When should an electroencephalogram (EEG) be ordered?

When to Order an Electroencephalogram (EEG)

An EEG should be ordered emergently in patients suspected of having nonconvulsive status epilepticus or subtle convulsive status epilepticus, patients who have received a long-acting paralytic, or patients who are in a drug-induced coma. 1

Primary Indications for EEG

Urgent/Emergency EEG Indications

• Suspected nonconvulsive status epilepticus (NCSE) - particularly in patients with:

  • Altered mental status after a motor seizure 1
  • Persistent altered consciousness without clear explanation 1, 2
  • Subtle motor activity with altered consciousness 2

• Refractory status epilepticus - when seizures continue despite initial treatment 1

• Post-cardiac arrest - during therapeutic hypothermia and within 24 hours of rewarming to exclude nonconvulsive seizures in comatose patients 1

• Drug-induced coma - for monitoring of pharmacologically managed sedation 1

• Patients with long-acting paralytics - where clinical examination is limited 1

Other Important Clinical Indications

• Comatose patients with subarachnoid hemorrhage (SAH) - to detect delayed cerebral ischemia when neurological examination is unreliable 1

• Suspected viral encephalitis - especially when there is uncertainty between psychiatric or organic cause 1

• ICU patients with unexplained altered mental status - particularly those with:

  • Severe sepsis
  • Renal/hepatic failure
  • Unexplained neurological deficits 1

• First unprovoked seizure evaluation - especially when:

  • Loss of consciousness is prolonged and inconsistent with syncope
  • Episode is accompanied by clonic movements or spasms
  • Episode is followed by postictal confusion or stertor
  • Episode results in head injury or tongue biting
  • Episode is preceded by premonitory signs consistent with epilepsy 3

When EEG is NOT Recommended

• Uncomplicated syncope - when syncope is the most likely cause of transient loss of consciousness 1

• No clinical suspicion of epilepsy - in the absence of clinical signs suggesting seizure activity 3

• Routine screening in all patients with suspected encephalitis (only indicated when there is uncertainty about organic vs. psychiatric cause) 1

Continuous vs. Routine EEG Monitoring

• Continuous EEG (cEEG) is preferred over routine EEG when feasible in:
  • Critically ill patients with unexplained impaired mental status 1, 4
  • Patients at high risk for nonconvulsive seizures 5, 4
  • Routine EEG will miss nonconvulsive seizures in approximately half of cases compared to prolonged monitoring 1

Common Pitfalls and Caveats

1. Misdiagnosis of epilepsy vs. syncope:

   • Movements can occur in both epilepsy and syncope
   • In syncope, movements only occur after loss of consciousness and fall
   • In epilepsy, movements typically last ~1 minute; in syncope, only seconds 1

2. Delayed diagnosis of nonconvulsive status epilepticus:

   • NCSE can present solely as altered mental status without motor manifestations
   • EEG is the only definitive test to diagnose this condition 1, 2

3. Limited availability of emergency EEG:

   • Most facilities require neurologic consultation before obtaining emergent EEG
   • Average response time from request to initial EEG reading is approximately 3 hours 1
   • This may delay diagnosis of time-critical conditions

4. Interpretation challenges:

   • EEG interpretation is a specialized skill within neurology
   • Neurologic consultation is typically recommended before ordering 1
   • Quantitative EEG algorithms may help support expert review but should not replace it 1

The decision to order an EEG should be guided by clinical suspicion of seizure activity or other neurological conditions where EEG would provide critical diagnostic information affecting patient management and outcomes.