Question 11

College Answer

a)
 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.

b)
 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.

c)
 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.

Discussion

Rationale for the use of somatosensory evoked potentials in the comatose survivor of cardiac arrest

• Peripheral nerve stimulation should evoke a central response even in the presence of sedation or hypothermia
• The absence of such a response suggests severe damage to the cortex
• Bilateral absence of response suggests global rather than focal damage
• Ergo, SSEP should act as sensitive diagnostic tool to detect severe brain injury after cardiac arrest

Practice of somatosensory stimulation and evoked potential measurement

• Both median nerves are stimulated at the wrist with a bipolar surface electrode
• Alternative site is the tibial nerve
• Stimulus repeats at 2-5 Hz, with a duration of 0.2msec
• Surface electrodes read cortical activity at the scalp
• Evoked potentials are peaks of electrical activity which follow the peripheral stimulus with a predictable latency.
• The responses are named after their polarity (N for negative, P for positive) and their latency.
• N20 indicates a negative response over primary somatosensory cortex at ∼20 ms post stimulation.

Advantages of somatosensory evoked potentials

• Non-invasive
• Portable
• Less confounded by sedation or hypothermia than EEG (in fact, not influenced by sedatives, analgesics, paralysing agents or metabolic insults)
• Bilaterally absent N20 SSEP during hypothermia is a good predictor for absent N20 SSEP after rewarming, which means you can do SSEPs during the period of hypothermia (Bouwes et al, 2010)
• Reproduceable
• Interpretation is guided by specific criteria, rather than subjective expertise.

Evidence supporting the prognostic value of SSEPs

• 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.
• Recent (2014) consensus statement on prognostication following cardiac arrest suggested that SSEPs are prognostic at > 72 hours in cooled patients and at >24 hours in non-cooled patients
• Among a total 287 patients with bilaterally absent N20 SSEPs, only one was a false positive result (Young et al, 2005)
• Post hoc analysis by independent interpreters has suggested that the false positive was simply interpreted inaccurately in the first instance.

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.