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.
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
- 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)
- 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.