Somatosensory evoked potentials, specifically the bilateral absence of the N20 wave, is among the most accurate predictive markers of poor neurological outcome following cardiac arrest. Along with bilaterally absent corneal reflexes and bilaterally absent pupillary reflexes, SSEP has the lowest false positive rate of any post-arrest prognostic test, around 2-4%.
Question 11 from the second paper of 2014 sems to have been a nasty surprise to around 81% of the candidates. Few were able to say 10 marks worth of anything about SSEPs. The other SSEP topic in the recent history was a vague question about the utility of SSEPs in the ICU (Question 2 from the first paper of 2005), which dates almost 10 years back. So, it is no surprise that nobody got it. Oh's Manual doesnt even have an index entry for this topic.
A critical evaluation of SSEPs in post-resuscitation anoxic coma
This is an attempt to write a well-referenced "model" answer for Question 11 from the second paper of 2014.
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)
- Lots of SSEP studies can be performed, but the only one validated for prognostication in cardiac arrest is the bilateral N20 wave.
- 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)
- Sequentially reproduceable
- Interpretation is guided by specific criteria, rather than subjective expertise.
- Good inter-interpreter agreement; low variability.
Disadvantages of SSEPs
- Not widely available
- Specific expertise is required to interpret them
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.
- 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.
- The meta-analysis by Golan et al (2014) rated SSEPs as one of the three best predictive tests of neurological outcome (with a false positive rate of 4%).
- Upon post-hoc analysis of the TTM trial (33°C vs 36°C) performed by Dragancea et al (2015), bilateral absence of SSEP N20s had a 2.6% false positive rate.
Professional organisation-level support for the use of SSEPs
- 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
- An even more recent (2016) set of post-arrest management guidelines from Canada have given SSEPs a lukewarm thumbs up (a "conditional" recommendation, on the basis of "low quality evidence"). The basis of the Canadians' reluctance to give this practice more of a boost was the strong predictive value of simple things like pupillary and corneal reflexes, which are much cheaper than routine SSEP and EEG testing.