Sensory evoked potentials and the search for the Holy Grail method to predict the outcome after hypoxic-ischemic coma.

C ischemia after cardiac arrest is one of the most common causes of coma in the United States. At one year after hypoxic-ischemic coma, 53% died, 32% remain in persistent vegetative state (PVS) and only 32% have displayed some evidence of cognition.1 The search for a reliable method to confidently predict outcome post cardiac arrest is crucial for early decisions surrounding continued life support. In addition, the use of hypothermia after cardiac arrest has called into question some of our previously held notions on predicting outcome after cardiac arrest. The use of somatosensory evoked potentials (SEPs) is a noninvasive, objective bedside technique to detect the integrity of the somatosensory pathway.2 Adults in coma from hypoxic-ischemic encephalopathy with absent evoked potential responses have <1% chance of awakening.1 The short – latency SEP peak (SLSEP) N20 is generated in the primary somatosensory cortex where thalamocortical (TC) cells make synaptic connections with the superficial and deep pyramidal cell layers. Therefore, N20 peak reflects the thalmocortical pathways in which somatosensory neurons are functioning as receivers.3 SEPs have maintained their predictive utility in the era of hypothermia. In adults comatose patients admitted after cardiac pulmonary resuscitation (CPR) and treated with induced therapeutic hypothermia (TH) the absence of cortical N20 responses had a positive predictive value of poor outcome of 1.00.4 Despite the high specificity of N20 absence to predict poor outcome, its presence does not indicate good neurological outcome. Middle-latency somatosensory evoked potentials (MILSEP) include the N35, P45 and N60 components that follow the N20 response. An intact MILSEP thus reflects additional cortical interactions. In a previous study a preserved MILSEP N60 response was suggestive of a good outcome. The authors recommended the use of both MLSEP and SLSEP to improve predictive power.5 The return of consciousness is mainly determined by sufficient recovery of the synaptic transmission function of pyramidal cells. Therefore, the combined use of N20 and continuous electerencephalography (cEEG) will have better predictive value for the neurological outcome. The authors reasoned that intact SEPs reflect intact cortex which may recover while cEEG monitoring may be able to track synaptic function recovery.3 In this issue Zanatta et al.6 retrospectively reviewed the functional outcome of 17 patients with post anoxic coma after cardiac arrest. All patients were evaluated with serial SEPs. Patients with intact MILSEP responses had a good outcome while patients without N20/P25 (SLSEP) but with an increase in blood pressure remained in Sensory evoked potentials and the search for the Holy Grail method to predict the outcome after hypoxic-ischemic coma