Critically evaluate the role of EEG and Evoked Potentials in the critically ill.

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College Answer

The Electro-EncephaloGram is the recording of brain electrical activity from standard sites on the scalp.  It is commonly used in ICU to evaluate patients with abnormal movements or neurological impairment.  Studies have not been directed at the use or not of EEGs and their effect on outcome except for prognostication (eg. after cardiac arrests).  The EEG is useful to distinguish between potential causes of encephalopathy (eg. metabolic or drug induced) and to establish the presence of and guide therapy for potential epileptiform activity (eg. generalised or focal).   More recent applications include prognostication (eg. after cardiac arrests) where it still has significant limitations regarding sensitivity and specificity.  The more widespread and successful use of BiSpectral monitoring in anaesthesia to limit the incidence of awareness in high risk patients (“B-Aware” Myles Lancet 2004), may have relevance for some components of ICU practice.

 

Somato-Sensory Evoked Potentials (SSEP) are the averaged electrical responses in the CNS to somatic stimulation (usually from median nerve at the wrist, or nerves in the leg).  The predominant use in Intensive Care has been to evaluate patients after cerebral hypoxic insult (eg cardiac arrest).  In this setting median nerve SSEPs (eg. Bilateral absence of the N20 component) have been used in normothermic patients, comatose for at least 72 hours after cardiac arrest, to predict poor outcome with 100% specificity (see metanalysis: Zandbergen Lancet 1998).

 

Discussion

 

A 2012 article by Eric S Rosenthal -"The utility of EEG, SSEP, and other neurophysiologic tools to guide neurocritical care"- seems tailor-made to answer this question, even though it was published almost seven years after the paper.

I have used it to compile the tabulated answer below. It is a merger of two tables, available in the respective sections:

 

Modality

Advantages

Disadvantages

Advantages and Disadvantages of EEG and SSEPs in ICU
EEG
  • 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, butlow inter- and intra-rater reliability.
  • EEG is confounded by sedation and hypothermia

 

 


SSEPs
  • This is a low-yield investigation
  • It requires specialist interpretation
  • Following cardiac arrest, SSEPs have a better capacity to identify patients with poor outcome than to predict good outcome. 
  • Intermediate test results are common, and difficult to interpret.
  • Coexisting spinal lesions may affect cortical response
  • Confounded by noise from muscle activity (easier to perform in paralysed patients)

 


 

 

EEG:

  • Advantages
    • Characteristic findings can distingusih between different causes of encephalopathy
    • Can detect non-convulsive status epilepticus
    • 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 patients
    • Can be used to confirm brain death
    • Non-invasive
  • Disadvantages
    • This is a low-yield investigation
    • It requires specialist interpretation
    • It is confounded by sedation and hypothermia

 

Somatosensory Evoked Potentials (SSEP):

  • Advantages:
  • Disadvantages:
    • This is a low-yield investigation
    • It requires specialist interpretation
    • In subarachnoid haemorrhage or traumatic brain injury, it is of limited value

 

References

References

Tjepkema-Cloostermans, Marleen Catharina, J. Horn, and M. J. A. M. Putten. "The SSEP on the ICU: Current applications and pitfalls." Netherlands journal of critical care 17.1 (2013): 5-9.

 

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

 

Zandbergen, Eveline GJ, et al. "Systematic review of early prediction of poor outcome in anoxicischaemic coma." The Lancet 352.9143 (1998): 1808-1812.

 

Zandbergen, E. G. J., et al. "SSEPs and prognosis in postanoxic coma Only short or also long latency responses?." Neurology 67.4 (2006): 583-586.

 

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.