Does hypothermia influence the predictive value of bilateral absent N20 after cardiac arrest?

Bilateral absent N20 responses of median nerve somatosensory evoked potentials (SEPs) reliably predict poor prognosis after cardiac arrest. However, the studies supporting this fact were carried out before hypothermia was established as standard treatment. Recent evidence suggests that hypothermia treatment affects the predictive value of clinical findings in cardiac arrest patients, raising the question whether the predictive value of N20 responses has changed as well.

We retrospectively studied 185 consecutive patients treated with hypothermia after cardiac arrest. SEP recordings were available for 112 patients. SEPs were classified as bilateral absent N20, pathologic N20, or normal. Baseline and follow-up information were obtained from our database.

We identified 36 patients with bilateral absent N20, 35 (97%) of whom had poor outcome. One patient had prolonged high amplitude peripheral SEP, but bilaterally absent N20 3 days after cardiac arrest and regained consciousness with normal cognitive functions and reproducible N20 responses. One further patient had minimally detectable N20 at day 3 and recovered consciousness and normal N20 responses on follow-up.

Our data indicate that recovery of consciousness and cognitive functions is possible in spite of absent or minimally present N20 responses more than 24 hours after cardiac arrest in a very small proportion of patients. N20 responses may recover beyond this time window. The predictive value of bilateral absent N20 responses needs to be reevaluated in larger prospective studies. Until these studies are available, decisions to stop therapy in cardiac arrest survivors should not be based on N20 responses alone.