Evoked Potentials in Critical Care

Reviewed and revised 14 April 2016

OVERVIEW

  • Evoked potentials are the electrical signals generated by the nervous system in response to stimuli
  • Somatosensory evoked potentials (SSEPs) consist of a series of waves that reflect sequential activation of neural structures along the somatosensory pathways
  • SSEPs are of particular interest in critical care, due to their role in neuroprognostication

TYPES OF EVOKED POTENTIALS

  • somatosensory (SSEPs or SEPs)
  • brainstem auditory
  • visual
  • motor (cortical or spinal)

METHOD

SSEPs

  • SSEPs are assessed as normal, abnormal (increased latency or reduced amplitude), or absent on each side, providing the basis for a 6-point ordinal scale in which both hemispheres are considered
  • non-invasive, rapidly acquired and portable — can be recorded at the patient’s bedside
  • stimuli applied to a peripheral nerve (median at wrist or posterior tibial nerve at ankle)
  • low amplitude current used
  • duration 20 msec
  • resultant sensory cortical response recorded at the scalp
  • repeated to remove the background EEG and environmental electrical noise thereby producing visualized reproducible evoked responses
  • SSEP components typically are named by their polarity and typical peak latency in the normal population. N20 indicates a negative response over primary somatosensory cortex at ∼20 ms post stimulation.

Motor evoked potentials

  • can be cortical or spinal
  • allows assessment of descending motor tracts

INFORMATION GATHERED

  • cortical response peaks
  • conduction delay (latency)
  • loss of EPs occurs under conditions of profound cerebral ischaemia or mechanical trauma (thus are highly specific)
  • bilateral loss of cortical SSEPs (N20) is a strong predictor of poor outcome

SPECIFIC SITUATIONS WHERE EVOKED POTENTIALS ARE USEFUL

In general, evoked potentials may useful in these settings:

  • hypoxic-ischaemic brain injury and neuroprognostication after cardiac arrest
  • traumatic brain injury
  • brain stem integrity assessment
  • spinal cord integrity (scoliosis surgery)

SSEPs for hypoxic-ischaemic brain injury

  • Bilaterally absent short latency peaks (N20 peaks) have 100% predictive value for poor outcome (death or severe disability), with false positive rate nearly 0% and narrow confidence intervals
  • SSEP is the most reliable test to predict poor outcome in this patient group
  • SSEP does not predict good outcome
  • Pre-test probability for poor outcome essential: use test only in patients who remain unconscious following hypoxic-ischaemic insult (M score ≤ 3)
  • Validated to use as early as 24 hours after cardiac arrest
  • SSEP not influenced by sedatives, analgesics, paralysing agents or metabolic insults

Traumatic brain injury

  • conflicting data
  • SSEPs can assist in the prognosis, but should never be considered in isolation. If used, SSEPs should be integrated with other neurophysiologic tools and clinical examination

Other conditions

  • no prognostic role in SAH or septic encephalopathy

PROS AND CONS OF SSEPs

Advantages

  • Non-invasive
  • Absence of N20 responses are useful in prognostication to identify poor neurological recovery:
    • confirms diagnosis of severe hypoxic brain injury after cardiac arrest
    • after traumatic brain injury
  • Able to monitor over time (serial measurements)
  • SSEPs are less confounded by sedation or hypothermia than EEG
  • Neuromuscular blockade may improve the reliability of SSEP measurements by reducing interference from muscular activity
  • SSEPs can be used to identify cortical blindness

Disadvantages

  • Need to rule out subcortical and spinal lesions that may affect cortical response
    • e.g. in TBI, transient N20 disappearance may result from focal midbrain dysfunction due to oedema
  • Overall, tends to be a low-yield investigation
  • Interpretation is user dependent
    • Requires specialist interpretation
    • Interobserver agreement for SSEPs in anoxic-ischaemic coma is moderate to good but is influenced by noise
    • can be affected by electrical interference from muscle artefacts or from the ICU environment
  • limited availability
  • cannot be used to predict good outcome
  • Intermediate test results are common, and difficult to interpret
  • Affected by hypothermia (see below)
  • Most of prognostic accuracy studies on SSEPs in post-anoxic coma were not blinded, which may have led to an overestimation of the SSEP prognostic accuracy due to a self-fulfilling prophecy

HYPOTHERMIA AND SSEPs

  • Hypothermia affects SSEP test results
    • delayed peaks (prolongation conduction times)
    • no consistent effect on voltages (amplitudes)
  • After rewarming of the patient SSEPs have comparable test characteristics as compared with studies done before therapeutic hypothermia and as such have been validated for prognostication following hypoxic-ischaemic brain injury after rewarming with similar low false positive rate

EVIDENCE

Hypoxic-ischaemic encephalopathy

  • Bilaterally absent short latency peaks (N20 peaks) have 100% predictive value for poor outcome (death or severe disability), with false positive rate nearly 0% and narrow confidence intervals
  • The ERC/ESICM consensus statement authored by Sandroni et al (2014) on prognostication following cardiac arrest suggested that SSEPs are prognostic at:
    • > 72 hours in cooled patients, and
    • >24 hours in non-cooled patients
  • Young et al (2005) found only one was a false positive result among 287 patients with bilaterally absent N20 SSEPs. Furthermore, subsequent post hoc analysis by independent interpreters suggested that the false positive was due to inaccurate interpretation

References and Links

LITFL

Journal articles

  • Amantini A, Carrai R, Lori S, Peris A, Amadori A, Pinto F, Grippo A. Neurophysiological monitoring in adult and pediatric intensive care. Minerva Anestesiol. 2012 Sep;78(9):1067-75. Epub 2012 Jun 7. Review. PubMed PMID: 22672930. [Free Full Text]
  • Guérit JM. Neurophysiological testing in neurocritical care. Curr Opin Crit Care. 2010 Apr;16(2):98-104. doi: 10.1097/MCC.0b013e328337541a. Review. PubMed PMID: 20168224.
  • Rosenthal ES. The utility of EEG, SSEP, and other neurophysiologic tools to guide neurocritical care. Neurotherapeutics. 2012 Jan;9(1):24-36. doi: 10.1007/s13311-011-0101-x. Review. PubMed PMID: 22234455; PubMed Central PMCID: PMC3271154.
  • Kane N, Oware A. Somatosensory evoked potentials aid prediction after hypoxic-ischaemic brain injury. Practical neurology. 2015. [pubmed]
  • Sandroni C, Cariou A, Cavallaro F. Prognostication in comatose survivors of cardiac arrest: an advisory statement from the European Resuscitation Council and the European Society of Intensive Care Medicine. Intensive care medicine. 40(12):1816-31. 2014. [pubmed] [free full text]
  • Tjepkema-Cloostermans MC, Horn J, van Putten, MJAM. The SSEP on the ICU: Current applications and pitfalls. Netherlands journal of critical care, 17(1):5-9. 2013. [free full text]
  • Young GB, Doig G, Ragazzoni A. Anoxic-ischemic encephalopathy: clinical and electrophysiological associations with outcome. Neurocritical care. 2(2):159-64. 2005. [pubmed] [free full text]

FOAM and web resources


CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

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