures for precise localization of seizure onset site.
  • •Interpretation is done by epileptologists and/or neurophysiologists with special expertise in interpretation of intracranial EEGs.


  • •Basic brain rhythms:
    • 1.Awake: Alpha 8.5 to 13 Hz; seen in children above the age of 3 yr and adults of all ages.
    • 2.Sleep: Drowsiness (stage 1); stage 2 with sleep spindles and K complexes, slow wave sleep (SWS) with high amplitude delta (1-4 Hz) frequency activity and REM sleep (rapid eye movements seen with low amplitude mixed frequency); sleep architecture remains stable throughout life.
  • •Other normal waveforms and variants can be seen and interpreted carefully to avoid misdiagnosis.
  • •Normal findings:
    • 1.Symmetric and continuous background
    • 2.A posterior dominant rhythm more than 8 Hz while awake
    • 3.Symmetric sleep with stages 1, 2, SWS, and REM sleep
  • •Abnormal findings:
    • 1.Awake rhythm slower than alpha (<8 Hz)—suggestive of encephalopathy with severity depending on the degree of slowing (delta 1-4 Hz activity suggestive of severe)
  • •Certain patterns can also provide guidance on etiology:
    • 1.Triphasic waves are suggestive of metabolic etiology.
    • 2.Generalized suppression can suggest hypothermia, anoxia, or medications (including anesthesia).
  • •Fluctuating patterns of unilateral, bilateral, or generalized periodic patterns that may or may not be epileptiform:
    • 1.Focal pattern may suggest underlying structural lesion.
    • 2.Generalized patterns may suggest severe encephalopathies and other etiologies and with epileptiform discharges may suggest nonconvulsive status epilepticus.
  • •Epileptiform discharges: 5 , 9 Spikes and sharp waves are typically suggestive of underlying epilepsy.
    • 1.Can be focal, multifocal, or generalized depending on the type of epilepsy.
    • 2.Specific patterns in epilepsy (e.g., generalized 3 Hz spike and wave activity seen in absence epilepsy of childhood).
    • 3.Lateralized and periodic patterns—can develop into seizures in ∼ 50%; typically associated with an underlying structural lesion.
    • 4.Evolving patterns suggestive of seizures: Changes in the background rhythm, typically in frequency and morphology that may or may not be associated with clinical correlate. Typically, these are associatedwith epileptiform discharges but can be subtle in EEGs from critically ill patients.
    • 5.EEG in the evaluation of brain death:
      • a.Technical standards exist for EEG recording in brain death. 10
      • b.Specific guidelines exist for evaluation of brain death.

Limitations & Pitfalls

  • •Scalp EEGs may not always detect seizures if the cortical generator is situated deep and the signals are attenuated.
  • •Artifacts related to movement, electrical interference, or other equipment of concomitant procedure may distort the recording and limit the ability to interpret the study accurately.

Pearls & Considerations

  • •When used appropriately, the EEG can provide invaluable information for diagnosis and treatment in the outpatient and inpatient (ICU, EMU) settings.
  • •It is important to recognize that a normal EEG does not rule out seizures as the study is done in real-time and may not capture the seizure or epileptiform activity during the period of testing.
  • •EEG in the ICU setting is invaluable in early identification and treatment of nonconvulsive seizures/status epilepticus.
  • •Intracranial EEG offers precise localization of seizure onset for planning surgical intervention in medically refractory epilepsy.
  • •EEG is not routinely used as part of brain death criteria but can be an ancillary test.


  • 1. Tatum, et al.: Clinical utility of EEG in diagnosing and monitoring epilepsy in adults. Clin Neurophysiol 2018; 129 (5): pp. 1056-1082.
  • 2. ABRET, Neurodiagnostic Credentialing and Education,
  • 3. National Association of Epilepsy Centers (
  • 4. Sinha, et al.: American clinical neurophysiology society guideline 1: minimum technical requirements for performing clinical electroencephalography. J Clin Neurophysiol 2016; 33 (4): pp. 303-307.
  • 5. Herman, et al.: Consensus statement on continuous EEG in critically Ill adults and children, part I: indications. J Clin Neurophysiol 2015; 32 (2): pp. 87-95.
  • 6. Herman, et al.: Consensus statement on continuous EEG in critically Ill adults and children, part II: personnel, technical specifications, and clinical practice. J Clin Neurophysiol 2015; 32 (2): pp. 96-108.
  • 7. Hirsch, et al.: ACNS standardized critical care EEG Terminology: 2021 version. J Clin Neurophysiol 2021; 38 (1): pp. 1-29.
  • 8. Jobst, et al.: Intracranial EEG in the 21st century. Epilepsy Curr 2020; 20 (4): pp. 180-188.
  • 9. Sam M.C., So E.L.: Significance of epileptiform discharges in patients without epilepsy in the community. Epilepsia 2001; 42 (10): pp. 1273-1278.
  • 10. Stecker, et al.: American clinical neurophysiology society guideline 6: minimum technical standards for EEG recording in suspected cerebral death. J Clin Neurophysiol 2016; 33 (4): pp. 324-327.
  • 11. Wijdicks, et al.: Evidence-based guideline update: determining brain death in adults– Report of the Quality standards Subcommittee of the American Academy of Neurology. Neurology 2010; 74 (23): pp. 1911-1918.

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