Blurred lines: Reduced discrimination of gray white junctions observed on CT in out-of-hospital cardiac arrest (OHCA) portends poor prognosis.

Therapeutic hypothermia (TH) is a powerful tool that improves neurologic outcomes in a subset of patients suffering out of hospital cardiac arrest (OHCA). However, TH may delay the ability to perform a clinical neurologic exam and other tests, like EEG, that allow prognostication in comatose survivors of OHCA.1 In contrast, neuroimaging may be better suited to early prognostication during TH.2 In this issue of Resuscitation, Langkjaer and colleagues analyzing acute Head Computed Tomography (CT) scans when available in a large cohort of OHCA survivors (n = 341).3 “Reduced discrimination” between gray and white matter and/or cerebral edema predicted a significantly greater 30 day mortality. This association held true for patients receiving scans early (<24 h from arrest), and for those receiving scans later (≥24 h, a median of 3 [IQR 2–5]days).3The majority (70%), but not all, patients underwent TH. This is the largest study to date addressing the question of CT after cardiac arrest. The results agree with previous investigations. In a large cohort of cardiac arrest patients (n = 240), decreased gray matter attenuation to white matter attenuation ratio (GWR) on CT within 24 h of arrest, was associated with poorer survival.4 Subjects with severe cerebral edema, defined by GWR < 1.20, had very low survival with conventional care, including hypothermia. This association of GWR with outcome held true when the CT was performed within an hour of arrest in patients that were being treated with TH, Kim et al. found a threshold of an average GWR <1.14 predicted a poor neurological outcome with a high specificity and high positive predictive value.5 If return of spontaneous circulation (ROSC) is regained quickly (within 20 min), patients had fewer signs of ischemia (loss of gray-white boarder) on CTs obtained within an hour of resuscitation and better outcomes.6 As such, the European Society of Intensive Care medicine recommends “using the presence of a marked reduction of the GM/WM ratio or sulcal effacement on brain CT within 24 h after ROSC to predict a poor outcome in patients who are comatose after resuscitation from cardiac arrest both TH-treated or non-TH-treated,”.7 The Langkjaer study has some important limitations. The authors did not measure GWR in their cohort, which would have allowed an objective measure facilitating comparison with previous studies. There was not a standard protocol regarding which patients received scans and when those patients were scanned, potentially introducing a selection bias. CTs were obtained in younger patients with fewer co-morbidities, and these patients were more likely to undergo hypothermia. ICU physicians were not blinded to the results of the CTs, potentially introducing a self-fulfilling prophecy,8 namely that families of patients with reduced discrimination might be counseled to withdraw care. Unfortunately, we do not know the role the CT played in withdrawal of support decisions. Despite the widespread availability and ease of obtaining head CTs, questions remain on the optimal timing with regard to prognosis. Is it necessary to calculate a GWR, or is merely detecting “reduced discrimination” enough as this paper suggests? The answers to these important questions await prospective studies using a standardized head CT protocol in comatose survivors of OHCA with or without TH.