The College had asked about this in  Question 19from the first paper of 2011.Specifically, they wanted indications for EEG and the particular waveforms you might expect in hypoxic brain injury. The answer was derived specifically from one source: Box 49.2 in Oh's Manual (page 556), in the chapter by Balasubramanian Venkatesh ("Disorders of consciousness").

Earlier in this century, the college wanted their candidates to "critically evaluate the role" of EEG in Question 2 from the first paper of 2005.

Thus:

Indication

Findings

Associations

Indications for EEG in the ICU, and Associated Findings
Non-convulsive status epilepticus Epileptiform discharges

Confirms the diagnosis; respects the definition (as epileptiform activity on EEG in the absence of motor manifestations)

Continuous EEG monitoring Epileptiform discharges

Monitoring of therapy for refractory status epilepticus (essentially confirms that the level of anaesthesia is sufficiently deep to suppress epileptiform activity, when motor manifestations are subtle or absent).

If there are seizures, and you are trying to detect them, how long do you need to monitor the comatose patient?

  • 56% of seizures occur in the first hour
  • 88%of seizures occur in the first 24 hours
  • 93% of seizures occur in the first 48 hours
Hepatic encephalopathy Triphasic waves
Early - alpha-wave slowing
Late - high-amplitude irregular delta waves.

Unfortunately, triphasic waves are also seen with uremic encephalopathy and with medication toxicities (e.g., lithium, valproate and baclofen).

Some EEG-specific guidelines have been synthesised (ISHEN) which acknowledge the limitation of visual "reading" of EEG data, and instead recommend computerised semiquantitiative interpretation of the relative power of frequency bands and the mean dominant frequency (which does have some prognostic value).

Ischaemic encephalopathy
  • Presence of theta activity
  • Diffuse slowing
  • Burst suppression
  • Alpha coma

The AAN recommendations for prognostication in hypoxic brain injury did not recommend the routine use of EEG, because it is "strongly but not invariably associated with poor outcome". The new 2014 consensus statement is more optimistic about it, specifcally about the absence of EEG reactivity and status epilepticus within the first 24-48 hours.

Burst suppression and alpha coma are frequently seen in severe hypoxic brain injury, and can be associated with a poor outcome. However, the false positive rate is high. The new recommendations are "do not use".

Herpes encephalitis
  • Periodic sharp spikes

Oh's Manual mentions this, but it seems to be based on studies form the 1970s. These features are not diagnostic of HSV encephalitis (i.e. one would not abandon the LP in favour of EEG) but they certainly seem to be consistently associated.

Advantages and limitations of the EEG in the ICU

Advantages

Disadvantages

Advantages and Disadvantages of EEG in ICU
  • Non-invasive
  • Characteristic findings can distinguish between different causes of encephalopathy
  • Can detect specific conditions:
    • Non-convulsive status epilepticus
    • Herpes encephalitis
    • Hepatic encephalopathy
    • Ischaemic encephalopathy
    • SAH-associated vasospasm
  • Can monitor response to antiepileptic treatment
  • Can localise epileptiform activity to a focus
  • Can detect organised activity in patients with locked-in syndrome
  • Can monitor awareness in anaesthetised or paralysed patients
  • Can be used to confirm brain death
  • This is a low-yield investigation
  • It requires specialist interpretation
  •  Experienced interpreters have very high confidence in their EEG interpretations, but low inter- and intra-rater reliability.
  • EEG is confounded by sedation and hypothermia

References

Kennedy, Jeffrey D., and Elizabeth E. Gerard. "Continuous EEG monitoring in the intensive care unit." Current neurology and neuroscience reports 12.4 (2012): 419-428.

Kaplan, Peter W., and Andrea O. Rossetti. "EEG patterns and imaging correlations in encephalopathy: encephalopathy part II." Journal of Clinical Neurophysiology 28.3 (2011): 233-251.

Guerit, Jean‐Michel, et al. "Neurophysiological investigations of hepatic encephalopathy: ISHEN practice guidelines." Liver International 29.6 (2009): 789-796.

Wijdicks, E. F. M., et al. "Practice Parameter: Prediction of outcome in comatose survivors after cardiopulmonary resuscitation (an evidence-based review) Report of the Quality Standards Subcommittee of the American Academy of Neurology."Neurology 67.2 (2006): 203-210.

Smith, J. B., et al. "A distinctive clinical EEG profile in herpes simplex encephalitis." Mayo Clinic Proceedings. Vol. 50. No. 8. 1975.

Sutter, Raoul, et al. "Electroencephalography for diagnosis and prognosis of acute encephalitis." Clinical Neurophysiology (2014).

Tyler, Kenneth L. "Update on herpes simplex encephalitis." Rev Neurol Dis 1.4 (2004): 169-178.

Guérit, J-M., et al. "Consensus on the use of neurophysiological tests in the intensive care unit (ICU): electroencephalogram (EEG), evoked potentials (EP), and electroneuromyography (ENMG)." Neurophysiologie Clinique/Clinical Neurophysiology 39.2 (2009): 71-83.

Grant, Arthur C., et al. "EEG interpretation reliability and interpreter confidence: a large single-center study." Epilepsy & Behavior 32 (2014): 102-107.

Rosenthal, Eric S. "The utility of EEG, SSEP, and other neurophysiologic tools to guide neurocritical care." Neurotherapeutics 9.1 (2012): 24-36.

Robinson, Lawrence R., et al. "Predictive value of somatosensory evoked potentials for awakening from coma*." Critical care medicine 31.3 (2003): 960-967.

Harris, Catherine. "Neuromonitoring Indications and Utility in the Intensive Care Unit." Critical Care Nurse 34.3 (2014): 30-40.