a) What are short-latency (N20) somatosensory evoked potentials (SSEPs)?
b) Describe how SSEPs can be used for prognostication in patients with hypoxic-ischaemic brain injury.
c) Explain whether, and if so how, induced hypothermia impacts on the validity of SSEP results.
Evoked potentials are the electrical signals generated by the nervous system in response to sensory stimuli.
Somatosensory evoked potentials (SSEPs) consist of a series of waves that reflect sequential activation of neural structures along the somatosensory pathways.
Somatosensory evoked potentials are usually derived from the median nerve and the tibial nerve
SSEP components typically are named by their polarity and typical peak latency in the normalb population. N20 is a negativity that typically peaks at 20 milliseconds after the stimulus.
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.
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.
Hypothermia affects SSEP test results: mainly 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.
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.
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.